Mycorrhizal Fungi as a Biocontrol Agent
Arbuscular mycorrhizae fungi (AMF) are the symbiotic fungi that predominate in the roots and soils of agricultural crop plants. The AMF form beneficial symbioses in most terrestrial ecosystems and crop production systems. Ninty percent of land plant species are colonized by one or more of the mycorrhizal fungi species ranging from flowering to non flowering plants, while only a few plant families do not form this association. The relationship between mycorrhiza and plant is very widely spread among terrestrial vascular plants. The AMF must have a host to complete its life cycle and this association has been found to be mutually beneficial; thus, the fungus assists the plant in mineral nutrients uptake, while the plant supplies the fungus with carbon as a result of this relation. The negative-antagonistic interaction of AMF with various soilborne plant pathogens is the reason for their use as a bio-control agents. Many workers have observed an antagonistic effect of AMF against some fungal pathogens.
Received: August 28, 2010;
Accepted: September 17, 2010;
Published: November 12, 2010
Mycorrhizal fungi are a major component of the agricultural natural resource
and they are members of the fungus kingdom. Asymbiotic association of fungus
and roots has been discovered in Monotropa Hypopity by Franciszek Kamienski
(1881). The studies of the Polish botanist Frank 1885 had initiated worldwide
interest on a fungus-root (Myco-rhiza). Also, he gave the name MYCORRHIZA to
the peculiar association between root trees and ectomycorrhizal fungi. The AMF
play an important function in the reduction of plant pathogens (Whipps,
2004; St-Arnaud et al., 1994; Azcon-Aguilar
and Barea, 1997), such as Rhizoctonia solani (Yao
et al., 2002) and Pythium ultimum and Phytophthora
species (Trotta et al., 1996; Cordier
et al., 1996). In different crops the AMF have also been shown to
reduce bacterial diseases (Dehne, 1982), for example,
Glomus mossease suppressed Ralstonia solanacearum, bacterial wilt
causal organism on tomato (Tahat et al., 2009).
There have been a few studies of the potential role of Arbuscular Mycorrhizal Fungi (AMF) for the protection of plant from pathogens.
IDENTIFICATION, DISTRIBUTION AND CLASSIFICATION
Mycorrhizal fungi association widely varied in structures and functions, but
the Arbuscular Mycorrhizae (AM) are the most common interactions (Harrier,
2001). Six genera of arbuscular mycorrhizal fungi have been recognized based
on morphological characteristics of a sexual spores and also based on various
biochemical studies as well as molecular methods (Peterson
et al., 2004). Further, various criteria have been used for the identification
of AMF like hyphal character, auxiliary cells subtending hyphae, spore or sporcarp
ontogeny, morphology, germination, shield spore wall, biochemical, molecular
and immunological characteristics (Mukerji et al., 2002).
Few species of host roots synthesize a yellow pigment when colonized by mycorrhizal
fungi which is considered as a sign of infection (Peterson
et al., 2004). AMF are zygomycetous belonging to the genera Glomus,
Gigaspora, Sclerocystis, Acaulospora, Entrophospora
and Scutellospora (Garbaye, 1994).
The classification of AMF is based on the structure of their soil-borne resting
spore, biochemical properties and molecular studies (Morton
and Benny, 1990). The latest classification of AMF contains 4 orders with
9 families (Table 1) (Sieverding and Oehl,
2006). Plant species belonging to the cruciferae and chenopodiaceae are
not known to form AMF symbiosis (Smith and Read, 1997).
The AMF reproduce asexually by spore production. There is no evidence that
AMF reproduce sexually (Kuhn et al., 2001).
TYPES OF MYCORRHIZAL FUNGI
Seven different types of mycorrhizal fungi association have been recognized and the most important types are:
Endo-mycorrhizae: Endo-mycorrhizae represent a group of fungi that are
associated with most agricultural crops and provide biological protection against
soil-borne diseases (Smith and Read, 2008). They occur
in most ecosystems of the world and are found in many important crop species
(wheat, maize, rice, grape, soybean and cotton) and horticultural species roses,
petunias and lilies) (Peterson et al., 2004). AMF
are obligatory biotrophs feeding on the products of their live plant host and
those fungi are not specialized to their potential hosts. The host plant receives
mineral nutrients from outside the roots depletion zone via the extraradical
fungal mycelium, while the AMF obtains photo-synthetically produced carbon compound
from the host (Smith and Read, 1997).
