Arbuscular mycorrhizal colonization and activation of plant defense responses against phytopathogens

Abstract

Arbuscular mycorrhizal fungi (AMF) are potentially mutualistic biotrophs of plants and improve water supply and nutrient uptake in host plants. In exchange of this, it takes a part of photosynthate from the host plant to fulfill its metabolic requirements. Despite having its own immune system, plant gets attacked by various pathogens and therefore needs support to overcome such challenges and to become stabilized in such hostile environment. AMF colonization helps the plants either directly or indirectly to face the challenges of biotic and abiotic stresses. Several physiological and biochemical changes occur in the host plant and mycorrhizosphere following colonization of roots by AMF, and AMF colonization also affects interactions of the host plants with a diverse range of both above- and belowground organisms. Protective effects of AMF colonization against pests, pathogens, and stem or root parasitic plants were described in many agriculturally important crop species. These mechanisms not only improve plant nutrition consumption and competition but also play a significant role in plant defense activation. Successful establishment of mycorrhizal species on host leads to regulation of the JA and SA signaling pathways, and it itself explains the range of protection conferred by this symbiosis. Defense activation following colonization by mycorrhizal species is associated with moderate activation of host transcription factors such as MAP kinases. Further, several other defense-related compounds are also accumulated such as PR proteins, Β-1,3-glucanases, phytoalexins, and phenolics, and deposition of callose also occurs leading to protection against various pathogens. In the present chapter, we discussed the major defense signaling aspects during plant-pathogen interactions mediated through mycorrhizal colonization in the host plant roots.