Mycorrhizoremediation—an enhanced form of phytoremediation

Mycorrhizoremediation—an enhanced form of phytoremediation 1 2 first-author †E-mail:a.khan@uws.edu.aulasara37@hotmail.com †E-mail:a.khan@uws.edu.aulasara37@hotmail.com a.khan@uws.edu.au Department of Environmental Science, Faculty of Science, Allama Iqbal Open University, Islamabad 44000, Pakistan Department of Environmental Science, Faculty of Science, Allama Iqbal Open University, Islamabad 44000, Pakistan University of Western Sydney, Fac. Sci., Sch. Sci., Penrith, NSW 1797, Australia University of Western Sydney, Fac. Sci., Sch. Sci., Penrith, NSW 1797, Australia Study of plant roots and the diversity of soil micro biota, such as bacteria, fungi and microfauna associated with them, is important for understanding the ecological complexities between diverse plants, microbes, soil and climates and their role in phytoremediation of contaminated soils. The arbuscular mycorrhizal fungi (AMF) are universal and ubiquitous rhizosphere microflora forming symbiosis with plant roots and acting as biofertilizers, bioprotactants, and biodegraders. In addition to AMF, soils also contain various antagonistic and beneficial bacteria such as root pathogens, plant growth promoting rhizobacteria including free-living and symbiotic N-fixers, and mycorrhiza helping bacteria. Their potential role in phytoremediation of heavy metal (HM) contaminated soils and water is becoming evident although there is need to completely understand the ecological complexities of the plant-microbe-soil interactions and their better exploitation as consortia in remediation strategies employed for contaminated soils. These multitrophic root microbial associations deserve multi-disciplinary investigations using molecular, biochemical, and physiological techniques. Ecosystem restoration of heavy metal contaminated soils practices need to incorporate microbial biotechnology research and development. This review highlights the ecological complexity and diversity of plant-microbe-soil combinations, particularly AM and provides an overview on the recent developments in this area. It also discusses the role AMF play in phytorestoration of HM contaminated soils, i.e. mycorrhizoremediation. Phytoremediation Plant growth promoting rhizobacteria Arbuscular mycorrhizae (AM) Mycorrhizoremediation Mycorrhiza helping bacteria Ecosystem restoration Phytorestoration Heavy metal (HM) INTRODUCTION 1 Biosphere pollution by heavy metals and nucleotides was accelerated dramatically during the last few decades due to mining, smelting, manufacturing, treatment of agricultural soils with agro-chemicals and soil sludge, etc. Problems associated with the contamination of soil and water such as animal welfare, health, fatalities and disruptions of natural ecosystems are well documented (He et al., 2005 2005 ). Heavy metals such as Pb, Cr, As, Cu, Cd, and Hg, being added to our soils through industrial, agricultural and domestic effluents, persist in soils and can either be adsorbed in soil particles or leached into ground water. Human exposure to these metals through ingestion of contaminated food or uptake of drinking water can lead to their accumulation in humans, plants and animals. Lead, Copper, Zinc and Cadmium are also found naturally in soils and they can cause significant damage to environment and human health as a result of their mobility and solubilities. They can occur in soil and water in several forms and their speciation in soils is determined by sequential extraction using specific extract ants, which solubilize different phases of metals (Shuman, 1985 1985 ). The physical and chemical characteristics of soil determine the speciation and mobility of heavy metals (Kabata-Pendias and Pendias, 1992 1992 ). SOIL AND RHIZOSPHERE 1 Many soil microbes originate in the soil or are closely associated with the soil environment and have substantial impact on mankind (Doyle and Lee, 1986 1986 ). We know very little about the enormous diversity of soil microbes, their properties, and behaviour in the soil environment. Soil microorganisms inhabiting the rhizosphere environment interact with plant roots and mediate nutrient availability, e.g. those forming useful symbiotic associations with the roots and contribute to plant nutrition. Implications of plants and their symbionts like mycorrhizal fungi, N-fixing rhizobia, and free living rhizosphere population of bacteria which promote plant growth need to be fully exploited and encouraged by inoculating nutrient poor agricultural soils with appropriate microbes (Khan, 2002a 2002a ). Rhizosphere and microorganisms 2 1. Glomalian fungi 3 Glomales comprise one of the oldest group of fungi, older than land plants. The first land plants, Bryophytes, appeared in the Mid Silurian era (476~430 My old). The oldest fossil evidence of Bryophyte-like land plants, 100 My ago in the Early Devonian, had AM like infections even before roots evolved (Phipps and Taylor, 1996 1996 ). The first land plants were most likely evolved from algae but no fossil records are available for the rootless freshwater Charophycean algae, which are the probable ancestors of land plants, to show if they were mycorrhizal. Mosses, liverworts, and hornworts often contain structures like hyphae, vesicles, and arbuscules, characteristics of AM fungi (Schüßler, 2000 2000 ). Sphenophytes , Lycopodophytes , and Pteridophytes are among the first land plants with roots which originated in the mid Devonian era, and AM associations are reported in these plants (Brundrett, 2002 2002 ). Both living and Triassic fossil Cycades had AMF in their roots. AM associations are ubiquitous in the living angiosperms, which probably arose in the early Cretaceous era (Taylor and Taylor, 1993 1993 ). It is hypothesized that AM fungi were instrumental in the colonization of land by ancient plants (Simon et al., 1993 1993 ). This hypothesis is supported by observation that AM can now be found worldwide in the angiosperms, gymnosperms as well as ferns, suggesting that the nature of the association is ancestral. Furthermore, the origin of AM fungi coincides with that of vascular plants suggesting the nature of the association and supporting the hypothesis that AMF were instrumental in the colonization of land by ancient plants. It is now established that universal and ubiquitous symbiotic arbuscular mycorrhizal (AM) fungi, belonging to Glomales, form symbiotic relationships with roots of 80%~90% land plants in natural and agricultural ecosystems (Brundrett, 2002 2002 ), including halophytes, hydrophytes and xerophytes (Khan, 1974 1974 ; Khan and Belik, 1995 1995 ; Khan, 2003 2003 ), and are known to benefit plant nutrition, growth and survival, due to their greater exploitation of soil for nutrients (Smith and Read, 1997 1997 ). These associations represent a key factor in the below ground networks which influence diversity and plant community structure (van der Heijden et al., 1998 1998 ; Burrows and Pfleger, 2002 2002 ; O’Connor et al., 2002 2002 ; Chaudhry et al., 2005 2005 ), but we know very little about the enormous AM fungal diversity in soils and their properties and behaviour in the soil (Khan, 2002a 2002a ). The degree of benefit to each partner in any AMF-plant host interaction depends not only on the particular plant and AMF species involved but also on the rhizobacteria and soil abiotic factors. 2. Glomalin and heavy metal sequestration 3 Heavy metals in soil are associated with a number of soil components which determine their behaviour in the soil and influence their bioavailability (Boruvka and Drabek, 2004 2004 ). The cell wall components such as free amino, hydroxyl, carboxyl and other groups of soil fungi can bind to potentially toxic elements such as Cu, Pb, Cd, etc. (Kapoor and Viraraghavan, 1995 1995 ). Many filamentous fungi can sorb these trace elements and are used in their commercial biosorbants (Morley and Gadd, 1995 1995 ). The proteins in the cell walls of AM fungi appear to have similar ability to sorb potentially toxic elements by sequestering them. There is evidence that AMF can withstand potentially toxic elements. Gonzalez-Chavez et al.