Many endomycorrhizal fungi form terminal or intercalary vesicles in the root
cortex. When the vesicles are expanded the thin walled structures, which are
not septum and it's contain a large quantity of lipids. They may be oval, spherical,
or lobed in shapes and may become thick walled and resting spores (Pirozynski
and Dalphe, 1989). The term arbuscular mycorrhiza replaced the earlier term
vesicular arbuscular mycorrhizae (VAM) because some endomycorrhiza produce vesicles,
but all form arbuscules (Strack et al., 2003).
Ecto-mycorrhizae: Ecto-mycorrhizal (ECM) fungus forms a thick mantle
structure within the intercellular spaces of root cortex and a sheath around
the feeder root acting as an interface for channeling of nutrients from the
plant to the fungus and vice versa (Kumar and Satyanarayana,
2002). Ectomycorrhizal fungi do not penetrate living cells in host roots,
but can only surround them. The extensive mycelium produced by ectomycorrhizal
may function in transferring nutrients directly from the decaying leaves (Suverch
et al., 1991).
Ectomycorrhizas are most common in ornamental and forest trees species in the
family Pinaceae, Myrtaceae, Salicaeae, Dipterocarpecae, Fagaceae and Gentum
plants (Shalini et al., 2000). Ectomycorrhizas
are distinguished by the presence of mantle and the hartig net. Hartig net (develops
in cortical cells or epidermal cells. Hartig net consists of branch systems
which can provide a large surface contact between cells of the two symbionts
(Peterson et al., 2004).
Other types of mycorrhizal fungi include (Ecto-endo Mycorrhiza, Ericoid Mycorrhiza,
Monotropoid, Arbutoid mycorrhizas and Orchid mycorrhiza) (Smith
and Read, 2008).
THE FUNCTIONS OF ARBUSCULAR MYCORRHIZAL FUNGI
Mycorrhizal fungi offer protection against pathogens (bio-control agents):
Soil borne pathogens were controlled by using several agricultural practices
methods, such as resistant cultivars, seed certification, chemical fungicide,s
crop rotation and soil fumigation etc. There are many problems associated with
controlling pathogens with long-term persistent survival structures due to difficulties
in reducing pathogen inoculum and lack of good sources of plant resistance (Azcon-Aguliar
and Barea, 1997). Therefore, many researchers were trying to use alternate
approaches based on either manipulating or adding microorganisms to enhance
plant protection against pathogens (Grosch et al.,
2005). The beneficial microorganisms (antagonistic bacteria) (e.g., Pseudomonas
fluorescens, Bacillus subtilis, etc.) and fungi (e.g., AMF, Trichoderma,
etc.) compete with plant pathogens for nutrients and space, by producing antibiotics,
by parasitizing pathogens, or by inducing resistance in the host plants. These
microbes have been used for biocontrol of pathogens (Berg
et al., 2007).
The extensive use of chemicals to control diseases poses a serious threat to
the present day plant production systems (Dehne, 1982).
Currently the use of beneficial microorganisms is one of the alternative management
practices reviewed to have protective effect against plant soilborne pathogens
(Brimmer and Boland, 2003; Mukerji
et al., 2002).
The protective effect of mycorrhizal symbioses against root pathogenic fungi
has been tested by many researchers (Caron, 1989; Dehne,
1982). Disease reduction within host plants colonized by AMF is the result
and output of the complex interactions between pathogens, AMF and plant (Harrier
and Watson, 2004). AMF symbiosis has been shown to reduce the damage caused
by soilborne pathogens (Azcon-Aguilar et al., 2002).
