A Tripartite Amplification Loop Involving WRKY75, Salicylic Acid and ROS Accelerates Leaf Senescence in Arabidopsis

Leaf senescence is a highly coordinated and complicated process with the integration of numerous internal and environmental signals. Salicylic acid (SA) and reactive oxygen species (ROS) are two well-defined inducers of leaf senescence, whose contents progressively and inter-dependently increase during leaf senescence via a yet unknown mechanism. Here, we have characterized a newly identified positive regulator of leaf senescence, WRKY75, and demonstrated that knock-down or knock-out of WRKY75 delays, while over-expression of WRKY75 accelerates age-dependent leaf senescence. The WRKY75 transcription is induced by age, SA, H2O2, as well as multiple plant hormones. Meanwhile, WRKY75 is able to promote SA production by inducing the transcription of SA INDUCTION-DEFICIENT 2 (SID2), and suppress H2O2 scavenging partly by repressing the transcription of CATALASE 2 (CAT2). Genetic analysis reveals that the SID2 mutation or an increase of catalase activity rescues the precocious leaf senescence phenotype evoked by WRKY75 over-expression. Based on these results, we propose a “tripartite amplification loop” model in which WRKY75, SA and ROS undergo a gradual but self-sustained rise driven by three interlinked positive feedback regulations. This tripartite amplification loop provides a molecular framework connecting the upstream signals, such as age, ethylene, JA and ABA, to the downstream regulatory network executed by those SA-responsive and H2O2-responsive transcription factors during leaf senescence.