( 2004 2004 ) showed that glomalin produced on hyphae of AMF can sequester them. AMF play a significant ecological role in the phytostabilization of potentially toxic trace element polluted soils by sequestration and, in turn, help mycorrhizal plants survive in polluted soils. One of these components is Glomalin, a glycoprotrien produced by the hyphae of AMF fungi (Wright and Upadhyaya, 1998 1998 ), which is released into soil from AMF hyphae (Driver et al., 2005 2005 ). These authors, using an in vitro system of Ri T-DNA transformed roots infected with Glomus intraradices, an AMF, showed that glomalin is tightly bound in AMF hyphal and spore walls. Small amounts ( < 20%) of glomalin were found to be adhered to soil via release into liquid medium from hyphae and not through passive secretion and their function is physiological in the course of the life of the organism. It has been hypothesized that glomalin has a role in the immobilization (‘filtering’) of heavy metals at the soil-hypha interface, i.e. before entry into fungal-plant system. The extra-radical mycelium of AMF, in addition to its crucial role in enhancing nutrition of host plant, also plays a role in soil particle aggregation and soil stability (Dodd et al., 2000 2000 ; Wright and Upadhyaya, 1998 1998 ). There has been few analytical studies of AM in polluted soils. While some workers observed that the external mycelium of AMF was the main site for trace element localization (Kaldorf et al., 1999 1999 ; Turnau, 1998 1998 ), others reported selective exclusion of toxic and non-toxic elements by adsorption onto chitinous cell walls (Zhou, 1999 1999 ), or onto extra-cellular glycoprotein, glomalin (Wright and Upadhyaya, 1998 1998 ), or intra-cellular precipitation. All these mechanisms have implications in reducing a plant’s exposure to potentially toxic elements, i.e. mycorrhizoremediation technology. Gonzalez-Chavez et al.( 2002 2002 ) studied the form and localization of Cu accumulation in the extra-radical mycelium of three AM fungi isolated from the same polluted soil contaminated with Cu and As. The authors reported differential capacity of AMF to sorb and accumulate Cu as determined by TEM and SEM. However, the nature of accumulation and mechanisms involved require further studies in order to better understand the participation of AMF in plant tolerance and its ecological significance in polluted soils. The AMF can be screened for their ability to produce maximum levels of extra-radical mycelium in polluted soils (Joner et al., 2000 2000 ), and to utilize adapted AMF to help accumulate HM both within the plant roots (phytoaccumulation) and the extra metrical fungal mycelium. Most of the information available in literature concerning fairly recently discovered soil compound, glomalin, an iron-containing glycol-soil-proteinaceous substance produced by AMF, is in relation to its role in soil aggregation (Rillig et al., 2003 2003 ). Glomalin makes a large contribution to active soil organic C pools. It is quite recalcitrant and we know very little about its chemical structure. Glomalin plays a vital part in sorption and sequestration of potentially toxic elements, reducing their bioavailability (Gonzalez-Chavez et al., 2004 2004 ). It has been suggested that this sequestration could be important for heavy metal biostabilization in heavy metal polluted soils (Khan, 2005 2005 ). Glomalin attaches to soil and helps stabilize aggregates. Mycorrhizae may be of great significance for the survival of plants at HM polluted sites. Literature data on glomalin content in HM polluted soils are extremely scarce and there is a need to gather data on this aspect of mycorrhizoremediation, i.e use of mycorhizal plants in the phytoremediation of HM contaminated soils. AM AND PHYTOREMEDIATION 1 Despite the importance of the role that AM play in soil-microbe-plant interactions, relatively few studies have focused on their potential in phytoremediation efforts. This is first due to the fact that earlier phytoremediation studies focused on the predominantly non-mycorrhizal plant families such as Brassicaceae and Caryophyllaceae , and second AM have not been considered by earlier workers as important component of phytoremediation practices. It is possible to improve the phytoremediation capabilities of plants by inoculating them with appropriate AM fungi. Significance of arbuscular mycorrhizae 2 AM associations are important in natural and managed ecosystems due to their nutritional and non-nutritional benefits to their symbiotic partners. They can alter plant productivity, because AMF can act as biofertilizers, bioprotectants, or biodegraders (Xavier and Boyetchko, 2002 2002 ). AMF are known to improve plant growth and health by improving mineral nutrition, or increasing resistance or tolerance to biotic and abiotic stresses (Clark and Zeto, 2000 2000 ; Turnau and Haselwandter, 2002 2002 ). Their potential role in phytoremediation of heavy metal contaminated soils and water is also becoming evident (Chaudhry et al., 1998 1998 ; Khan et al., 2000 2000 ; Khan, 2001 2001 ; Jamal et al., 2002 2002 ; Hayes et al., 2003 2003 ). AMF modify the quality and abundance of rhizosphere microflora and alter overall rhizosphere microbial activity. Following host root colonization, the AMF induces changes in the host root exudation pattern, which alters the microbial equilibrium in the mycorrhizosphere (Pfleger and Linderman, 1994 1994 ). These interactions can be beneficial or harmful to the partner microbes involved and to the plant, and sometimes may enhance plant growth, health, and productivity (Paulitz and Linderman, 1989 1989 ; Lynch, 1990 1990 ). Giovannetti and Avio ( 2002 2002 ) reviewed and analysed important data on the main parameters affecting AM fungal infectivity, efficiency, and ability to survive, multiply and spread, which may help in utilizing obligate biotrophic AMF in biotechnological exploitation and sustainable agriculture. There is a need to understand and better exploit AM symbionts in the different world ecosystems. Although AMF are ubiquitous, it is probable that natural AM associations are not efficient in increasing plant growth (Fitter, 1985 1985 ). Cropping sequences as well as fertilization and plant pathogen management practices also dramatically affect the AMF propagules in the soil and their effects on plants (Bethlenfalvay and Linderman, 1992 1992 ). The propagation system used for horticultural fruit and micro-propagated plants, can benefit most from AM biotechnology. Micropropagated plants can withstand transplant stress from in vitro to in vivo systems, if they are inoculated with appropriate AMF (Lovato et al., 1996 1996 ; Azcon-Aguilar et al., 2002 2002 ). In order to use AMF in sustainable agriculture, knowledge of the factors such as fertilizer inputs, pesticide use, soil management practices, etc. influencing AMF communities is essential (Bethlenfalvay and Linderman, 1992 1992 ; Allen, 1991 1991 ; 1992 1992 ). This area deserves further research and efforts because sound scientific knowledge is necessary for the improvement of AM biotechnology aimed at selecting infective and efficient inoculants to be used as biofertilizers, bioprotectants, and biostimulants in sustainable agriculture, horticulture, and forestry. The potential of arbuscular mycorrhizal fungi (AMF) to enhance plant growth is well recognized but not exploited to the fullest extent. They are rarely found in nurseries due to the use of composted soil-less media, high levels of fertilizer and regular application of fungicide drenches. The potential advantages of the inoculation of plants with AM fungi in horticulture, agriculture, and forestry are not perceived by these industries as significant. This is partially due to inadequate methods for large-scale inoculum production. Monoxenic root-organ in vitro culture methods for AMF inocula production have also been attempted by various workers (Mohammad and Khan, 2002 2002 ; Fortin et al., 2002 2002 ) but these techniques, although useful in studying various physiological, biochemical, and genetic relationships, have limitations in producing inocula of AM fungi for commercial purposes. Pot cultures in pasteurized soils, have been the most widely used method for producing AMF inocula but are time consuming, bulky, and often not pathogen free. To overcome these problems, soil-free methods such as soil-less growth media, aeroponics, hydroponics and axenic cultures of AM fungi have been used successfully to produce AMF-colonized root inocula (Sylvia and Jarstfer, 1994a 1994a ; 1994b 1994b ; Mohammad et al., 2000 2000 ; Mohammad and Khan, 2002 2002 ). Substrate-free colonized roots produced by these methods can be sheared and used for large-scale inoculation purposes. Mohammad et al.