Arbuscular Mycorrhizal (AM) Fungi

AM fungi occur over a wide range of agro climatic conditions and are geographically ubiquitous. They form symbiotic relationships with roots of about 90% land plants in natural and agricultural ecosystems (Brundrett, 2002). The AM association has been observed in 200 families of plants representing 1,000 genera and about 3, 00,000 plant species (Bagyaraj, 1991) and it is as normal for the roots of plants to be mycorrhizal as it is for the leaves to photosynthesize (Mosse, 1986). These fungi are included in the phylum Zygomycota, order Glomales (Redecker et al., 2000) but recently they have been placed into the phylum ‘Glomeromycota’ (Schussler et al., 2001). The Glomeromycota is divided into 4 orders, 8 families, 10 genera and 150 species. The common genera are Acaulospora, Gigaspora, Glomus and Scutellospora (Schussler, 2005). They are characterized by the presence of extra radical mycelium branched haustoria like structure with in the cortical cells termed as arbuscules and are the main site of nutrient transfer between two symbiotic partners (Hock and Verma, 1995; Smith and Read, 1997).

AM fungi colonize plant roots and ramify into surrounding soil extending the root depletion zone and the root system. They supply water and mineral nutrients from the soil to the plant while AM is benefited from carbon compounds provided by the host plant (Smith and Read, 1997; Siddiqui et al., 1999). AM fungi are associated with improved growth of host plant species due to increased nutrient uptake, production of growth promoting substances, tolerance to draught, salinity and synergistic interactions with other beneficial microorganisms (Sreenivasa and Bagyaraj, 1989).
The soil conditions prevalent in sustainable agriculture are likely to be more favorable to AM fungi than those under conventional agriculture (Bethlenfalvay and Schuepp, 1994; Smith and Read, 1997). Any agricultural operation that disturbs the natural ecosystem will have repercussions on the mycorrhizal system (Mosse, 1986). The preceding crops affect the growth and yield of subsequent crops (Karlen et al., 1994). The inclusion of non-mycorrhizal crops within rotations has been shown to decrease both AM fungal colonization and yield of following crops (Douds and Galvez, 1997; Arihawa and Karasawa, 2000). In addition to crop sequence, varietals selection, cultivation and fallowing have all been shown to affect mycorrhizal activity (Ocampo et al., 1980; Hetrick et al., 1996; McGonigle and Miller, 2000). However, impact of soluble fertilizers on colonization and function of AM fungi is contradictory. The application of soluble phosphorus decrease root colonization (Abbott and Robson, 1984) with occasional reports of increases (Gryndler et al., 1990). Similarly, contradictory results have also been reported with nitrogen fertilizer (Baltruschat and Dehne, 1988; Gryndler et al., 1990; Liu et al., 2000).