Phytophthora parasitica proliferation greatly reduced when tomato root
colonized by Glomus mosseae and P. parasitica compared with non-mycorrhizal
tomato roots (Cordier et al., 1996). Trotta
et al. (1996) found that phosphate by AMF may contribute to lessen
the damage by P. parasitica in tomato. The presence of AMF successfully
delays the time required by Ganoderma boninense to infect and kill oil
palm plant and the seedlings were more resistant to G. boninense (Rini,
2001). AMF has shown no indirect interaction with soilborne pathogen through
antagonism, mycoparasitism and or antibiosis (Harrier and
Watson, 2004). Different mechanisms have been reported to explain bio-control
by AMF including biochemical changes in plant tissues, microbial changes in
rhizosphere, nutrient status, anatomical changes to cells, changes to root system
morphology and stress alleviation (Hooker et al.,
1994). Therefore, those mechanisms by which AMF could control the soil borne
pathogen are listed below:
Enhancing plant nutrition uptake: Improvements in plant growth followed
by root colonization by AMF occurs as a result of enhancement of the mineral
nutrient status of plants. Some reports indicate that phosphorus induced changes
in root exudation could reduce the germinations of pathogen spores (Graham,
1982; Sharma et al., 2007). Some studies suggest
that competition for space between AMF and pathogen, AMF may increase host tolerance
to pathogen by increasing the uptake of essential nutrients rather than phosphorus
which are otherwise deficient in the non-mycorrizal plants (Gosling
et al., 2006). The AMF spores germinate and thick-walled hyphae penetrate
the host root causing internal infection. After penetrating into the root, the
hyphae spread inter- and/or intra-cellularly in the root cortex without damaging
the integrity of the cells (Strack et al., 2003).
The increasing nutrient uptake resulted in more vigorous plants; thus, the plant
itself may be more resistant or tolerant to pathogen attack (Linderman,
Damage compensation: It is suggested that AMF increase host tolerance
of pathogen attack by compensating for the loss of root functional and biomass
caused by soilborne pathogens (Linderman, 1994) including
fungi and nematodes (Cordier et al., 1996). This
illustrates an indirect contribution to the biological control through the conservation
of root system function both by AMF hyphae growing out into the soil and increasing
the root absorbing surface area as well as by the maintenance of root cell activity
through arbuscules formation (Gianinazzi-Person et al.,
Soil microbial population interactions: The first report attempted to
specifically study the interaction of plant pathogenic fungus and a species
of AM fungus was that of Safir (1968). The role of AMF
in improving plant nutrition and their interactions with other soil biota have
been investigated with reference to the host plant growth. Few informations
known about how these interactions affect soil structure (Schreiner
and Bethlenfalvay, 1995). Plants colonized by AMF differ from non-mycorrhizal
plant in rhizosphere microbial community, resulted in alterations in root respiration
rate quality and quantity of the exudates (Marschner et
Hyphae emerging from spores in the presence of bacteria smoothly developed
small vesicles, longer and more branched than those without bacteria (Khan,
2005). The growth and health of plants influenced by the microbial shifts occur
in the mycorrhizosphere (Azcon-Aguilar et al., 2002).
This effect has not been specifically evaluated as mechanisms for AM-associated
biocontrol, but there are indications that such a mechanism does operate (Linderman,
1994). Some reports suggest that AMF alter the composition of functional
groups of microbes in the mycorrhizosphere, including the numbers and/or activity
of pathogens antagonists (Secilia and Bagyaraj, 1987).
No alteration was observed in the total numbers of actinomycetes or bacteria
isolated from Trifolium subterraneum L. and Zea mays colonized
by Glomus fasciculatum. However, there was a change in the functional
groups of these microbes, including more facultative anaerobic bacteria in mycorrhizosphere
of AMF colonized T. subterraneum. The total number of bacteria isolated
from rhizoplane of T. subterraneum and Zea mays increased as a
result of AMF colonization (Meyer and Linderman, 1986).
The population of Fusarium oxysporum in the mycorrhizosphere soil of
tomato reduced in AM plants relative to non-mycorrhizal one (Johansson
et al., 2004).
Plant root systems colonized by AMF differ in their effects on the bacterial
community composition within the rhizosphere and rhizoplane (Burke
et al., 2002). Several biotic and a biotic factors are very important
for determination of efficiency of AMF as a disease control agent. The most
important factors are, soil moisture, soil contents, host genotype, mycorrhizal
level inoculums, inoculation time of mycorrhiza, mycorrhizal fungi species virulence,
inoculums potential of pathogen and soil microflora (Singh
et al., 2000).
Systemic bio-protection of plant against take all disease of barely plant depends
on a high degree of AMF root colonization (Khaosaad et
al., 2007). The use of AMF resulted in resistance increment against
the wilt pathogen Fusarium oxysporum f. sp. Lini (Dugassa
et al., 1996). Two zones of interactions can be defined (1) The rhizoplane
and the surrounding rhizosphere soil and (2) The mycosphere (Bansal
and Mukerj, 1994). Mycorrhization Helper Bacteria (MHB) certainly improve
the ability of mycorrhiza fungi to colonize plant roots (Fitter
and Garbaye, 1994).