( 2004 2004 ) compared the growth responses of wheat to sheared root and pot-culture inocula of AMF at different P levels under field conditions, and concluded that P fertilization can be substituted by AMF inoculum produced aeroponically to an extent of 5 kg/ha under field conditions. Arbuscular mycorrhiza-rhizobacteria interactions 2 The increased microbial activity in the rhizosphere soil affects the plant. A range of stimulated rhizosphere microorganisms such as saprophytes, pathogens, parasites, symbionts, etc., carry out many activities which are important to plant health and growth. Some of these microbes affect plant root morphology and physiology by producing plant growth-regulating hormones and enzymes. Others alter plant nutrient availability and biochemical reactions undertaken by them. AM fungi have differential effects on the bacterial community structure in the mycorrhizosphere (Paulitz and Linderman, 1989 1989 ; Marschner and Baumann, 2003 2003 ). AMF improve P-nutrition by scavenging available P through the large surface area of their hyphae. PGPR (plant growth promoting rhizobacteria) may also improve plant P-acquisition by solubilizing organic and inorganic P sources through phosphatase synthesis or by lowering pH of the soil (Rodríguez and Fraga, 1999 1999 ). Garbaye ( 1994 1994 ) defined MHB (mycorrhization helper bacteria) as “bacteria associated with mycorrhizal roots and mycorrhizal fungi which collectively promote the establishment of mycorrhizal symbioses”. There is growing evidence that diverse microbial populations in the rhizosphere play a significant role in sustainability issues (Barea, 2000 2000 ; Barea et al., 2002 2002 ), and that the manipulation of AMF and certain rhizobacteria like PGPR and MHB is important. Vivas et al.( 2003 2003 ) used a dual AM fungus-bacterium inoculum to study the effect of drought stress induced in lettuce grown in controlled-environment chambers. Their results showed that there was a specific microbe-microbe interaction that modulates the effectivity of AMF on plant physiology. The authors concluded that plants must be mycorrhizal in nutrient-poor soils and that mycorrhizal effects can be improved by co-inoculation with MHB such as Bacillus spp. Results of this study by Vivas and co-researchers show that co-inoculation of selected free-living bacteria isolated from adverse environments and AM fungi can improve the formation and function of AM symbiosis, particularly when the plant growth conditions are also adverse. Both AMF and PGPR complement each other in their role in N-fixation, phytohormone production, P-solubilization, and increasing surface absorption. Behl et al.( 2003 2003 ) studied the effects of wheat genotype and Azobacter survival on AMF and found that the genotype tolerant to abiotic stresses had higher AMF infection and a cumulative effect of plant-AMF-PGPR interaction was found. Similar observations were made by Chaudhry and Khan ( 2002 2002 ; 2003 2003 ), who studied the role of symbiotic AMF and PGPR N-fixing bacterial symbionts in sustainable plant growth on nutrient-poor heavy metal contaminated industrial sites and found that the plants surviving on such sites were associated with N-fixing rhizobacteria and had a higher arbuscular mycorrhizal infection, i.e. a cumulative and synergistic effect. The MHB cannot be ignored when studying mycorrhizal symbioses in their natural ecosystems. They are quite common and, as Garbaye ( 1994 1994 ) said, they are found every time they are looked for, and they seem to be closely associated with the mycorrhizal fungi in the symbiotic organs. They are adapted to live in the close vicinity of the AM fungi as high frequencies of MHB populations are isolated from the mycorrhizae. Some MHB isolates also promoted ectomycorrhizae formation in four conifers (Garbaye et al., 1992 1992 ), indicating that the MHB effect is not plant-specific. Various researchers showed that MHB’s are fungus selective (Garbaye, 1994 1994 ). Mosse ( 1962 1962 ) showed that cell-wall degrading enzyme producing Pseudomonas sp. enhanced the germination of AM fungal spores of Glomus mosseae and promoted the establishment of AM on clover roots under aseptic conditions. These observations were later supported by other workers (Mayo et al., 1986 1986 ; Linderman and Paulitz, 1990 1990 ). Enriched soil microbial communities in the mycorrhizosphere are often organized in biofilms and probably horizontal gene transfer (HGT) between co-inhabiting microbial species and between plant-to-microbe occurs (van Elsas et al., 2003 2003 ). Many plant associated Pseudomonas rhizobacteria produce signal molecules for quorum sensing regulation, which were absent from soil-borne strains (Elasri et al., 2001 2001 ). This indicates that quorum sensing systems exist and are required in the mycorrhizosphere. Microbial colonies on the root surfaces consist of many populations or strains and positive and negative inter-population signaling on the plant root occur (Pierson et al., 2002 2002 ), which may play an important role in the efficiency of the use of biofertilizers. In addition to the above described interactions between AM fungi and rhizobacteria, certain bacteria-like organisms (BLOs) reside in the AM fungal cytoplasm, first described by Mosse ( 1962 1962 ). Khan ( 1971 1971 ) illustrated AMF spores, collected from the semi-arid areas of Pakistan, containing one to ten small spherical ‘endospores’ without any subtending hyphae of their own. Ultra structural observations clearly revealed their presence in many field-collected AM fungal isolates. Because of their unculturable nature, further investigation of BLO’s was hampered but current advanced electron and confocal microscopic and molecular analysis techniques have allowed us to know a bit more about their endosymbiotic nature. Minerdi et al.( 2002 2002 ) reported the presence of intracellular and endosymbiotic bacteria belonging to the genus Burkholderia (a genus known to fix N) in fungal hyphae of many species of Gigasporaceae . The authors used genetic approaches to investigate the presence of N-fixing genes and their expression in this endosymbiont and found nif HDK genes in the endosymbiont Burkholderia and their RNA messengers demonstrating that they possess a molecular basis for N-fixation. This discovery, as stated by Minerdi et al.( 2002 2002 ), indicates that a fungus which improves P-uptake, might also fix N through specialized endobacteria. Endosymbiont Bukholderia may have an impact on AMF-PGPR-MHB-Plant associations and metabolism. This finding suggests a new application scenario worth pursuing. Recent methodological developments in molecular and microscopical techniques together with those in genomes, bioinformatics, remote-sensing, proteomics, etc. will be of help in understanding the complexity of interactions existing between diverse plants, microbes, climates, and soil. Plant growth promoting rhizobacteria 2 Rhizobacteria include mycorrhization helper bacteria (MHB) and plant growth promoting rhizobacteria (PGPR), which assist AMF to colonize the plant roots (Klyuchnikov and Kozherin, 1990 1990 ; Andrade et al., 1997 1997 ), P-solubilizers, free-living and symbiotic nitrogen fixers, antibiotic producing rhizobacteria, plant pathogens, predators and parasites (Sun et al., 1999 1999 ). The most common bacteria in the mycorrhizosphere are Pseudomonas (Vosátka and Gryndler, 1999 1999 ), while different bacterial species exist in the hyphosphere. Like AMF, rhizobacteria such as pseudomonads, are also ubiquitous members of the soil microbial community and have received special attention as they also exert beneficial effect on plant growth by suppressing soil-born pathogens, synthesizing phytohormones, and promoting plant growth (Weller, 1988 1988 ; Glick, 1995 1995 ; Kapulnik, 1996 1996 ; Chin-A-Woeng et al., 2003 2003 ). Many fluorescent Pseudomonas strains have been reported as plant growth enhancing beneficial rhizobacteria. They are studied extensively in agriculture for their role in crop improvement as they stimulate plant growth either by producing plant growth promoting hormones, fixing atmospheric N 2 or suppressing plant pathogens. Rhizospheric component of PGPR bacteria adhere to the root surfaces, use root exudates for growth, synthesize amino acids and vitamins, and establish an effective and enduring root colonization (Lugtenberg and Dekkers, 1999 1999 ). However, quantitative and qualitative variations in root exudates during plant growth could affect the rhizospheric competency of introduced PGPRs. The development of AM symbiosis, also influence PGPR dynamics. Pseudomonas fluorescens 92rk increased mycorrhizal colonization of tomato roots by Glomus mosseae BEG12 suggesting that strain 92rk behaves as MHB (Toro et al., 1997 1997 ; Singh and Kapoor, 1998 1998 ). Many researchers have reported additive effects on plant growth by AMF and PGPR (Edwards et al., 1998 1998 ; Galleguillos et al., 2000 2000 ; Gamalero et al., 2003 2003 ) but the mechanisms by which MHB and PGPR stimulate AM colonization are still poorly understood (Barea et al., 1997 1997 ). Various hypotheses have been suggested which include physical, chemical, physiological, and even direct stimulatory or antagonistic relations between AMF and other mycorrhizosphere microbes (Bansal et al., 2002 2002 ). AM and PGPR symbioses not only induce physiological changes in the associated plant, but also modify the morphological architecture of the roots such as total root length and root tip numbers (Atkinson et al., 1994 1994 ). Gamalero et al.