Chapter 8 Interactions Between Nonpathogenic Fungi and Plants

ABSTRACT This chapter deals with the interactions established between nonpathogenic fungi/biocontrol agents and plants and their involvement in the reinforcement of plant defenses. Among the nonpathogenic fungi, one can distinguish between the true endophytic and the free-living opportunistic symbiotic fungi. As for the first group, the majority of the literature available corresponds to mycorrhizal fungi. Historically, these studies were focused on the influence of the mycorrhizal symbiosis on plant nutrition and later evolved to studies on plant immunity. Other endophytic fungi that trigger enhanced plant resistance correspond to nonpathogenic binucleate Rhizoctonia spp. and Piriformospora indica. The second group includes fungi that were considered to be free-living microorganisms that reduce plant diseases by direct competition with plant pathogens through different antagonistic mechanisms. More recently, such fungi were proven to opportunistically establish symbiotic relationships with plants and to enhance plant defense by inducing systemic resistance. The most studied genus corresponds to Trichoderma spp., but information is also available on the mode of action of nonpathogenic Fusarium oxysporum, Penicillium spp., Phoma spp., and Pythium oligandrum strains. Information about the mechanisms of action of this broad range of nonpathogenic fungi is summarized, paying special attention to the genes, proteins, and signaling pathways involved, and common patterns of defense mechanisms induced by beneficial fungi in plants are proposed.