Vesicular-arbuscular mycorrhizae in relation to plant disease.

Vesicular-arbuscular mycorrhizae (VAM) enhance plant growth through increased nutrient uptake, stress tolerance and disease resistance. As an integral part of the root system, they interact with other microorganisms in soil and result in increased root exudation approaching about 25% of the plant dry matter production. Roots support a multitude of microorganisms that, in concert, can have profound influence on growth and survival of the plant. VAM fungi can alter the root exudation pattern, enhance chitinolytic activity and alter photosynthetic/respiratory deficiencies. VAM-positive plants are known to exhibit varied resistance towards soil-borne and foliar pathogens. The known interactions include a number of mechanisms, such as exclusion of the pathogen, lignification of plant cell walls, changed phosphate nutrition resulting in altered exudation by roots, and formation of inhibitory low molecular weight compounds. The purpose of this review is to discuss VAM-plant-pathogen interactions and the possible mechanisms involved in altered resistance. Based on these observations, a working model is proposed to explain the VAM-disease interaction under varied environmental conditions.