Competition for colonization and infection sites: Physical competition
between endomycorrhizal fungi and rhizosphere microorganisms to occupy more
space in the root architecture is the first mechanism to explain the interaction
between AMF and soil microorganisms (Bansal and Mukerji,
1996). Mycorrhizal fungi depend on the plant host photosynthates, so the
competition for carbon compounds maybe, a cause of the pathogen suppression
in mycorrhizal plant. The interaction between AMF and Phytophthora in
tomato plant has shown that the pathogen dose not penetrate arbuscular containing
cells (Cordier et al., 1998). Dehne
(1982) documented how AM fungi and root pathogens colonize in the same host
tissues and how they develop in different root cortical cells, indicating some
sort of or competition for space. The interaction of Glomus mosseae and
phytophthora nicotiana var. parastica in tomato was shown to increase
the AMF at the root apex site.
Morphological and anatomical changes: Root morphology system can be
altered due to the colonization of root by AMF (Tahat et
al., 2008). Roots colonized by AMF are more highly branched compared
to non colonized plants and also the adventitious root diameters are larger
(Berta et al., 1993), which can provide more infection
sites for a pathogen (Hooker et al., 1994). Dugassa
et al. (1996) found that the infection of tomato and cucumber by Fusarium
wilt might slow down due to the morphological changes in the root cells of the
endodermis of AM plants which include lignifications incensement. The raising
lignifications may protect the roots from penetration by other pathogens, while
elevating of phenolic metabolism within the host plant (Miranda, 1996).
The colonization of tomato root by Glomus mosseae lead to a bigger root
size and more branching which increase the number of root tips, length, surface
area and root volume (Tahat et al., 2008). Root
damage by Gaeumannomyces graminis var. tritici was systemically
reduced when barley plants showed high degree of mycorrhizal root colonization.
Allowing mycorrhizal root infection exhibited no affect on Gaeumannomyces
graminis var.tritici infection (Khaosaad et
Competition for host photosynthates: The growth of AMF and root pathogen
depends on the host photosynthates and they compete for the carbon compounds
received by the root (Smith and Read, 1997). When AMF have
primary access to the photosynthates, the higher carbon demand may inhibit the
pathogen growth (Linderman, 1994). AMF is dependent on
the host plant for carbon source. 4-20% net photosynthates of host are transferred
to the fungus; nevertheless, there is only a limited data to support this mechanism
(Smith and Read, 2008).
Changes in chemical constituents of plant tissues (root exudates): Phytoalexins
toxic components are not detected during the first stages of AM formation but
can be detected in the later stages of symbiosis (Miranda, 1996). Wall-bound
peroxidase activity has been detected during the initial stage of AM colonization
(Azcon-Aguilar et al., 2002). Phytophthora parasitica
development decreased in Glomus mosseae and non G. mosseae
parts of tomato mycorrhizal root systems in association with plant cell defense
responses and accumulation of phenolics. Cortical cells containing G. mosseae
are immune to the pathogen and exhibit a localized resistance response (Cordier
et al., 1998). Corresponding proteins involved in plant defense responses
have been studied in AMF symbioses; these include hydroxyproline-rich glycoproteins,
phenolics peroxidases, chitinase, B-1-3 glucanases-callose deposition and PR-phathogenesis
related proteins (Miranda, 1996).
Root exudates play an important role in AMF establishment symbiosis (Vierheilig
et al., 2003). The germination of Fusarium oxysporum f.sp., Lycopersici
was inhibited in the presence of root exudates from tomato (Scheffknecht
et al., 2006). Root exudates from mycorrhizal strawberry plants suppressed
the sporulation of Phytophthora fragariae (in) in vitro study
(Norman and Hooker, 2000). Differential growth of Fusarium
oxysporum f.sp chrysanthemi, Trichoderma harzianum, Clavibactor
michiganesis and Pseudomonas chlororaphis was explained by substances
released from Glomus intraradices under in vitro culture conditions
(Filion et al., 1999). Grandmaison
et al. (1993) suggested that phenolic compounds bound to cell wall
could be indirectly responsible for the resistance of AMF roots to pathogenic
fungi since they increased the resistance of cell wall to the action of digestive
Nutrient uptake: The primary goal of AMF inoculation is to increase
and enhance the yield and production of plants (Brundrett
and Juniper, 1995). The main benefits of AMF are enhancing plant the acquisition
of mineral nutrients and increasing the ability of host plants to withstand
or reduce acquisition of toxic elements to growth (Clark,
1997). AMF provide a greater effective root surface area to explore greater
volumes of soil and to overcome water and nutrient depletion zones around active
root surfaces (Smith and Read, 2008).