( 2003 2003 ) found the highest values of root architecture parameters in tomato roots inoculated with two strains of P . fluorescens (92rk and P190r) and AMF Glomus mosseae BEG12. The authors ascribed these findings both to modification of root architecture, due to PGPR and AMF, and to a greater absorption surface area due to extrametrical mycelium of AMF. PGPR have also been shown to induce systemic resistance (ISR) to fungal, bacterial, and viral pathogens in various crops such as bean, tomato, radish, and tobacco (Zhang et al., 2002 2002 ). Co-occurrence of AMF and PGPR/MHB 2 Co-existence of certain fungi and rhizobacteria has been known since 1896, when Janse ( 1896 1896 ) reported bacteria and fungi in the same pieces of legumes roots. These fungi were also described as arbuscular mycorrhizal by Jones ( 1924 1924 ), and it was Asai ( 1944 1944 ) who first stated that root nodulation by the rhizobacteria was dependent on the formation of mycorrhizae by the AM fungi. These earlier observations were confirmed by subsequent researchers (Filion et al., 1999 1999 ). Multifaceted interactions of AM fungi with various micro-organisms and micro-fauna in the mycorrhizosphere may be positive or negative (Facelli et al., 1999 1999 ). The positive/synergistic interactions between mycorrhizosphere AM fungi and various N-fixing and P-solubilizing bacteria is the basis of application of these microbes as biofertilizer and bio-protectant agents (Bansal et al., 2002 2002 ). These microbes are regulated by AMF for their own benefit, which in turn benefit the host plant. Meyer and Linderman ( 1986 1986 ) reported enhanced mycorrhization of clover in the presence of PGPR rhizobacterium Pseudomonas putida . Similar observations were made later by several other researchers (Suresh and Bagyaraj, 2002 2002 ). All these studies suggest that colonization of plant roots by AM fungi significantly influences the mycorrhizosphere microorganisms, including PGPR. Duc et al.( 1989 1989 ) were the first to report that some pea mutants defective in nodulation also did not form AM symbiosis. Since the first report of legume nodulation mutants to be non-mycorrhizal, a range of nodulation-defective mutant legumes and non-legume mutated crops, have been tested (Barker et al., 2002 2002 ). Many recent reports have confirmed the similarities between nodulation and mycorrhiza-formation processes (Peterson and Guinel, 2000 2000 ; Resendes et al., 2001 2001 ). There is enough evidence available now which indicates positive interaction between AM fungi and PGPR (Chanway et al., 1991 1991 ). There are several reports on the interactions between AMF and Rhizobium species (Albrecht et al., 1999 1999 ) which suggest that the interaction is synergistic, i.e. AM fungi improve nodulation due to enhanced P-uptake by the plant. In addition to this principal effect of AMF on P-mediated nodulation, other secondary effects include supplying trace elements and plant hormones, which play an important role in nodulation and N 2 -fixation. Current researches are directed to gaining understanding on the role AMF play in the expression of Nod genes in Rhizobia (Harrison, 1997 1997 ). Barea and his associates (Azcon-Aguilar and Barea, 1994 1994 ; Azcon-Aguilar et al., 2002 2002 ; Barea, 2000 2000 ; Barea et al., 2002 2002 ) achieved many significant findings in this area. Synergistic interactions between AMF and asymbiotic N 2 fixing bacteria such as Azobacter chroococum , Azospirillum spp. and Acetobacter diazotrophicus have also been reported by many researchers (Suresh and Bagyaraj, 2002 2002 ). Synergistic interactions between P-solubilizing bacteria and AMF and their effect on plant growth were studied by many workers during the last three decades. Duponnois and Plenchette ( 2003 2003 ) studied the effects of MHB Pseudomonas monteillii strain HR13 on the frequency of AM colonization of Australian Acacia species and reported a stimulatory effect. The authors recommend dual inoculation to facilitate controlled mycorrhization in nurseries where Acacia species are grown for forestation purposes. Modern researches have provided evidence that the genetic pathway of AM symbiosis is shared in part by other root-microbe symbioses such as N-fixing rhizobia (Peterson and Guinel, 2000 2000 ). Many researchers reported unsuccessful attempts to select an appropriate Rhizobium strain for inoculating legumes owing to the failure of the selected strain(s) to survive and compete for nodule occupancy with indigenous native strains under low P and moisture contents (Bottomley, 1992 1992 ). In this context, the role of AM fungi as P suppliers to legume root nodules appears to be of great relevance. Requena et al.( 1997 1997 ) found a specific AM fungus, Rhizobacterium sp. combination for effective nodulation and N 2 fixation in a mycotrophic legume Anthyllis cytisoides in Mediterranean semi-arid ecosystems in Spain. They reported Glomus intraradices to be more effective with Rhizobium sp. NR4, whereas G . coronatum was more effective when co-inoculated with strain NR9. Such specificity in interactions between AM fungi, Rhizobium and PGPR have been described by various researchers, indicating that it is important to consider the specific functional compatibility relationships between AMF, PGPR and MHB and their management when used as biofertilizers. New techniques applied in molecular ecology have resulted in the identification of members of unculturable Archea in the mycorrhizospheres but their role in the hyphosphere is not known (Bomberg et al., 2003 2003 ). This means, as pointed out by Sen ( 2003 2003 ), that analyses of PGPR distribution and activities must now be extended to accommodate Crearchaeotal microbes as well. Areas like host-microbe-specificity and microbial-linked control of plant diversity and productivity still needs to be elucidated at gene, organismal, and ecosystem scales (Sen, 2003 2003 ). CONCLUSION 1 AM are ubiquitous and most plants are colonized by AMF in nature, i.e., mycorrhizosphere is the rule, not the exception. Thus if we are to understand the rhizosphere reactions and interactions, we must understand the mycorrhizosphere. Mycorrhization helper bacteria (MHB) might be exploited to improve mycorrhization, and AMF to improve nodulation and stimulate PGPR. It is anticipated that future commercial biofertilizers would contain PGPR, MHB, and AMF. Requena et al.( 1997 1997 ) found that AM fungus Glomus coronatum , native in the desertified semi-arid ecosystems in the southeast of Spain, was more effective than the exotic G . intraradices in AM/PGPR co-inoculum treatments. The indigenous isolates must be involved. This area merits greater attention. More extensive field investigations on this multi-agent biofertilizer will make this a popular technology among field workers in agriculture, forestry and horticulture. Manipulation of microorganisms in the mycorrhizosphere for the benefit of plant growth requires research at the field level (Khan, 1975 1975 ; 2002b 2002b ). In order to exploit microbes as biofertilizers, biostimulants and bioprotectants against pathogens and heavy metals, ecological complexity of microbes in the mycorrhizosphere needs to be taken into consideration and optimization of rhizosphere/mycorrhizosphere systems need to be tailored. Smith ( 2002 2002 ) stressed the need to better integrate information on root and soil microbe distribution dynamics and activities with known spatial and physiochemical properties of soil. This, as pointed out by Smith ( 2002 2002 ), should be achieved through greater collaborative efforts between biologists, soil chemists and physicists. References C Albrecht , R Geurts , T Bisseling . Legume nodulation and mycorrhizae formation, two extremes in host specificity meet . The EMBO Journal , 1999 . 18 ( 2 ): 281 - 288 . 9889184 DOI: 10.1093/emboj/18.2.281 http://doi.org/10.1093/emboj/18.2.281 . Legume nodulation and mycorrhizae formation, two extremes in host specificity meet MF Allen . The Ecology of Mycorrhiza , 1991 . : Cambridge Cambridge Uni. Press , 184 . The Ecology of Mycorrhiza MF Allen . Mycorrhizal Functioning: An Integrative Plant-Fungal Process , 1992 . : Routledge, NY Chapman & Hall Inc. , 534 . Mycorrhizal Functioning: An Integrative Plant-Fungal Process G Andrade , KL Mihara , RG Linderman , GJ Bethlenfalvay . Bacteria from the rhizosphere and hyphorhizosphere soils of different arbuscular mycorrhizal fungi . Plant and Soil , 1997 . 