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.

中文翻译：

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MKK4 / MKK5-MPK1 / MPK2级联通过拟南芥中NPR1的磷酸化介导SA激活的叶片衰老。

内源性水杨酸（SA）调节叶片衰老的机制仍然不清楚。在这里，我们提供直接的证据表明，通过化学诱导的异果酸合酶1（ICS1）的上调，内源性SA水平的提高可以通过介导关键的SA信号成分，显着地加速老叶的衰老过程。光致病相关基因的非表达子（NPR1）在拟南芥中。重要的是，通过利用这种化学诱导的叶片衰老系统，我们确定了SA / NPR1介导的叶片衰老所需的促分裂原活化蛋白激酶（MAPK）级联MKK4 / 5-MPK1 / 2。MKK4 / 5和MPK1 / 2均表现出SA诱导的激酶活性，而MPK1 / 2是MKK4 / 5的直接靶标。mkk4 mkk5和mpk1 mpk2的双突变体显示出延迟的叶片衰老，而激酶基因的组成型过表达导致叶片过早衰老。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，抗病性的诱导显着受损。综上所述，我们的数据表明MKK4 / 5-MPK1 / 2级联在拟南芥中通过复杂的调控网络调节SA信号传导中起关键作用。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，抗病性的诱导在激酶基因的单突变体和双突变体中显着受损。综上所述，我们的数据表明MKK4 / 5-MPK1 / 2级联在拟南芥中通过复杂的调控网络调节SA信号传导中起关键作用。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，抗病性的诱导显着受损。两者合计，我们的数据表明MKK4 / 5-MPK1 / 2级联在拟南芥中通过复杂的调控网络调节SA信号中起关键作用。在激酶基因的单突变体和双突变体中，疾病抗性的诱导显着受损。两者合计，我们的数据表明MKK4 / 5-MPK1 / 2级联在拟南芥中通过复杂的调控网络调节SA信号中起关键作用。在激酶基因的单突变体和双突变体中，疾病抗性的诱导显着受损。两者合计，我们的数据表明MKK4 / 5-MPK1 / 2级联在拟南芥中通过复杂的调控网络调节SA信号中起关键作用。