AtVQ25 promotes salicylic acid-related leaf senescence by fine-tuning the self-repression of AtWRKY53

Most mechanistic details of chronologically ordered regulation of leaf senescence are unknown.Regulatory networks centered on AtWRKY53 are crucial for orchestrating and integrating various senescence-related signals.Notably,AtWRKY53binds to its own promoter and represses transcription of AtWRKY53,but the biological significance and mechanism underlying this selfrepression remain unclear.In this study,we identified the VQ motif-containing protein AtVQ25as a cooperator of AtWRKY53.The expression level of AtVQ25 peaked at mature stage and was specifically repressed after the onset of leaf senescence.AtVQ25-overexpressing plants and atvq25 mutants displayed precocious and delayed leaf senescence,respectively.Importantly,we identified AtWRKY53 as an interacting partner of AtVQ25.We determined that interaction between AtVQ25 and AtWRKY53 prevented AtWRKY53from binding to W-box elements on the AtWRKY53promoter and thus counteracted the selfrepression of AtWRKY53.In addition,our RNA-sequencing data revealed that the AtVQ25-AtWRKY53 module is related to the salicylic acid(SA)pathway.Precocious leaf senescence and SA-induced leaf senescence in AtVQ25-overexpressing lines were inhibited by an SA pathway mutant,atsid2,and Nah G transgenic plants;AtVQ25-overexpressing/atwrky53 plants were also insensitive to SA-induced leaf senescence.Collectively,we demonstrated that AtVQ25 directly attenuates the self-repression of AtWRKY53 during the onset of leaf senescence,which is substantially helpful for understanding the timing of leaf senescence onset modulated by AtWRKY53.

The NPR1-WRKY46-WRKY6 signaling cascade mediates probenazole/salicylic acid-elicited leaf senescence in Arabidopsis thaliana

Endogenous salicylic acid(SA) regulates leaf senescence, but the underlying mechanism remains largely unexplored. The exogenous application of SA to living plants is not efficient for inducing leaf senescence. By taking advantage of probenazole(PBZ)-inducedbiosynthesisof endogenous SA, we previously established a chemical inducible leaf senescence system that depends on SA biosynthesis and its core signaling receptor NPR1 in Arabidopsis thaliana. Here,using this system, we identified WRKY46 and WRKY6 as key components of the transcriptional machinery downstream of NPR1 signaling. Upon PBZ treatment, the wrky46 mutant exhibited significantly delayed leaf senescence. We demonstrate that NPR1 is essential for PBZ/SA-induced WRKY46 activation, whereas WRKY46 in turn enhances NPR1 expression. WRKY46 interacts with NPR1 in the nucleus, binding to the W-box of the WRKY6 promoter to induce its expression in response to SA signaling. Dysfunction of WRKY6 abolished PBZ-induced leaf senescence, while overexpression of WRKY6 was sufficient to accelerate leaf senescence even under normal growth conditions, suggesting that WRKY6 may serve as an integration node of multiple leaf senescence signaling pathways. Taken together,these findings reveal that the NPR1-WRKY46-WRKY6 signaling cascade plays a critical role in PBZ/SA-mediated leaf senescence in Arabidopsis.

Expression of the inactive ZmMEK1 induces salicylic acid accumulation and salicylic acid-dependent leaf senescence

Leaf senescence is the final leaf developmental process that is regulated by both intracellular factors and environmental conditions. The mitogen-activated protein kinase（MAPK） signaling cascades have been shown to play important roles in regulating leaf senescence; however, the component（s） downstream of the MAPK cascades in regulating leaf senescence are not fully understood. Here we showed that the transcriptions of Zm MEK1, Zm SIMK1, and Zm MPK3 were induced during dark-induced maize leaf senescence.Furthermore, in-gel kinase analysis revealed the 42 k Da MAPK was activated. Zm MEK1 interacted with Zm SIMK1 in yeast and maize mesophyll protoplasts and Zm SIMK1 was activated by Zm MEK1 in vitro. Expression of a dominant negative mutant of Zm MEK1 in Arabidopsis transgenic plants induced salicylic acid（SA） accumulation and SA-dependent leaf senescence.Zm MEK1 interacted with Arabidopsis MPK4 in yeast and High-Im pact activated MPK4 in vitro. SA treatment accelerated darkinduced maize leaf senescence. Moreover, blockage of MAPK signaling increased endogenous SA accumulation in maize leaves. These findings suggest that Zm MEK1-Zm SIMK1 cascade and its modulating SA levels play important roles in regulating leaf senescence.