192 ( 1 ): 71 - 79 . DOI: 10.1023/A:1004249629643 http://doi.org/10.1023/A:1004249629643 . Bacteria from the rhizosphere and hyphorhizosphere soils of different arbuscular mycorrhizal fungi T Asai . Die bedeutung der mikorrhiza fur das pflanzenleben . Japanese J Bot , 1944 . 12 359 - 408 . . Die bedeutung der mikorrhiza fur das pflanzenleben D Atkinson , G Berta , JE Hooker . S Gininazzi , H Scheupp . Impact of Mycorrhizal Colonization on Root Architecture, Root Longevity, and the Formation of Growth Regulators . Impact of Arbuscular Mycorrhizas on Sustainable Agriculture and Natural Ecosystems , 1994 . : Basel Birkhauser Verlag , 89 - 99 . . Impact of Mycorrhizal Colonization on Root Architecture, Root Longevity, and the Formation of Growth Regulators C Azcon-Aguilar , JM Barea . MA Allen . Interactions between Mycorrhizal Fungi and Other Rhizosphere Microorganisms . Mycorrhizal Functioning: An Integrative Plant-fungus Process , 1994 . : London Chapman and Hall , 163 - 198 . . Interactions between Mycorrhizal Fungi and Other Rhizosphere Microorganisms C Azcon-Aguilar , MC Jaizme-Vega , C Calvet . S Gianinazzi , H Schuepp . The Contribution of Arbuscular Mycorrhizal Fungi to the Control of Soil-borne Plant Pathogens . Mycorrhizal Technology: from Genes to Bioproducts―Achievements and Hurdles in Arbuscular Mycorrhiza Research , 2002 . : Basel Birkhauser , 187 - 198 . . The Contribution of Arbuscular Mycorrhizal Fungi to the Control of Soil-borne Plant Pathogens M Bansal , BP Chamola , N Sarwar , 等 . KG Mukerji , BP Chamola , J Singh . Mycorrhizosphere: Interactions between Rhizosphere Microflora . Mycorrhizal Biology , 2002 . : New York Kluwer Academic/Planum Publishers , 143 - 152 . . Mycorrhizosphere: Interactions between Rhizosphere Microflora JM Barea . P Toutant , E Balazs , E Galante , 等 . Rhizosphere and Mycorrhizae of Field Crops . Biological Resource Management, Connecting Science and Policy (OECD) , 2000 . : Berlin, Heidelberg, New York INRA and Springer , . Rhizosphere and Mycorrhizae of Field Crops JM Barea , C Azon-Aguilar , R Azcon . AC Gange , VK Brown . Interactions between Mycorrhizal Fungi and Rhizosphere Microorganisms within the Context of Sustainable Soil-plant System . Multrophic Interactions in Terrestrial Ecosystems , 1997 . : Oxford Blackwell , 65 - 77 . . Interactions between Mycorrhizal Fungi and Rhizosphere Microorganisms within the Context of Sustainable Soil-plant System JM Barea , M Gryndler , P Lemananceau , 等 . S Gianinazzi , H Schuepp , JM Barea , 等 . The Rhizosphere of Mycorrhizal Plants . Mycorrhizal Technology in Agriculture: from Genes to Bioproducts , 2002 . : Basel Birkhauser , . The Rhizosphere of Mycorrhizal Plants SJ Barker , S Duplessis , D Tagu . The application of genetic approaches for investigations of mycorrhizal symbioses . Plant and Soil , 2002 . 244 ( 1/2 ): 85 - 95 . DOI: 10.1023/A:1020293627422 http://doi.org/10.1023/A:1020293627422 . The application of genetic approaches for investigations of mycorrhizal symbioses RK Behl , H Sharma , V Kumar , N Narula . Interactions among mycorrhiza, Azobacter chroococcum and root characteristics of wheat varieties . J Agron & Crop Sci , 2003 . 189 ( 3 ): 151 - 155 . DOI: 10.1046/j.1439-037X.2003.00026.x http://doi.org/10.1046/j.1439-037X.2003.00026.x . Interactions among mycorrhiza, Azobacter chroococcum and root characteristics of wheat varieties GJ Bethlenfalvay , RG Linderman . Mycorrhizae in Sustainable Agriculture , 1992 . : St Paul, Minnesota, USA The American Phytopathological Society , 124 Special Publication No. 54 . Mycorrhizae in Sustainable Agriculture M Bomberg , G Jurgens , A Saano , R Sen , S Timonen . Nested PCR detection of Archaea in defined compartments of pine mycorrhizospheres developed in boreal forest humus microcosms . FEMS Microbiol Ecol , 2003 . 43 ( 2 ): 163 - 171 . DOI: 10.1111/j.1574-6941.2003.tb01055.x http://doi.org/10.1111/j.1574-6941.2003.tb01055.x . Nested PCR detection of Archaea in defined compartments of pine mycorrhizospheres developed in boreal forest humus microcosms L Boruvka , O Drabek . Heavy metal distribution between fractions of humic substances in heavy polluted soils . Plant Soil Environ , 2004 . 50 339 - 345 . . Heavy metal distribution between fractions of humic substances in heavy polluted soils PG Bottomley . G Stacey , RH Burris , HJ Evans . Ecology of Bradyrhizobium and Rhizobium and Rhizobium . Biological Nitrogen Fixation , 1992 . : Lodon Chapman and Hall , 293 - 348 . . Ecology of Bradyrhizobium and Rhizobium and Rhizobium MC Brundrett . Coevolution of roots and mycorrhizas of land plants . New Phytol , 2002 . 154 ( 2 ): 275 - 304 . DOI: 10.1046/j.1469-8137.2002.00397.x http://doi.org/10.1046/j.1469-8137.2002.00397.x . Coevolution of roots and mycorrhizas of land plants RL Burrows , FL Pfleger . Arbuscular mycorrhizal fungi respond to increasing plant diversity . Can J Bot , 2002 . 80 ( 2 ): 120 - 130 . DOI: 10.1139/b01-138 http://doi.org/10.1139/b01-138 . Arbuscular mycorrhizal fungi respond to increasing plant diversity CP Chanway , R Turkington , FB Holl . Ecological implications of specificity between plants and rhizosphere micro-organisms . Adv Ecol Res , 1991 . 21 122 - 170 . . Ecological implications of specificity between plants and rhizosphere micro-organisms TM Chaudhry , AG Khan . RC Rajak . Role of Symbiotic Organisms in Sustainable Plant Growth on Heavy Metal Contaminated Industrial Sites . Biotechnology of Microbes and Sustainable Utilization , 2002 . : Jodhpur, India Scientific Publishers , 270 - 279 . . Role of Symbiotic Organisms in Sustainable Plant Growth on Heavy Metal Contaminated Industrial Sites TM Chaudhry , AG Khan . GR Gorban , N Lepp . Plants Growing on Abandoned Mine Site and Their Root Symbionts . Proceedings 7th International Conference on the Biogeochemistry of Trace Elements , 2003 . : Uppsala, Sweden Swedish University of Agricultural Sciences , 134 - 135 . . Plants Growing on Abandoned Mine Site and Their Root Symbionts TM Chaudhry , WJ Hayes , AG Khan , CS Khoo . Phytoremediation-focusing on accumulator plants that remediate metal-contaminated soil . Aust J Ecotox , 1998 . 4 37 - 51 . . Phytoremediation-focusing on accumulator plants that remediate metal-contaminated soil MS Chaudhry , Z Batool , AG Khan . Preliminary assessment of plant community structure and arbuscular mycorrhizas in rangeland habitats of Cholistan Desert, Pakistan . Mycorrhiza , 2005 . 15 ( 8 ): 606 - 611 . 16133251 DOI: 10.1007/s00572-005-0002-0 http://doi.org/10.1007/s00572-005-0002-0 . Preliminary assessment of plant community structure and arbuscular mycorrhizas in rangeland habitats of Cholistan Desert, Pakistan TFC Chin-A-Woeng , GV Bloemberg , BJJ Lugtenberg . Phenazines and their role in biocontrol by Pseudomonas bacteria . New Phytol , 2003 . 157 ( 3 ): 503 - 523 . DOI: 10.1046/j.1469-8137.2003.00686.x http://doi.org/10.1046/j.1469-8137.2003.00686.x . Phenazines and their role in biocontrol by Pseudomonas bacteria RB Clark , SK Zeto . Mineral acquisition by arbuscular mycorrhizal plants . J Plant Nutri , 2000 . 23 867 - 902 . . Mineral acquisition by arbuscular mycorrhizal plants JC Dodd , CL Boddington , A Rodríguez , C Gonzalez-Chavez , I Mansur . Mycelium of arbuscular mycorrhizal fungi (AMF) from different genera: form, function and detection . Plant and Soil , 2000 . 226 ( 2 ): 131 - 151 . DOI: 10.1023/A:1026574828169 http://doi.org/10.1023/A:1026574828169 . Mycelium of arbuscular mycorrhizal fungi (AMF) from different genera: form, function and detection RJ Doyle , NC Lee . Microbes, warfare, religion and human institutions . Can J Microbiol , 1986 . 32 193 - 200 . 3518891 . Microbes, warfare, religion and human institutions JD Driver , WE Holben , MC Rilling . Characterization of glomalin as a hyphal wall component of arbuscular mycorrhizal fungi . Soil Biol Biochem , 2005 . 37 ( 1 ): 101 - 106 . DOI: 10.1016/j.soilbio.2004.06.011 http://doi.org/10.1016/j.soilbio.2004.06.011 . Characterization of glomalin as a hyphal wall component of arbuscular mycorrhizal fungi G Duc , A Trouvelot , V Gianinazzi-Pearson , S Gianinazzi . First report of non-mycorrhizal mutant (myc − ) obtained in pea ( Pisum sativum L.) and fababean ( Vicia faba L.) . Plant Sci , 1989 . 60 ( 2 ): 215 - 222 . DOI: 10.1016/0168-9452(89)90169-6 http://doi.org/10.1016/0168-9452(89)90169-6 . First report of non-mycorrhizal mutant (myc−) obtained in pea (Pisum sativum L.) and fababean (Vicia faba L.) R Duponnois , C Plenchette . A mycorrhiza helper bacterium enhances extomycorrhizal and endomycorrhizal symbiosis of Australian Acacia species . Mycorrhiza , 2003 . 13 85 - 91 . 