State Key Laboratory of Genetic Engineering and Fudan Center for Genetic Diversity and Designing Agriculture, School of Life Sciences, Fudan University, Shanghai, 200438, China.
Plant Mol Biol. 2020 Mar;102(4-5):463-475. doi: 10.1007/s11103-019-00958-z. Epub 2020 Jan 8.
The mechanism by which endogenous salicylic acid (SA) regulates leaf senescence remains elusive. Here we provide direct evidence that an enhancement of endogenous SA level, via chemical-induced upregulation of ISOCHORISMATE SYNTHASE 1 (ICS1), could significantly accelerate the senescence process of old leaves through mediation of the key SA signaling component NON EXPRESSOR OF PATHOGENESIS RELATED GENES 1 (NPR1) in Arabidopsis. Importantly, by taking advantage of this chemically induced leaf senescence system, we identified a mitogen-activated protein kinase (MAPK) cascade MKK4/5-MPK1/2 that is required for the SA/NPR1-mediated leaf senescence. Both MKK4/5 and MPK1/2 exhibited SA-induced kinase activities, with MPK1/2 being the immediate targets of MKK4/5. Double mutants of mkk4 mkk5 and mpk1 mpk2 displayed delayed leaf senescence, while constitutive overexpression of the kinase genes led to premature leaf senescence. Such premature leaf senescence was suppressed when they were overexpressed in an SA synthesis defective mutant (sid2) or signaling detective mutant (npr1). We further showed that MPK1, but not MPK2, could directly phosphorylate NPR1. Meanwhile, MPK1 also mediated NPR1 monomerization. Notably, induction of disease resistance was significantly compromised in the single and double mutants of the kinase genes. Taken together, our data demonstrate that the MKK4/5-MPK1/2 cascade plays a critical role in modulating SA signaling through a complex regulatory network in Arabidopsis.
内源性水杨酸（SA）调节叶片衰老的机制仍不清楚。在这里，我们提供了直接的证据，通过化学诱导异分支酸合酶 1（ICS1）的上调来增强内源性 SA 水平，可以通过关键的 SA 信号成分 NON EXPRESSOR OF PATHOGENESIS RELATED GENES 1（NPR1）在拟南芥中显著加速老叶的衰老过程。重要的是，利用这种化学诱导的叶片衰老系统，我们鉴定了一个丝裂原激活蛋白激酶（MAPK）级联 MKK4/5-MPK1/2，它是 SA/NPR1 介导的叶片衰老所必需的。MKK4/5 和 MPK1/2 都表现出 SA 诱导的激酶活性，而 MPK1/2 是 MKK4/5 的直接靶标。mkk4 mkk5 和 mpk1 mpk2 的双突变体表现出延迟的叶片衰老，而激酶基因的组成型过表达导致叶片提前衰老。当它们在 SA 合成缺陷突变体（sid2）或信号检测突变体（npr1）中过表达时，这种过早的叶片衰老被抑制。我们进一步表明，MPK1 而不是 MPK2 可以直接磷酸化 NPR1。同时，MPK1 也介导 NPR1 单体化。值得注意的是，在激酶基因的单突变体和双突变体中，对疾病抗性的诱导显著受损。总之，我们的数据表明，MKK4/5-MPK1/2 级联在通过拟南芥中复杂的调控网络调节 SA 信号中起关键作用。
