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Abstract

Reactive oxygen species are, at high levels, harmful to plant cells, whereas, at a balanced and controlled level, they are involved in transduction of oxidative signaling. The apoplast, also known as the extracellular matrix plays a vital role in transferring the extracellular signals to internal organelles and the signal from cell to cell. Moreover, having a lower level of antioxidants renders it a favorable compartment for ROS production and maintenance. Multiple routes for H2O2 production exist in the apoplast; however, a prominent source is the NADPH oxidases, also named respiratory burst oxidase homologs (RBOHs). Many stimuli, such as pathogens, extracellular ATP, hormones, and abiotic stresses can trigger the RBOH-dependent ROS production in the apoplast. RBOHD and RBOHF are extensively studied and proposed typical roles in stomatal conductance and pathogen responses. In this review, we provide an update on RBOH enzymes and the regulatory mechanisms controlling the key enzyme of the family, RBOHD. Furthermore, we performed a phylogenetic analysis by including the RBOH protein family of Arabidopsis and some important crop species which have not been studied before, such as Brasssica rapa, Brassica oleracea, Oriza sativa, Glycine max, and Zea mays to characterize the evolutionary relationship of these isoforms in order to better classify them. We further describe how ROS are perceived and transduced inside the cell and how this interlinks with other systemic signals, including hormones, and abiotic stress responses such as those toward light and salt stress.