12682830 . A mycorrhiza helper bacterium enhances extomycorrhizal and endomycorrhizal symbiosis of Australian Acacia species SG Edwards , JPW Young , AH Fitter . Interactions between Pseudomonas fluorescens biocontrol agents and Glomus mosseae , an arbuscular mycorrhizal fungus, within the rhizosphere . FEMS Microbiology Letters , 1998 . 166 ( 2 ): 297 - 303 . DOI: 10.1111/j.1574-6968.1998.tb13904.x http://doi.org/10.1111/j.1574-6968.1998.tb13904.x . Interactions between Pseudomonas fluorescens biocontrol agents and Glomus mosseae, an arbuscular mycorrhizal fungus, within the rhizosphere M Elasri , S Delorme , P Lemanceau , G Stewart , B Laue , E Glickmann , PM Oger , Y Dessaux . Acyle-homoserine lactone production is more common among plant-associated Pseudomonas spp. than among soilborne Pseudomonas spp . App Environ Microbiol , 2001 . 67 ( 3 ): 1198 - 1209 . DOI: 10.1128/AEM.67.3.1198-1209.2001 http://doi.org/10.1128/AEM.67.3.1198-1209.2001 . Acyle-homoserine lactone production is more common among plant-associated Pseudomonas spp. than among soilborne Pseudomonas spp E Facelli , JM Facelli , SE Smith , MJ Mclaughlin . Interactive effects of arbuscular mycorrhizal symbiosis, intraspecific competition and resource availability on Trifolium subterraneum cv. Mt. Barker . New Phytol , 1999 . 141 ( 3 ): 535 - 547 . DOI: 10.1046/j.1469-8137.1999.00367.x http://doi.org/10.1046/j.1469-8137.1999.00367.x . Interactive effects of arbuscular mycorrhizal symbiosis, intraspecific competition and resource availability on Trifolium subterraneum cv. Mt. Barker M Filion , M St-Arnaud , JA Fortin . Interactive effects of arbuscular mycorrhizal fungus Glomus intraradices and different rhizosphere microorganisms . New Phytol , 1999 . 141 ( 3 ): 525 - 533 . DOI: 10.1046/j.1469-8137.1999.00366.x http://doi.org/10.1046/j.1469-8137.1999.00366.x . Interactive effects of arbuscular mycorrhizal fungus Glomus intraradices and different rhizosphere microorganisms AH Fitter . Functioning of vesicular-arbuscular mycorrhiza on growth and water relations of plants . New Phytol , 1985 . 89 599 - 608 . . Functioning of vesicular-arbuscular mycorrhiza on growth and water relations of plants JA Fortin , G Bécard , S Declerck , Y Dalpé , M St-Arnaud , AP Coughlan , Y Piché . Arbuscular mycorrhiza on root-organ cultures . Can J Bot , 2002 . 80 ( 1 ): 1 - 20 . DOI: 10.1139/b01-139 http://doi.org/10.1139/b01-139 . Arbuscular mycorrhiza on root-organ cultures C Galleguillos , C Aguirre , JM Barea , R Azcon . Growth promoting effect of two Sinorhizobium meliloti strains (a wild type and its genetically modified derivative) on a non-legume plant species in specific interaction with two arbuscular mycorrhizal fungi . Plant Sci , 2000 . 159 ( 1 ): 57 - 63 . 11011093 DOI: 10.1016/S0168-9452(00)00321-6 http://doi.org/10.1016/S0168-9452(00)00321-6 . Growth promoting effect of two Sinorhizobium meliloti strains (a wild type and its genetically modified derivative) on a non-legume plant species in specific interaction with two arbuscular mycorrhizal fungi E Gamalero , A Trotta , N Massa , A Copetta , MG Martinotti , G Breta . Impact of two fluorescent pseudomonads and an arbuscular mycorrhizal fungus on tomato plant growth, root architecture and P acquisition . Mycorrhiza , 2003 . 14 ( 3 ): 185 - 192 . 15197635 . Impact of two fluorescent pseudomonads and an arbuscular mycorrhizal fungus on tomato plant growth, root architecture and P acquisition J Garbaye . Helper bacteria: a new dimension to the mycorrhizal symbiosis . New Phytol , 1994 . 128 ( 2 ): 197 - 210 . DOI: 10.1111/j.1469-8137.1994.tb04003.x http://doi.org/10.1111/j.1469-8137.1994.tb04003.x . Helper bacteria: a new dimension to the mycorrhizal symbiosis J Garbaye , JL Churin , R Duponnois . Effects of substrate disinfection, fungicide treatments and mycorrhization helper bacteria on ectomycorrhizal formation of pedunculate oak ( Quercus robur ) inoculated with Laccaria laccata in two bare-root nurseries . Biol Fertil Soils , 1992 . 13 ( 1 ): 55 - 57 . DOI: 10.1007/BF00337239 http://doi.org/10.1007/BF00337239 . Effects of substrate disinfection, fungicide treatments and mycorrhization helper bacteria on ectomycorrhizal formation of pedunculate oak (Quercus robur) inoculated with Laccaria laccata in two bare-root nurseries M Giovannetti , L Avio . GG Khachatourians , DK Arora . Biotechnology of Arbuscular Mycorrhizae . Applied Mycology and Biotechnology. Vol. 2: Agriculture and Food Production , 2002 . : Amsterdam Elsevier , 275 - 310 . . Biotechnology of Arbuscular Mycorrhizae BR Glick . The enhancement of plant growth by free-living bacteria . Can J Microbiol , 1995 . 41 109 - 117 . . The enhancement of plant growth by free-living bacteria C Gonzalez-Chavez , J D’Haen , JJ Vangronsveld , JC Dodd . Copper sorption and accumulation by the extraradical mycelium of different Glomus spp. (arbuscular mycorrhizal fungi) isolated from the same polluted soil . Plant and Soil , 2002 . 240 ( 2 ): 287 - 297 . DOI: 10.1023/A:1015794622592 http://doi.org/10.1023/A:1015794622592 . Copper sorption and accumulation by the extraradical mycelium of different Glomus spp. (arbuscular mycorrhizal fungi) isolated from the same polluted soil MC Gonzalez-Chavez , R Carrillo-Gonzalez , SF Wright , K Nichols . The role of glomalin, a protein produced by arbuscular mycorrhizal fungi, in sequestering potentially toxic elements . Environ Pollution , 2004 . 130 ( 3 ): 317 - 323 . DOI: 10.1016/j.envpol.2004.01.004 http://doi.org/10.1016/j.envpol.2004.01.004 . The role of glomalin, a protein produced by arbuscular mycorrhizal fungi, in sequestering potentially toxic elements MJ Harrison . The arbuscular mycorrhizal symbiosis: an underground association . Trends in Plant Sci , 1997 . 2 ( 2 ): 54 - 60 . DOI: 10.1016/S1360-1385(97)82563-0 http://doi.org/10.1016/S1360-1385(97)82563-0 . The arbuscular mycorrhizal symbiosis: an underground association WJ Hayes , TM Chaudhry , RT Buckney , AG Khan . Phytoaccumulation of trace metals at the Sunny Corner mine, New South Wales, with suggestions for a possible remediation strategy . Aust J Toxicol , 2003 . 9 69 - 82 . . Phytoaccumulation of trace metals at the Sunny Corner mine, New South Wales, with suggestions for a possible remediation strategy ZL He , XE Yang , PJ Stoffella . Trace elements in agroecosystems and impacts on the environment . J Trace Elem Med Biol , 2005 . 19 ( 2-3 ): 125 - 140 . 16325528 DOI: 10.1016/j.jtemb.2005.02.010 http://doi.org/10.1016/j.jtemb.2005.02.010 . Trace elements in agroecosystems and impacts on the environment A Jamal , N Ayub , M Usman , AG Khan . Arbuscular mycorrhizal fungi enhance zinc and nickel uptake from contaminated soil by soybean and lentil . Int J Phytoremed , 2002 . 4 ( 3 ): 205 - 221 . . Arbuscular mycorrhizal fungi enhance zinc and nickel uptake from contaminated soil by soybean and lentil JM Janse . Les endophytes radicaux des quelques plantes Javanaises . Annals du Jardin Botanique Buitenzord , 1896 . 14 53 - 212 . . Les endophytes radicaux des quelques plantes Javanaises EJ Joner , R Briones , C Leyval . Metal-binding capacity of arbuscular-mycorrhizal mycelium . Plant and Soil , 2000 . 226 ( 2 ): 227 - 234 . DOI: 10.1023/A:1026565701391 http://doi.org/10.1023/A:1026565701391 . Metal-binding capacity of arbuscular-mycorrhizal mycelium FR Jones . A mycorrhizal fungus in the roots of legumes and some other plants . J Agric Res , 1924 . 29 459 - 470 . . A mycorrhizal fungus in the roots of legumes and some other plants A Kabata-Pendias , H Pendias . Trace Elements in Soils and Plants , 1992 . 2nd Ed. : Boca Raton, FL CRC Press , . Trace Elements in Soils and Plants M Kaldorf , AJ Kuhn , WH Schroder , U Hildebrandt , H Bothe . Selective element deposits in maize colonized by a heavy metal tolerance conferring arbuscular mycorrhizal fungus . J Plant Physiol , 1999 . 154 195 - 206 . . Selective element deposits in maize colonized by a heavy metal tolerance conferring arbuscular mycorrhizal fungus A Kapoor , T Viraraghavan . Fungal biosorption―an alternative treatment option for heavy metal bearing wastewater: a review . Biores Technol , 1995 . 53 ( 3 ): 195 - 206 . DOI: 10.1016/0960-8524(95)00072-1 http://doi.org/10.1016/0960-8524(95)00072-1 . Fungal biosorption―an alternative treatment option for heavy metal bearing wastewater: a review Y Kapulnik . Y Waisel , A Eshel , U Kafkafi . Plant Growth Promotion by Rhizosphere Bacteria . Plant Roots―The Hidden Half , 1996 . : New York Dekker , 769 - 781 . . Plant Growth Promotion by Rhizosphere Bacteria AG Khan . Occurrence of Endogone spores in West Pakistan soils . Trans Brit Mycol Soc , 1971 . 56 ( 2 ): 217 - 224 . . Occurrence of Endogone spores in West Pakistan soils AG Khan . The occurrence of mycorrhizae in halophytes, hydrophytes, and xerophytes, and of Endogone spores in adjacent soil . J Gen Microbiol , 1974 . 