Disentangling the Complexity of Mitogen-Activated Protein Kinases and Reactive Oxygen Species Signaling

For about 2 million years, molecular oxygen arising from photosynthetic processes has become pivotal to almost all organisms. Reactive oxygen species (ROS), the partially reduced or activated derivatives of oxygen (hydrogen peroxide [H₂O₂], HO·, ¹O₂, O₂¿), are the highly reactive by-products of aerobic metabolism. They arise from various chemical reactions and can lead to oxidative damage of cells. Plants possess a sophisticated ROS network, comprising antioxidative enzymes, antioxidants, and ROS-producing enzymes, which allow them to keep ROS levels under tight control. Moreover, as research of the past few years has shown, plants have developed efficient strategies for targeted production of ROS. For instance, ROS play a role in programmed cell death (PCD), development, and stress response. Mitogen-activated protein kinase (MAPK) cascades are key players in ROS signaling. Several studies have shown that MAPK signaling pathways are not only induced by ROS but can also regulate ROS production. MAPK cascades are signaling modules that minimally consist of a MAPK kinase kinase (MAPKKK/MEKK), a MAPK kinase (MAPKK/MKK), and MAPK. Upon a stimulus-triggered activation of a MAPKKK, the signal is transduced via phosphorylation-mediated activation of a corresponding downstream MAPKK, which in turn phosphorylates and thereby activates a specific MAPK. The Arabidopsis (Arabidopsis thaliana) genome contains more than 60 MAPKKKs, 20 MAPKs, and 10 MAPKs, which can, depending on the environmental stimulus or developmental stage, engage in different MAPK modules. With the characterization of mutants affected in pathogen response as well as the development and dynamics of stomata, the network of MAPK cascade activation and ROS is being disentangled. Here, we discuss the most recent insights into ROS production and perception involving MAPK-mediated signaling