Fungal symbiotic relations

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VAM fungi: improvement of water uptake and conference of drought tolerance in the host plant.

Mycorrhiza is a structural and functional relationship between fungi and plants occurring in nature. This association starts when a fungus infects a plant resulting in a nutritional symbiosis being established between the two. One type of fungal infection involves ubiquitous microscopic fungi that infect the plant root (Abbott and Robson, 1982). The fungi that form this vesicular-arbuscular mycorrhiza (VAM) are known as VAM fungi. The resulting symbiosis is beneficial to both the host plant and the VAM fungus. The fungus has a substrate to grow on and obtain nutrients and the plant improves its nutrient and water uptake. Other benefits occur at the ecosystem level reviewed by Jeffries and Barea (1994), but are not discussed here.

When VAM infections occur, the host plant generally allows the fungus to establish itself in its root system. Establishment of VAM infections is regulated and controlled by mechanisms in the plant and by environmental conditions (Azcon-Aguilar and Bago, 1994). VAM fungi are typically found in soils that are actively mineralizing (Read, 1983). Since the essential element P is immobile, there is usually an increase in VAM infections seen in this soil type. Arbuscular mycorrhizas are, by definition, not confined to the root systems, but that is where they are usually found. The infection procedure occurs by the formation of spores, hyphae and then appressoria (Giovannetti et al., 1994). The appressoria is a swelling or outgrowth of the hyphae used for attachment in the infection process (Emmett and Parbery, 1975). After forming appressoria, the second phase of VAM fungus infection occurs, involving penetration of the root into a potential host (Giovannetti et al., 1994). The process underlying penetration is not well understood and is an area fore more research. It is generally assumed that the fungal hyphae then spreads into the root cells, typically near the root tip where the cells are young (Brundrett and Kendrick, 1990). Since the root cell wall must be broken down there is probably secretion of degradation enzymes, however, this process has not been fully elucidated. Highly branched arbuscules form inside the plant root in the cortical layer which is indicative of a VAM infection. At this stage, it is postulated that developmental structures will form that will effect mineralization and nutrient uptake (Anderson, 1992)

An obvious benefit of a fungal infection in the root system is an increase in the surface area in the soil. Water uptake by a plant is facilitated by the mobility of water and by the perfusion of its roots through the soil. As a result, a limited root surface area will result in low water uptake. Microscopic fungi can be recognized by their extensive vegetative mycelia or hyphae. The extending hyphae of VAM fungi can further increase the surface area of the root system allowing an increase in water uptake. The extraradical hyphae of the VAM fungus found on the sea oat Uniola paniculata, was measured at 592 m/cm of colonised root length (Sylvia, 1986). A study by Allen (1982) showed that VAM infection of Bouteloua gracilis decreased the leaf resistance and increased transpiration by 100%, resulting in increased water uptake when compared with non-infected plants. The increased water uptake shown by VAM infected plants ...