水杨酸（SA）是正调控叶片衰老的主要植物激素之一。由于SA合成与信号组分相关突变体的衰老表型微弱，妨碍了对其分子机制的深入研究。烯丙异噻唑（PBZ）可诱导拟南芥体内SA合成增加，并诱发叶片早衰；PBZ诱发的衰老依赖于SA信号调控因子NPR1。初步遗传学分析发现MKK4/5-MPK1/2级联是SA触发的叶片衰老调控路径上的关键组分；生化分析显示MKK4/5和MPK1/2可受外源SA诱导磷酸化，且MPK1与NPR1存在蛋白水平的互作。在此基础上，本项目拟开展以下研究：1）探明MKK4/5-MPK1/2级联在介导SA触发的叶片衰老中的关键作用；2）分析MKK4/5-MPK1/2级联在叶片衰老进程中介导NPR1磷酸化的可能机制；3）探索MKK4/5-MPK1/2级联介导SA触发的抗病反应调控的可能机制。本研究将有助于阐明SA调控路径上的关键组分，并深化人们对磷酸化在SA信号调控中的作用机制的理解。

Salicylic acid (SA) is a positive regulator of leaf senescence. Due to the less obvious senescent phenotypes of SA biosynthetic gene and signaling pathway component mutants, the regulation of endogenous SA-triggered leaf senescence has not been extensively explored. Application of 3-allyloxy-1,2-benzisothiazole-1,1-dioxide (Probenazole; PBZ), a chemical inducer of systemic acquired resistance, could enhance endogenous SA biosynthesis and consequently induce leaf senescence through NONEXPRESSER OF PR GENES 1 (NPR1) in Arabidopsis. By taking advantage of this inducible system, we demonstrated that MKK4/5-MPK1/2 cascade might be required for SA-triggered leaf senescence. Both MKK4/5 and MPK1/2 kinases could be phosphorylated upon exogenous SA-induction, and MPK1 could interact with NPR1, as shown in preliminary biochemical experiments. With these preliminary results, we would like to further our study in the following directions: 1) to confirm the crucial role of MKK4/5-MPK1/2 cascade in mediating SA-triggered leaf senescence, 2) to explore the possible mechanism of MKK4/5-MPK1/2 cascade in directly phosphorylating NPR1 during leaf senescence, 3) to reveal the possible mechanism of MKK4/5-MPK1/2 cascade in inducing disease resistance associated with SA signaling. This study will certainly help to identify new components of SA signaling pathway involved in regulating SA-triggered leaf senescence and disease resistance, and eventually extend our understanding of the importance of phosphorylation in the regulation of SA signaling pathway.