Root Allies: Arbuscular Mycorrhizal Fungi Help Plants to Cope with Biotic Stresses

Abstract

Plant interaction with beneficial soil microorganisms usually promotes plant growth and increases the plant’s ability to cope with biotic and abiotic stresses. Among these beneficial microorganisms, arbuscular mycorrhizal fungi (AMF) are of major importance because they establish mutualistic symbiosis with most plant species. Profound physiological changes take place in host plants upon root colonization by AMF. These changes impact their communication with the environment, altering the outcome of plant interactions with a wide range of organisms below- and aboveground. Protective effects of the symbiosis against pathogens, pests, and parasitic plants have been described for many plant species, including agriculturally important crop varieties. Besides mechanisms such as improved plant nutrition and competition, experimental evidence supports a major role of plant defenses in the observed protection. The increase in plant resistance against aggressors is known as mycorrhiza-induced resistance (MIR). During mycorrhiza establishment, plant defense mechanisms have to be tightly regulated in order to achieve a functional symbiosis. As a consequence of this regulation, a mild, but effective, activation of the plant immune responses seems to occur, not only locally but also systemically. This activation may lead to a primed state of the plant that allows a more efficient activation of defense mechanisms in response to a potential attack. Here, we give an overview of AMF and their impact on plant interactions with microbial pathogens, herbivorous insects, and parasitic plants, and we summarize the current knowledge of the underlying mechanisms.