write a note on VAM
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Vesicular arbuscular mycorrhizae (VA mycorrhizae, or VAM) are the most abundant of a group of symbiotic fungi that infect plant roots. VA mycorrhizae obtain carbon from their plant hosts, and (usually -- any generalization about VA mycorrhizae is risky) increase the nutrient and water uptake of their hosts. Some species of VA mycorrhizae colonize only particular plant hosts, while others are less specific. VA mycorrhizae consist of both external and intraradical structures, with the most distinctive structures occurring inside the root. A description of VA mycorrhizal physiology, though, is perhaps better started with the external mycorrhizal structures.
The most visible VA mycorrhizal structure is the hyphal network. Hyphae are thin (from 2 µm in diameter to > 20 µm) hollow tubes of fungi, which in VA mycorrhizae have few cross walls (are aseptate) and have distinct angular projections (Sylvia in Norris et al. 1994). These tubes originally grow from fungal spores, extending short distances (on the order of millimeters) into the soil in search of the roots of host plants (Smith and Read 1997). If the hyphae do not encounter a plant root in close proximity to the spore, the mycorrhiza will die (other types of mycorrhizae can be cultured in agar, but VA mycorrhizae do not survive without a host) (Berch and Fortin 1983). Hyphae that encounter a host root form a structure called an appressorium, and penetrate the cell wall of the root either by mechanical pressure or through the release of cell wall-degrading enzymes (Varma 1998). Hyphae that enter host roots through these infection points can form extensive networks both inside the root and throughout the soil surrounding the root. Some hyphae exit the root to form runner hyphae, which track roots through the soil and can infect additional roots. Thinner, branched networks of hyphae called absorbing hyphae develop from these runner hyphae, extending into the surrounding soil and transporting nutrients from as much as 7 cm away to the host roots (Allen 1991, Bago et al. 1998). Although a second type of mycorrhizae, the ectomycorrhizae, tend to form more substantial networks of anastomosing hyphae, VA mycorrhizae can also form networks. When two sets of fan-shaped hyphae meet, they can join and connect two plants of the same, or of different, species (Friese and Allen 1991). Nuclei are sometimes exchanged between fungi through these networks, and can fuse as a form of sexual reproduction for the fungus (Allen 1991).
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