12. Priming Host Defense Against Biotic Stress by Arbuscular Mycorrhizal Fungi

Mycorrhizal symbiosis has an important impact on plant interactions with pathogens and insects. Direct competition has been suggested as mechanism by which arbuscular mycorrhizae (AM) fungi can reduce the abundance of pathogenic fungi in roots. Priming set the plant to an “alert” state in which defenses are not actively expressed but in which the response to an attack occurs faster and/or stronger compared to plants not previously exposed to the priming stimulus, efficiently increasing plant resistance. Thus, priming confers important plant fitness benefit thereby defense priming by AM has a great ecological relevance. With regard to its bioprotective properties, the mycorrhizal symbiosis has become a focal point of research as an alternative to chemical fertilizers and pesticides in sustainable agriculture. In this chapter, we summarize the information available regarding mycorrhiza-induced resistance (MIR) with special emphasis in those involving plant defense responses.