plant surface microbiology.pdf

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Ramesh Chander Kuhad et al.
roots (Torrisi et al. 1999). Extra radical AM hyphae augment the uptake of
nutrients from up to 12 cm away from the root surface (Cui and Caldwell
1996).
The network of hyphae may increase the availability of nutrients like N or
P from locked sources by decomposing large organic molecules (George et al.
1995). Mycorrhizal fungi are also known to develop bridges connecting the
root with the surrounding soil particles to improve both nutrient acquisitions
by the plant and soil structure (Varma 1995; Hodge 2000). Unlike N 2-fixing
bacteria that function as biological fertilizers, AM fungi do not add P to the
soil. They only improve its availability to the plant. There is evidence that
phosphatase activity is higher in the rhizosphere around AM than in nonmycorrhizal
roots. P uptake is enhanced with the increase in root colonization by
mycorrhizae.A system of barter operates, the colonized plant provides photosynthate
to the fungus, in return, its extraradical hypha makes more P available
to the host (Merryweather and Fitter 1995). Plants rely more on AM when
growing in soils deficient in P (Bationo et al. 2000). Depriving a plant in its
natural environment of mycorrhizae on a long-term basis can also reduce P
acquisition. Plants that are nonmycorrhizal invest more in their vegetative tissues
like shoots and roots. In contrast, in mycorrhizal plants, the functions of
the roots are taken over by the AM hyphae, thereby permitting the host plant
to invest its resources in reproductive organs.
Nitrogen occurs in the soil predominantly in the form of nitrate and
ammonia, which is water-soluble and readily available for absorption. Studies
with labelled N have revealed that the AM increases N uptake by plants (Bijbijen
et al. 1996; Faure et al. 1998; Mädder et al. 2000). AM fungal hyphae have
been credited with the uptake and transfer of large amounts of N from the soil
to the host (Johansen et al. 1996; Hodge et al. 2000). However, there is little reciprocal
transfer of N from the plant to the fungi, which makes uptake and
assimilation of N by the symbiont essential for its growth (Bijbijen et al. 1996).
Since AM form underground hyphal links between plants, N transfer between
plants by means of such links is possible. Using labelled 15 N, Frey and Schüepp
(1993) demonstrated that N flows from Trifolium alexandrium to Zea mays
via AM fungal network. AM are believed to enhance N 2-fixation by symbiotic
legumes by increasing root and nodule biomass, N 2-fixation rates, root N
absorption rates, and plant N and P content (Olesniewicz and Thomas 1999).
Mycorrhizae have also been reported to be involved in the uptake of other
micro- and macro-nutrients like K, S, Mg, Zn, Cu, Ca and Na (Díaz et al. 1996;
Hodge 2000).
Soil microorganisms, particularly saprophytic fungi affect the development
and function of AM symbiosis. Fracchia et al. (2000) investigated the
effect of the saprophytic fungus Fusarium oxysporum on AM colonization
and plant dry matter was studied in greenhouse and field experiments using
host plants, maize, sorghum, lettuce, tomato, wheat, lentil and pea and AM
fungi, Glomus mosseae, G. fasciculatum, G. intraradices, G. clarum and G.