Fig. 8

A working model of SA-AtNAP-SAG202-ICS1-SA positive feedback loop in leaf senescence and its convergence/divergence in defense response in Arabidopsis. At the onset of and during leaf senescence, AtNAP TF physically binds to the promoter of SAG202 to direct the target gene expression. Subsequently the SAG202 TF activates its direct target gene ICS1 that is involved in the SA biosynthesis. The produced SA in turn feedback upregulates both AtNAP and SAG202. When the SA levels increase to a threshold, S3H (encoding an SA 3-hydroxylase) and S5H is induced to prevent overaccumulation of SA (Zhang et al., 2013; Zhang et al., 2017b). Too high levels of SA will cause hypersensitive response (HR)-like fast cell death. Leaf senescence is a slow programmed cell death process to allow nutrients released from degradation of proteins and other macromolecules to be recycled to active growing region or storage organs. Insert: A diagram showing convergence and divergence between leaf senescence and defense response with regard to the newly uncovered SA-AtNAP-SAG202-ICS1-SA regulatory loop. SA-AtNAP-SAG202-ICS1-SA feedback loop is activated to modulate endogenous SA levels in leaf senescence but not in defense responses. In defense response part of the loop, SAG202-ICS1-SA is activated