NADPH Oxidases Have Key Roles in Mutulistic Associations with Rhizobia or with Mycorrhizal Fungi in Root Legumes

Symbiotic associations between plants and microorganisms appeared early in evolution, conferring great advantages to both partners, under limited nutrient conditions. Arbuscular mycorrhizal symbiosis (AMS) is one of the oldest known mutualistic relationship between roots and microorganisms, enhancing nitrogen and phosphorus acquisition into the plant. Legumes form a specialized organ called nodule, in presence of soil diazotroph bacteria (rhizobia), providing assimilable nitrogen to the plant host, a process known as root nodule symbiosis (RNS). Several reports indicate that RNS recruited molecular components from the signaling pathway of the AMS. This latter has been confirmed by several evidences, including legume mutants impaired in establishing both symbiotic associations. Recent reports indicate that members of the plant NADPH oxidase gene family, known as respiratory burst oxidase homologs (Rbohs) participate in both symbioses. RBOHs are membrane proteins specialized in reactive oxygen species (ROS) generation, and are produced at different stages of AMS and RNS. RBOH-mediated ROS have positive or negative roles in these interactions, depending on the oxidase isoform implicated in a particular process, and the type of symbiosis established. Herein, the participation of Rboh genes in both interactions is analyzed and discussed, proposing working models that explain the dynamics of such oxidases.