Towards Understanding Extracellular ROS Sensory and Signaling Systems in Plants
Figure 1
Apoplastic ROS sensory systems and downstream signaling. The action and role of ROS in the apoplast can be addressed by exposing plants to ozone (O3) which degrades in the cell wall to the same ROS that are produced by the activity of the plasma membrane NADPH oxidases (exemplified here by the D-isoform RBOHD). The ROS can be sensed by direct modification of plasma membrane proteins, for example, Receptor-like kinases (RLK) or ion channels, indirectly by RLKs by ROS-dependent modification of cell wall components, exemplified here by modification off a cell wall protein GRI [26, 33], or by transport of H2O2 through aquaporins. The apoplastic ROS signal can be transmitted directly to nucleus and/or amplified in the chloroplast, followed by retrograde signaling where the chloroplast envelope protein RETICULATA (RE) and the cytoplasmic type 2A protein phosphatases (PP2A) appear to be involved. In the nucleus specific transcription factors and coregulators (exemplified here by RCD1) specifically activate or inactivate target genes and create the apoplastic ROS-dependent transcriptional pattern [8].
