Figure 9

ROS, elicitor and hormone regulation of O ₃ -induced CRKs. O₃ enters the leaves through the stomata and immediately reacts with components of the cell wall to generate ROS. O₃ and the ROS induce an active production of ROS in the apoplast which is at least partly depending on membrane bound NADPH oxidases (RBOH), which produce . Similar ROS production in the apoplast takes place after infection of a plant with a pathogen or treatments with pathogen derived elicitors (PAMPs). ROS is hypothetically perceived via a "ROS receptor" which could sense ROS directly via protein modification, or via sensing of modified apoplastic proteins or other molecules that react with ROS. The perception of ROS initiates down-stream signalling events. H₂O₂ is also able to cross the plasma membrane and enter the cells. Inside the cell, the signalling pathway is split into two pathways. In the ROS pathway DND1/CNGC2 mediates a required step of the signalling pathway and JA and ET act as positive regulators, and SA and NPR1 are negative regulators. In the SA pathway ROS or pathogens activate SA biosynthesis via ICS1; and NPR1 is a required component. Since NPR1 is a positive regulator of the SA pathway and a negative regulator of the ROS pathway this implies that the separate signalling pathway use different transcription factors and promoter elements to regulate CRK expression, although it might be possible that two different transcription factors could converge on the same promoter element. In addition the pleiotropic nature of the dnd1 mutant, including high SA-levels, could change the place of DND1/CNGC2 in the model - constitutive SA signalling in dnd1 may limit the possibility for O₃ to activate the ROS pathway. Through the transcription factors EIN3 and EIL1 ET can repress SID2/ICS1 expression and SA levels. Increased ROS production in the chloroplast activates separate signalling pathway(s) leading to repression of CRK expression. One of these pathways could involve ABA and negative cross talk with the SA pathway.