81 7 - 14 . . The occurrence of mycorrhizae in halophytes, hydrophytes, and xerophytes, and of Endogone spores in adjacent soil AG Khan . FE Sanders , B Mosse , PB Tinker . Growth Effects of Vesicular-arbuscular Mycorrhiza on Crops in the Field . Endomycorrhizas , 1975 . : New York Academic Press , 419 - 439 . . Growth Effects of Vesicular-arbuscular Mycorrhiza on Crops in the Field AG Khan . Relationships between chromium biomagnification ratio, accumulation factor, and mycorrhizae in plants growing on tannery effluent-polluted soil . Environ Int , 2001 . 26 ( 5-6 ): 417 - 423 . 11392761 DOI: 10.1016/S0160-4120(01)00022-8 http://doi.org/10.1016/S0160-4120(01)00022-8 . Relationships between chromium biomagnification ratio, accumulation factor, and mycorrhizae in plants growing on tannery effluent-polluted soil AG Khan . MH Wong , AD Bradshaw . The Significance of Microbes . The Restoration and Management of Derelict Land: Modern Approaches , 2002 . : Singapore World Scientific Publishing , 80 - 92 . . The Significance of Microbes AG Khan . MH Wong , AD Bradshaw . The Handling of Microbes . The Restoration and Management of Derelict Land: Modren Approaches , 2002 . : Singapore World Scientific Publishing , 149 - 160 . . The Handling of Microbes AG Khan . MH Wong . Mycotrophy and Its Significance in Wetland Ecology and Wetland Management . Proceedings Croucher Foundation Study Institute: Wetland Ecosystems in Asia―Function and Management , 2003 . Hong Kong 11 - 15 . . Mycotrophy and Its Significance in Wetland Ecology and Wetland Management AG Khan . Role of soil microbes in the rhizospheres of plants growing on trace lement contaminated soils in phytoremediation . J Trace Elem Med Biol , 2005 . 18 ( 4 ): 355 - 364 . 16028497 DOI: 10.1016/j.jtemb.2005.02.006 http://doi.org/10.1016/j.jtemb.2005.02.006 . Role of soil microbes in the rhizospheres of plants growing on trace lement contaminated soils in phytoremediation AG Khan , M Belik . A Verma , B Hock . Occurrence and Ecological Significance of Mycorrhizal Symbioses in Aquatic Plants . Mycorrhiza: Structure, Function, Molecular Biology and Biotechnology , 1995 . : Heidelberg Springer-Verlag , 627 - 666 . . Occurrence and Ecological Significance of Mycorrhizal Symbioses in Aquatic Plants AG Khan , C Kuek , TM Chaudhry , CS Khoo , WJ Hayes . Role of plants, mycorrhizae and phytochelators in heavy metal contaminated land remediation . Chemosphere , 2000 . 41 ( 1-2 ): 197 - 207 . 10819202 DOI: 10.1016/S0045-6535(99)00412-9 http://doi.org/10.1016/S0045-6535(99)00412-9 . Role of plants, mycorrhizae and phytochelators in heavy metal contaminated land remediation AA Klyuchnikov , PA Kozherin . Dynamics of Pseudomonas fluorescens and Azospirillium brasilense populations during the formation of the vesicular arbuscular mycorrhiza . Microbiology , 1990 . 59 449 - 452 . . Dynamics of Pseudomonas fluorescens and Azospirillium brasilense populations during the formation of the vesicular arbuscular mycorrhiza RG Linderman , TC Paulitz . D Hornb . Mycorrhizal-rhizobacterial Interactions . Biological Control of Soil-borne Plant Pathogens , 1990 . : Wallingford CAB International , 261 - 283 . . Mycorrhizal-rhizobacterial Interactions PE Lovato , V Gianinazzi-Pearson , A Trouvelot , S Gianinazzi . The state of art mycorrhizas micropropagation . Adv Agric Sci , 1996 . 10 46 - 52 . . The state of art mycorrhizas micropropagation BJJ Lugtenberg , LC Dekkers . What makes Pseudomonas bacteria rhizosphere competent . Environ Microbiol , 1999 . 1 ( 1 ): 9 - 13 . 11207713 DOI: 10.1046/j.1462-2920.1999.00005.x http://doi.org/10.1046/j.1462-2920.1999.00005.x . What makes Pseudomonas bacteria rhizosphere competent JM Lynch . The Rhizosphere , 1990 . : West Sussex, UK John Wiley & Sons , . The Rhizosphere P Marschner , K Baumann . Changes in bacterial community structure induced by mycorrhizal colonization in split-root maize . Plant and Soil , 2003 . 251 ( 2 ): 279 - 289 . DOI: 10.1023/A:1023034825871 http://doi.org/10.1023/A:1023034825871 . Changes in bacterial community structure induced by mycorrhizal colonization in split-root maize K Mayo , R Davis , J Motta . Stimulation of germination of spores of Glomus versiforme by spore-associated bacteria . Mycologia , 1986 . 78 ( 3 ): 426 - 431 . . Stimulation of germination of spores of Glomus versiforme by spore-associated bacteria RJ Meyer , RG Linderman . Response of subterranean clover to dual inoculation with vesicular arbuscular mycorrhizal fungi and plant growth promoting rhizobacterium . Pseudomonas putida Soil Biol Biochem , 1986 . 18 ( 2 ): 185 - 190 . DOI: 10.1016/0038-0717(86)90025-8 http://doi.org/10.1016/0038-0717(86)90025-8 . Response of subterranean clover to dual inoculation with vesicular arbuscular mycorrhizal fungi and plant growth promoting rhizobacterium D Minerdi , V Bianciotto , P Bonfante . Endosymbiotic bacteria in mycorrhizal fungi: from their morphology to genome sequences . Plant and Soil , 2002 . 244 ( 1/2 ): 211 - 219 . DOI: 10.1023/A:1020211821753 http://doi.org/10.1023/A:1020211821753 . Endosymbiotic bacteria in mycorrhizal fungi: from their morphology to genome sequences A Mohammad , AG Khan . Monoxenic in vitro production and colonization potential of AM fungus Glomus intraradices . Indian J Exp Bot , 2002 . 40 1087 - 1091 . . Monoxenic in vitro production and colonization potential of AM fungus Glomus intraradices A Mohammad , AG Khan , C Kuek . Improved aeroponic culture of inocula of arbuscular mycorrhizal fungi . Mycorrhiza , 2000 . 9 ( 6 ): 337 - 339 . DOI: 10.1007/s005720050278 http://doi.org/10.1007/s005720050278 . Improved aeroponic culture of inocula of arbuscular mycorrhizal fungi A Mohammad , B Mitra , AG Khan . Effects of sheared-root inoculum of Glomus intraradices on wheat grown at different phosphorus levels in the field . Agric Ecosystems & Environment , 2004 . 103 ( 1 ): 245 - 249 . DOI: 10.1016/j.agee.2003.09.017 http://doi.org/10.1016/j.agee.2003.09.017 . Effects of sheared-root inoculum of Glomus intraradices on wheat grown at different phosphorus levels in the field GF Morley , GM Gadd . Sorption of toxic metals by fungi and clay minerals . Mycol Res , 1995 . 99 1429 - 1438 . . Sorption of toxic metals by fungi and clay minerals B Mosse . The establishment of vesicular-arbuscular mycorrhiza under aseptic conditions . J Gen Microbiol , 1962 . 27 509 - 520 . 14476553 . The establishment of vesicular-arbuscular mycorrhiza under aseptic conditions PJ O’Connor , SE Smith , F Smith . Arbuscular Mycorrhizas influence plant diversity and community structure in a semiarid herbland . New Phytol , 2002 . 154 ( 1 ): 209 - 218 . DOI: 10.1046/j.1469-8137.2002.00364.x http://doi.org/10.1046/j.1469-8137.2002.00364.x . Arbuscular Mycorrhizas influence plant diversity and community structure in a semiarid herbland TC Paulitz , RG Linderman . Interactions between fluorescent pseudomonads and VA mycorrhizal fungi . New Phytol , 1989 . 113 ( 1 ): 37 - 45 . DOI: 10.1111/j.1469-8137.1989.tb02393.x http://doi.org/10.1111/j.1469-8137.1989.tb02393.x . Interactions between fluorescent pseudomonads and VA mycorrhizal fungi RL Peterson , FC Guinel . Y Kapulnik , DD Douds . The Use of Pea Mutants to Study Regulation of Colonization by AM Fungi . Arbuscular Mycorrhiza: Physiology and Function , 2000 . : Dordrecht, the Netherlands Kluwer Academic Publishers , 147 - 171 . . The Use of Pea Mutants to Study Regulation of Colonization by AM Fungi FL Pfleger , RG Linderman . Mycorrhizae and Plant Health . Symposium Series, The American Phytopathological Society , 1994 . : St Paul, Minnesota, USA 344 . Mycorrhizae and Plant Health CJ Phipps , TN Taylor . Mixed arbuscular-mycorrhizae from the Triassic of Antarctica . Mycologia , 1996 . 88 707 - 714 . . Mixed arbuscular-mycorrhizae from the Triassic of Antarctica LS Pierson , EA Pierson , JE Morello . SA Leong , C Allen , EW Triplett . Positive and Negative Cross-communication among Rhizobacteria . Biology of Plant-microbe Interactions , 2002 . : St Paul, MN, USA International Society for Molecular Plant-microbe Interactions , 256 - 262 . . Positive and Negative Cross-communication among Rhizobacteria N Requena , I Jimenez , M Toto , JM Barea . Interactions between plant-growth-promoting rhizobacteria (PGPR), arbuscular mycorrhizal fungi and Rhizobium spp. in the rhizosphere of Anthyllis cytisoides , a model legume for revegetation in Mediterranean semi-arid ecosystem . New Phytol , 1997 . 