The chemical dialogue between plants and beneficial arbuscular fungi in disease resistance

Association between plants and beneficial microorganisms like arbuscular mycorrhizal fungi modifies plant fitness and activates plant immune responses. Despite some findings speculating that nutritional improvements or beneficial microbiota recruitment may influence defensive status, mycorrhizal associations activate a fine-tuning of immune responses affecting primary and secondary metabolism. These metabolic rearrangements impact almost all defense-related hormones with special relevance in the jasmonic acid-dependent responses. Furthermore, biochemistry of the plant switches to specific secondary metabolites such as oxylipins, phenolic acids, lignans, and flavonoids contributing to an improved defensive status against pathogens and herbivores. The so-called mycorrhizal-induced resistance (MIR) is not linked to growth promotion and seems to be mediated by defense priming in the shoots. MIR is not ubiquitous but context-dependent, in which the lifestyle of the pathogen or the insect determines the resistance level. This chapter discusses the physiological and molecular changes that occur after symbiosis establishment and the mechanisms regulating the interaction between arbuscular mycorrhizal plants and biotic stress. The interaction of the symbionts with the environment through volatile signals is also discussed, as well as the qualitative and quantitative activation of defenses following multiway interactions between beneficial microorganisms, the plant and pathogens, or herbivores.