Mycorrhizal plant roots have increased weight, length, number and layer diameters
than the non-mycorrhizal one (Hetrick et al., 1988).
Since, the average diameter of fungal hyphae is 3-4 μM, which is smaller
than the root hair diameter (>10 μM). Therefore, fungal hyphae penetrate
soil pores and contact with soil so that roots hair would not able to contact.
AM roots greatly enhance the acquisition of mineral nutrient in plant (Jakobsen,
1995). Mycorrhizal research has shown the increased nutrient uptake; mainly
Phosphorus (P), in mycorrhizl plants compared to non-mycorrhizal plants (Akthar
and Siddiqui, 2008). Soil phosphorus absorption by mycorrhizal plants is
complete and faster than the non-mycorrhizal plants, because the distance of
diffusion for HPO4-2 and H2PO4 ion
in the soil will be shorter to the hyphae than to the root (Li
et al., 1991). Some studies indicate that increased phosphorus nutrition
has the same effect as inoculation with AMF (Davies et
al., 1992). Numbers of physiological changes are induced in AMF root
as a result of an increase in nutrition uptake (Graham,
2001). Enhancement of phosphorus status is the major benefit of mycorrhizal
fungi. This is due to the uptake of phosphorus from the soil by the AMF and
then transfers it to the host plant (Smith et al.,
The improvement of P nutrition of plants has been the most recognized and well
established beneficial effect of mycorrhizas (Karandashov
and Bucher, 2005; Cardoso et al., 2006).
Phosphate is converted into polyphosphate by polyphosphate kinase in vacuoles
(incorporated) and is transported between the hyphal tips and a sink at the
symbiotic interface (Pearson and Tinker, 1975). Translocation
rate is affected by rates of net efflux of P at hyphal tips and net uptake (Johanson
et al., 1993). The abnormally high P loss from the arbuscules has
been explained and two mechanisms have been proposed in this connection: Firstly,
a high arbuscular P concentration will reduce hyphal re-absorption of lost P
and this is in accordance with low expression of high P affinity transporter
in the fungal tissue inside roots, compared to its expression level in the external
hyphae (Harrison and Van Buuren, 1995). Secondly, P
efflux may be promoted by altered operation of trans-membrane that is carrying
and opening of ion channels. Mycorrhizal fungi are able to mobilize P and N
from their organic substrates (Smith and Read, 1997).
AMF is the most efficient ecological factor in improving growth and N content
in legumes (Barea et al., 2002b). Enhanced nitrogen
(N) acquisition by AM plants has been reported. This enhancement has been explained
by high nitrogen demand because of enhanced phosphorus (George
et al., 1995). The hyphal of AMF have the capacity to take-up and
transport N from soil to root (Bago and Becard, 2002).
The uptake and translocation of nitrogen by hyphal fungus is regulated by host
plants demand for N (Hawkins and George, 2001).
AM hyphae absorb and translocated amounts of nitrogen when provides as NH+4
and NO-3 (George et al, 1992). Hyphae
took up about 40 % of the nitrogen applied as (NH4)2SO4
to a hyphal soil compartment, while some nitrogen was transported by hyphae
over a distance of 5 cm within six days (Fery et al.,
1994). In addition to phosphorus and Nitrogen, AM fungi are known to have
enhanced uptake of Zn, S and Ca (Clark and Zeto, 2000)
and also Iron (Fe) acquisition has been enhanced. It is found that AM plants
that are grown at low pH had higher Fe acquisition than AM plants grown at high
pH (Treeby, 1992). Manganese acquisition generally was
lower in AM plants compared to non AM plants (Azaizeh et
Enhance tolerance to heavy metals (bioremediation): The effect of AMF
plants on trace elements uptake was reported (Clark and
Zeto, 2000). The AMF have higher shoot concentrations of copper (Cu) and
zinc (Zn) when grown in soil with low concentration of these elements. Copper
and zinc concentrations increased in leaves of AM soybean plants compared to
nonmycorrhizal plants. Sulfur acquisition was enhanced in sorghum colonized
by Glomus fasciculatum compared to non colonized plants (Raju
et al., 1990). Boron content was increased in AM maize shoot in acidic
and alkaline soils while the acquisition of calcium (K), sodium (Ca) and magnesium
(Mg) was also increased compared to the non AM Gigaspora gigantia soybean
plants in low Phosphorus. At the same time Gigaspora gigantia colonized
maize plant was decreased (K) and Ca but increased Mg acquisition (Lambert
et al., 1979). Aleminum (Al) acquisition toxicity was lower in AM
switch grass grown in acidic soil compared to non AM plants (Clark,
The AMF were shown to enhance the acquisition of (Br) and (Cl) (Ellis
et al., 1995) and (Cd) (Copper and Tinker, 1978).
