Division of Biological Sciences, Cell and Developmental Biology Section, University of California, San Diego, La Jolla, CA 92093
Division of Biological Sciences, Cell and Developmental Biology Section, University of California, San Diego, La Jolla, CA 92093.
Proc Natl Acad Sci U S A. 2018 May 8;115(19):E4522-E4531. doi: 10.1073/pnas.1719659115. Epub 2018 Apr 23.
Abscisic acid (ABA) plays essential roles in plant development and responses to environmental stress. ABA induces subcellular translocation and degradation of the guanine nucleotide exchange factor RopGEF1, thus facilitating ABA core signal transduction. However, the underlying mechanisms for ABA-triggered RopGEF1 trafficking/degradation remain unknown. Studies have revealed that RopGEFs associate with receptor-like kinases to convey developmental signals to small ROP GTPases. However, how the activities of RopGEFs are modulated is not well understood. Type 2C protein phosphatases stabilize the RopGEF1 protein, indicating that phosphorylation may trigger RopGEF1 trafficking and degradation. We have screened inhibitors followed by several protein kinase mutants and find that quadruple-mutant plants in the calcium-dependent protein kinases (CPKs) disrupt ABA-induced trafficking and degradation of RopGEF1. Moreover, partially impairs ABA inhibition of cotyledon emergence. Several CPKs interact with RopGEF1. CPK4 binds to and phosphorylates RopGEF1 and promotes the degradation of RopGEF1. CPK-mediated phosphorylation of RopGEF1 at specific N-terminal serine residues causes the degradation of RopGEF1 and mutation of these sites also compromises the RopGEF1 overexpression phenotype in root hair development in Our findings establish the physiological and molecular functions and relevance of CPKs in regulation of RopGEF1 and illuminate physiological roles of a CPK-GEF-ROP module in ABA signaling and plant development. We further discuss that CPK-dependent RopGEF degradation during abiotic stress could provide a mechanism for down-regulation of RopGEF-dependent growth responses.
脱落酸（ABA）在植物发育和环境胁迫响应中发挥着重要作用。ABA 诱导 RopGEF1 的亚细胞易位和降解，从而促进 ABA 核心信号转导。然而，ABA 触发 RopGEF1 运输/降解的潜在机制尚不清楚。研究表明，RopGEFs 与受体样激酶结合，将发育信号传递给小 ROP GTPases。然而，RopGEFs 的活性如何被调节还不太清楚。2C 型蛋白磷酸酶稳定 RopGEF1 蛋白，表明磷酸化可能触发 RopGEF1 运输和降解。我们筛选了抑制剂，随后对几种蛋白激酶突变体进行了筛选，发现钙依赖性蛋白激酶（CPKs）的四重突变体植物破坏了 ABA 诱导的 RopGEF1 运输和降解。此外，部分破坏了 ABA 对子叶萌发的抑制作用。几种 CPKs 与 RopGEF1 相互作用。CPK4 结合并磷酸化 RopGEF1，并促进 RopGEF1 的降解。CPK 介导的 RopGEF1 特定 N 端丝氨酸残基磷酸化导致 RopGEF1 的降解，这些位点的突变也损害了 RopGEF1 在根毛发育中的过表达表型。我们的研究结果确立了 CPKs 在 RopGEF1 调控中的生理和分子功能及其相关性，并阐明了 CPK-GEF-ROP 模块在 ABA 信号转导和植物发育中的生理作用。我们进一步讨论了非生物胁迫过程中 CPK 依赖性 RopGEF 降解可能为依赖 RopGEF 的生长反应的下调提供一种机制。