136 ( 4 ): 667 - 677 . DOI: 10.1046/j.1469-8137.1997.00786.x http://doi.org/10.1046/j.1469-8137.1997.00786.x . Interactions between plant-growth-promoting rhizobacteria (PGPR), arbuscular mycorrhizal fungi and Rhizobium spp. in the rhizosphere of Anthyllis cytisoides, a model legume for revegetation in Mediterranean semi-arid ecosystem CM Resendes , RD Geil , FC Guinel . Mycorrhizal development in low nodulating pea mutant . New Phytol , 2001 . 150 ( 3 ): 563 - 572 . DOI: 10.1046/j.1469-8137.2001.00131.x http://doi.org/10.1046/j.1469-8137.2001.00131.x . Mycorrhizal development in low nodulating pea mutant MC Rillig , PW Ramsey , S Morris , EA Paul . Glomalin, an arbuscular-mycorrhizal fungal soil protein, respond to land-use change . Plant and Soil , 2003 . 253 ( 2 ): 293 - 299 . DOI: 10.1023/A:1024807820579 http://doi.org/10.1023/A:1024807820579 . Glomalin, an arbuscular-mycorrhizal fungal soil protein, respond to land-use change H Rodríguez , R Fraga . Phosphate solubilizing bacteria and their role in plant growth promotion . Biotech Advances , 1999 . 17 ( 4-5 ): 319 - 339 . DOI: 10.1016/S0734-9750(99)00014-2 http://doi.org/10.1016/S0734-9750(99)00014-2 . Phosphate solubilizing bacteria and their role in plant growth promotion A Schüßler . Glomus claroideum forms an arbuscular-mycorrhiza-like symbiosis with the hornwort Anthoceros punctatus . Mycorrhiza , 2000 . 10 ( 1 ): 15 - 21 . DOI: 10.1007/s005720050282 http://doi.org/10.1007/s005720050282 . Glomus claroideum forms an arbuscular-mycorrhiza-like symbiosis with the hornwort Anthoceros punctatus R Sen . The root-microbe-soil interface: new tool for sustainable plant production . New Phytol , 2003 . 157 ( 3 ): 391 - 398 . DOI: 10.1046/j.1469-8137.2003.00715.x http://doi.org/10.1046/j.1469-8137.2003.00715.x . The root-microbe-soil interface: new tool for sustainable plant production LM Shuman . Fractionation method for soil microelements . Soil Science , 1985 . 140 ( 1 ): 11 - 22 . . Fractionation method for soil microelements LK Simon , J Bousquet , RC Levesque , M Lalonde . Origin and diversification of endomycorrhizal fungi and coincidence with vascular land plants . Nature , 1993 . 363 ( 6424 ): 67 - 69 . DOI: 10.1038/363067a0 http://doi.org/10.1038/363067a0 . Origin and diversification of endomycorrhizal fungi and coincidence with vascular land plants S Singh , KK Kapoor . Effects of inoculation of phosphate solubilizing microorganisms and an arbuscular mycorrhizal fungus on mongbean growth under natural soil condition . Mycorrhiza , 1998 . 7 ( 5 ): 249 - 253 . DOI: 10.1007/s005720050188 http://doi.org/10.1007/s005720050188 . Effects of inoculation of phosphate solubilizing microorganisms and an arbuscular mycorrhizal fungus on mongbean growth under natural soil condition SE Smith . Soil microbes and plants―raising interest, mutual gains . New Phytol , 2002 . 156 ( 2 ): 142 - 144 . DOI: 10.1046/j.1469-8137.2002.00514.x http://doi.org/10.1046/j.1469-8137.2002.00514.x . Soil microbes and plants―raising interest, mutual gains SS Smith , DJ Read . Mycorrhizal Symbiosis , 1997 . 2nd Ed. : London Academic Press , . Mycorrhizal Symbiosis YP Sun , T Unestam , SD Lucase , KJ Johanson , L Kenne , R Finlay . Exudation-reabsorption in a mycorrhizal fungus, the dynamic interface for interaction with soil and soil micro-organisms . Mycorrhiza , 1999 . 9 ( 3 ): 137 - 144 . DOI: 10.1007/s005720050298 http://doi.org/10.1007/s005720050298 . Exudation-reabsorption in a mycorrhizal fungus, the dynamic interface for interaction with soil and soil micro-organisms CK Suresh , DJ Bagyaraj . AK Sharma , BN Johri . Mycorrhiza-microbe Interface: Effect on Rhizosphere . Arbuscular Mycorrhizae , 2002 . : Enfield, New Hampshire, USA Scientific Publishers , 7 - 28 . . Mycorrhiza-microbe Interface: Effect on Rhizosphere DM Sylvia , AG Jarstfer . Sheared root inoculum of vesicular arbuscular mycorrhizal fungi . App Environ Microbiol , 1994 . 58 229 - 232 . . Sheared root inoculum of vesicular arbuscular mycorrhizal fungi DM Sylvia , AG Jarstfer . AD Robson , LK Abbott , N Malajczuk . Production of Inoculum and Inoculation with Arbuscular Mycorrhizal Fungi . Management of Mycorrhizas in Agricture, Horticulture and Forestry , 1994 . : Dordrecht Kluwer , 231 - 238 . . Production of Inoculum and Inoculation with Arbuscular Mycorrhizal Fungi TN Taylor , EL Taylor . Biology and Evolution of Fossil Plants , 1993 . : Prentice Hall, NJ, USA Englewood Cliff , . Biology and Evolution of Fossil Plants M Toro , R Azcon , JM Barea . Improvement of arbuscular development by inoculation of soil with phosphate-solubilizing rhizobacteria to improve rock phosphate bioavailability (P 32 ) and nutrient cycling . App Environ Microbiol , 1997 . 63 4408 - 4412 . . Improvement of arbuscular development by inoculation of soil with phosphate-solubilizing rhizobacteria to improve rock phosphate bioavailability (P32) and nutrient cycling K Turnau . Heavy metal content and localization in mycorrhizal Euphorbia cyparissias from zinc wastes in Southern Polland . Acta Societatis Botanicorum Poloniae , 1998 . 67 105 - 113 . . Heavy metal content and localization in mycorrhizal Euphorbia cyparissias from zinc wastes in Southern Polland K Turnau , K Haselwandter . S Gianinazzi , H Schuepp . Arbuscular Mycorrhizal Fungi: An Essential Component of Soil Microflora in Ecosystem Restoration . Mycorrhizal Technology: from Genes to Bioproducts , 2002 . : Basel Birkhauser , 137 - 149 . . Arbuscular Mycorrhizal Fungi: An Essential Component of Soil Microflora in Ecosystem Restoration MGA van der Heijden , JN Klironomos , M Ursic , P Moutoglis , R Streitwolf-Engel , T Boller , A Weimken , JR Sanders . Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity . Nature , 1998 . 396 ( 6706 ): 69 - 72 . DOI: 10.1038/23932 http://doi.org/10.1038/23932 . Mycorrhizal fungal diversity determines plant biodiversity, ecosystem variability and productivity JD van Elsas , S Turner , MJ Bailey . Horizontal gene transfer in the phytosphere . New Phytol , 2003 . 157 ( 3 ): 525 - 537 . DOI: 10.1046/j.1469-8137.2003.00697.x http://doi.org/10.1046/j.1469-8137.2003.00697.x . Horizontal gene transfer in the phytosphere A Vivas , A Marulanda , JM Ruiz-Lozana , JM Barea , R Azcon . Influence of a Bacillus sp. on physiological activities of two arbuscular mycorrhizal fungi and on plant responses to PEG-induced drought stress . Mycorrhiza , 2003 . 13 ( 5 ): 249 - 256 . 14593518 DOI: 10.1007/s00572-003-0223-z http://doi.org/10.1007/s00572-003-0223-z . Influence of a Bacillus sp. on physiological activities of two arbuscular mycorrhizal fungi and on plant responses to PEG-induced drought stress M Vosátka , M Gryndler . Treatment with culture fractions from Pseudomonas putida modifies the development of Glomus fistulosum mycorrhizae and response of potato and maize plants to inoculation . App Soil Ecol , 1999 . 11 ( 2-3 ): 245 - 251 . DOI: 10.1016/S0929-1393(98)00151-6 http://doi.org/10.1016/S0929-1393(98)00151-6 . Treatment with culture fractions from Pseudomonas putida modifies the development of Glomus fistulosum mycorrhizae and response of potato and maize plants to inoculation DM Weller . Y Waisel , A Eschel , U Kafkafi . Biological Control of Soil-born Plant Pathogens in the Rhizosphere with Bacteria . Plant Roots: The Hidden Half , 1988 . : New York Dekker , 769 - 781 . . Biological Control of Soil-born Plant Pathogens in the Rhizosphere with Bacteria SF Wright , A Upadhyaya . A survey of soils for aggregate stability and glomalin, a glycoprotein produced by hyphae of arbuscular mycorrhizal fungi . Plant and Soil , 1998 . 198 ( 1 ): 97 - 107 . DOI: 10.1023/A:1004347701584 http://doi.org/10.1023/A:1004347701584 . A survey of soils for aggregate stability and glomalin, a glycoprotein produced by hyphae of arbuscular mycorrhizal fungi IJ Xavier , SM Boyetchko . GG Khachatourians , DK Arora . Arbuscular Mycorrhizal Fungi as Biostimulants and Bioprotectants of Crops . App. Mycol. and Biotechnol. Vol. 2: Agriculture and Food Production , 2002 . : Amsterdam Elsevier , 311 - 330 . . Arbuscular Mycorrhizal Fungi as Biostimulants and Bioprotectants of Crops S Zhang , MS Reddy , JW Kloepper . Development of assay for assessing induced resistance by plant-growth-promoting-rhizobacteria against blue mold of tobacco . Biol Control , 2002 . 23 ( 1 ): 79 - 86 . DOI: 10.1006/bcon.2001.0992 http://doi.org/10.1006/bcon.2001.0992 . Development of assay for assessing induced resistance by plant-growth-promoting-rhizobacteria against blue mold of tobacco JL Zhou . Zn biosorption by Rhizopus arrhizus and other fungi . App Microbiol Biotechnol , 1999 . 51 ( 5 ): 686 - 693 . DOI: 10.1007/s002530051453 http://doi.org/10.1007/s002530051453 . Zn biosorption by Rhizopus arrhizus and other fungi 10.1631/jzus.2006.B0503 2006-05-17 2006-05-29 2006-06-14 2022-01-05 Copyright © 2006, Journal of Zhejiang University Science 2006 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 07 07