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
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
). 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
; Turnau and Haselwandter, 2002
). Their potential role in phytoremediation of heavy metal contaminated soils and water is also becoming evident (Chaudhry et al., 1998
; Khan et al., 2000
; Khan, 2001
; Jamal et al., 2002
; Hayes et al., 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). 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; Lynch, 1990). Giovannetti and Avio (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). 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). 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; Azcon-Aguilar et al., 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; Allen, 1991; 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; Fortin et al., 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; 1994b; Mohammad et al., 2000; Mohammad and Khan, 2002). Substrate-free colonized roots produced by these methods can be sheared and used for large-scale inoculation purposes. Mohammad et al.(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
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; Marschner and Baumann, 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). Garbaye (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; Barea et al., 2002), and that the manipulation of AMF and certain rhizobacteria like PGPR and MHB is important. Vivas et al.(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) 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; 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) 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), indicating that the MHB effect is not plant-specific. Various researchers showed that MHB’s are fungus selective (Garbaye, 1994). Mosse (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; Linderman and Paulitz, 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). Many plant associated Pseudomonas rhizobacteria produce signal molecules for quorum sensing regulation, which were absent from soil-borne strains (Elasri et al., 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), 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). Khan (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) 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 nifHDK 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), 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
Rhizobacteria include mycorrhization helper bacteria (MHB) and plant growth promoting rhizobacteria (PGPR), which assist AMF to colonize the plant roots (Klyuchnikov and Kozherin, 1990
; Andrade et al., 1997
), P-solubilizers, free-living and symbiotic nitrogen fixers, antibiotic producing rhizobacteria, plant pathogens, predators and parasites (Sun et al., 1999
). The most common bacteria in the mycorrhizosphere are Pseudomonas
(Vosátka and Gryndler, 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; Glick, 1995; Kapulnik, 1996; Chin-A-Woeng et al., 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 N2 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). 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; Singh and Kapoor, 1998). Many researchers have reported additive effects on plant growth by AMF and PGPR (Edwards et al., 1998; Galleguillos et al., 2000; Gamalero et al., 2003) but the mechanisms by which MHB and PGPR stimulate AM colonization are still poorly understood (Barea et al., 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).
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). Gamalero et al.(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).
Co-occurrence of AMF and PGPR/MHB
Co-existence of certain fungi and rhizobacteria has been known since 1896, when Janse (1896
) reported bacteria and fungi in the same pieces of legumes roots. These fungi were also described as arbuscular mycorrhizal by Jones (1924
), and it was Asai (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
). Multifaceted interactions of AM fungi with various micro-organisms and micro-fauna in the mycorrhizosphere may be positive or negative (Facelli et al., 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
). These microbes are regulated by AMF for their own benefit, which in turn benefit the host plant. Meyer and Linderman (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
). All these studies suggest that colonization of plant roots by AM fungi significantly influences the mycorrhizosphere microorganisms, including PGPR.
Duc et al.(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). Many recent reports have confirmed the similarities between nodulation and mycorrhiza-formation processes (Peterson and Guinel, 2000; Resendes et al., 2001). There is enough evidence available now which indicates positive interaction between AM fungi and PGPR (Chanway et al., 1991).
There are several reports on the interactions between AMF and Rhizobium species (Albrecht et al., 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 N2-fixation. Current researches are directed to gaining understanding on the role AMF play in the expression of Nod genes in Rhizobia (Harrison, 1997). Barea and his associates (Azcon-Aguilar and Barea, 1994; Azcon-Aguilar et al., 2002; Barea, 2000; Barea et al., 2002) achieved many significant findings in this area. Synergistic interactions between AMF and asymbiotic N2 fixing bacteria such as Azobacter chroococum, Azospirillum spp. and Acetobacter diazotrophicus have also been reported by many researchers (Suresh and Bagyaraj, 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) 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).
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). 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) found a specific AM fungus, Rhizobacterium sp. combination for effective nodulation and N2 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). This means, as pointed out by Sen (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).