(Co), (Cs) and (Ni) (Rogers and Williams, 1986). AM hyphae can uptake trace
elements in very low concentration including Zn, Fe and Cu from the soil solution
(Bago et al., 1996).
Mycorrhizosphere: Mycorrhizosphere refers to the zone of soil influence
by mycorrhizal association (Oswald and Ferchau, 1968).
The first reported about the role of the mycorrhizosphere in biocontrol of pathogens
was by Meyer and Linderman (1986). They found that extracts
of rhizosphere soil from mycorrhizal plants reduced sporangia formation of Phytophthora
cinnamomi in comparison with extracts of rhizosphere soil from non-mycorrhizal
plants. These authors postulated thateither the sporulation-inducing microorganisms
were missing or that the number of sporulation-inhibiting microorganisms increased.
The changes in root exudates affect the microbial communities around the roots
leading to the formation of mycorrhizosphere (Varama et
al., 1999). In mycorrhizosphere the appearance of mycorrhizae exert
a strong influence on the microflora in the rhizosphere. The mycorrhizosphere
microbiota differs qualitatively as well as quantitatively from the non rhizosphere
mycorrhizal plants. Mycorrhizosphere has two components:
layer of soil surrounds the mycorrhizal roots
layer of soil surrounding AMF hyphae in the soil referred to as the hyphosphere
Data have been acquired that rhizosphere bacteria have efficient impacts on
AMF growth (Azcon-Aguilar et al., 2002). The interaction
between AMF and other soil microbes in mycorrhizosphere can be classified as
positive (synergistic) as well as negative (antagonistic) interaction (Mukerji
et al., 2002). The positive interaction of AMF with Plant growth promoting
bacteria (PGPR), phosphorus-solubilising bacteria and N2-fixing bacteria
can enhance the AMF spores germination and the plant growth (Mayo
et al., 1986). Negative interaction is related to the ability of
AMF to suppress and inhibit the occurrence of various pathogens (Dehne,
Arbuscular mycorrhizal fungi and beneficial soil microorganisms interactions:
The interaction between mycorrhizal fungi and other soil organisms are complex
and often poorly understood; they may be inhibitory or stimulatory (Fitter
and Garbaye, 1994). The PGPR interact with mycorrhiza in the mycorrhizosphere.
Inoculation of Glomus faciculatum has shown a positive influence on actinomycetes
population in tomato rhizosphere. The survival of Azotobacter paspali
increased in mycorrhizosphere (Barea et al., 2002b).
Higher bacterial population and number of nitrogen fixer such as streptomycin
were reported and it has been detected that plants in the presence of AMF and
bacteria produced more phytohormones (Secilia and Bagyaraj,
The relationship between Phosphate-Solubilizing Bacteria (PSB) and AMF is well
reported (Barea et al., 2002a). The PSB can survive
longer in mycorrhizospher root. A plant with higher concentration of P benefits
the bacterial symbiont and nitrogenase functioning (Barea
et al., 1993). Dual inoculation of AMF and PSB significantly increased
microbial biomass and N and P accumulation in plant tissues (Barea
et al., 2002a, b). Mycorrhizae increased
nitrogen nutrition in plant by facilitating the use of nitrogen forms that are
difficult for mycorrhizal plants to exploit. Many ryhizobium strains improve
processes involved in AM formation (mycelia growth, spore germination) (Barea,
The authors are grateful to Prof Dr. Lynette Abbott (University Of Western Ausralia), for her notes and ideas presented here.
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