ABSTRACT
Host reaction to fungal symbiont or pathogen may begin even before the establishment of any physical contact. Many steps leading to initial contact, formation of appresoria, production of inter-and intracellular structures, and molecular interactions that maintain these two types of biotrophic partnerships could be similar. Infection by fungal pathogen results in activation of an array of defense mechanisms that restrict its multiplication. Hypersehsitive cell death, oxidative burst, accumulation of phytoalexins and induction of pathogenesis-related proteins are a few examples of host reaction to the pathogen. Many of these reactions occur even during the establishment of symbiotic associations with mycorrhiza or rhizobia (Duffy and Cassais, 2000, 2003). Defense reactions are triggered in host tissue with the invasion of mycorrhizal fungus, but only in a transient and uncoordinated manner that fades away as symbiosis progresses (Gianinazzi-Pearson and Gianinazzi, 2003). Whereas, several other host reactions and end reactions can be diametrically opposite. At least broadly, if host Interaction with fungal pathogen leads to deleterious effects, a symbiotic relationship often imparts physiological benefits. The physiological and molecular basis for such interactive effects on biotrophic partners are being studied; yet there exist numerous lacunae in our knowledge. A comparative study of fungal pathogenesis and symbiosis may provide us with some helpful leads in selectively enhancing symbiosis; at the same time, repressing pathogenesis. For example, a carefully tailored breeding program that enables the development of crop genotypes with resistance to fungal pathogenesis, simultaneously preserving/enhancing mycorrhizal component will have immense applied value in practical agriculture and forestry. Also, a comparative study of molecular aspects could aid in search and easier identification of chemicals that selectively inhibit pathogenesis without any detriment to AM/ECM fungus. A review of recent advances in molecular biology of hosts reactions to fungal pathogens and symbionts-a few examples of mycorrhiza mediated host resistance to fungal pathogenesis and comparison of nutrient exchange at biotrophic interfaces-have been included in this chapter.
