Reactive oxygen species (ROS) in mycorrhizal fungi and symbiotic interactions with plants

The interaction of plant roots with mycorrhizal fungi leads to the formation of a novel structure, the ectomycorrhiza. Ectomycorrhizal symbioses benefit both organisms by mutual nutrient exchange and improve the host´s stress resilience. Here, we review the roles of reactive oxygen species and antioxidative systems in the fungal partner and the host plant for the establishment of the symbiosis and for the induction of local and systemic mycorrhizal resistance. Ectomycorrhizal symbioses can be formed with different fungal species each equipped with a distinct set of extracellular oxidative enzymes. While one function of the extracellular enzymes is the access to recalcitrant nutrient sources, some examples suggest neofunctionalization to enable intraradical colonization. The colonization also involves small secreted proteins, NADPH oxidases, H2O2 production, and affects the plant catalase, superoxide dismutase and peroxidase-based defenses. The fungal antioxidative defense is mainly based on glutathione and thioredoxin-based systems but its role in host colonization is largely unexplored. Among signals likely involved in systemic responses are jasmonates and salicylic acid in the plant, and volatile organic compounds released by the fungus. An emerging player with a role in plant-fungal interactions is nitric oxide. Higher stress tolerance of the mycorrhizal fungal species generally confers higher stress tolerance to the host and can vary among different isolates of the same fungal species. This review emphasizes that many puzzle parts are present, but that a complete picture has not yet emerged. We discuss further studies required to increase our understanding of the inter-kingdom dialog of both organisms.