Arabidopsis WRKY1 promotes monocarpic senescence by integrative regulation of flowering,leaf senescence,and nitrogen remobilization

Monocarpic senescence,characterized by whole-plant senescence following a single flowering phase,is widespread in seed plants,particularly in crops,determining seed harvest time and quality.However,how external and internal signals are systemically integrated into monocarpic senescence remains largely unknown.Here,we report that the Arabidopsis thaliana transcription factor WRKY1 plays essential roles in multiple key steps of monocarpic senescence.WRKY1 expression is induced by age,salicylic acid(SA),and nitrogen(N)deficiency.Flowering and leaf senescence are accelerated in the WRKY1 overexpression lines but are delayed in the wrky1 mutants.The combined DNA affinity purification sequencing and RNA sequencing analyses uncover the direct target genes of WRKY1.Further studies show that WRKY1 coordinately regulates three processes in monocarpic senescence:(1)suppressing FLOWERING LOCUS C gene expression to initiate flowering,(2)inducing SA biosynthesis genes to promote leaf senescence,and(3)activating the N assimilation and transport genes to trigger N remobilization.In summary,our study reveals how one stress-responsive transcription factor,WRKY1,integrates flowering,leaf senescence,and N remobilization processes into monocarpic senescence,providing important insights into plant lifetime regulation.

Constitutive activation of AtMEK5,a MAPK kinase,induces salicylic acid- independent cell death in Arabidopsis thaliana

AtMEK5 DD is an active mutant of AtMEK5, a MAP kinase kinase in Arabidopsis. Induction of AtMEK5 DD expression in transgenic plants leads to activation of 44 and 48 kD MAPKs and causes a rapid cell death. To compare the cell death induced by the expression of AtMEK5 DD with the HR-cell death induced by avirulence pathogen infection, we analyzed the activation of downstream MAP Kinase and induction of PR genes expression in permanent transgenic Arabidopsis plants. In-gel kinase activity assay revealed that the infection of Pseudomonas syringae DC3000 harboring Avr Rpt2 gene also lead to activation of 44 and 48 kD MAPKs. PAL, PR1 and PR5 were strongly induced in plants undergo- ing HR-cell death caused by the infection of P. syringae DC3000, while only the expression of PR5 was strongly in- duced in transgenic plants expressing AtMEK5 DD protein. NahG protein in AtMEK5 ×NahG plants cannot suppress DD the cell death induced by AtMEK5 . And AtMEK5 DD DD pro- tein expressed AtMEK5 ×NahG plants showed no signifi- DD cant change in salicylic acid (SA) level. All these suggest that the cell death induced by the activation of AtMEK5 is sali- cylic acid-independent.

水杨酸对离体青稞叶片衰老的缓解效应

以青稞幼苗离体叶片为试材,在暗培养条件下,研究了水杨酸处理对离体青稞叶片叶绿素总量、丙二醛(MDA)、脯氨酸、可溶性蛋白质含量的影响。结果显示,在离体青稞叶片衰老过程中,叶绿素总量和可溶性蛋白质含量均不断下降,MDA和脯氨酸含量升高。用800 mg/L的水杨酸处理可延缓离体青稞叶片叶绿素总量和可溶性蛋白质含量的降低,抑制MDA、脯氨酸含量的上升。表明水杨酸降低了离体青稞叶片膜脂过氧化程度,延缓了离体青稞叶片衰老的进程。