Calcium (Ca2+)-dependent protein kinases (CDPKs or CPKs) are a unique family of Ca2+-sensor/kinase-effector proteins with diverse functions in plants. In Arabidopsis thaliana, CPK28 contributes to immune homeostasis by promoting degradation of the key immune signaling receptor-like cytoplasmic kinase BOTRYTIS-INDUCED KINASE 1 (BIK1), and additionally functions in vegetative-to-reproductive stage transition. How CPK28 controls these seemingly disparate pathways is unknown. Here, we identify a single phosphorylation site in the kinase domain of CPK28 (Ser318) that is differentially required for its function in immune homeostasis and stem elongation. We show that CPK28 undergoes intra- and inter-molecular auto-phosphorylation on Ser318 and can additionally be trans-phosphorylated on this residue by BIK1. Analysis of several other phosphorylation sites demonstrates that Ser318 phosphorylation is uniquely required to prime CPK28 for Ca2+ activation at physiological concentrations of Ca2+, possibly through stabilization of the Ca2+-bound active state as indicated by intrinsic fluorescence experiments. Together, our data indicate that phosphorylation of Ser318 is required for the activation of CPK28 at low intracellular [Ca2+ ] to prevent initiation of an immune response in the absence of infection. By comparison, phosphorylation of Ser318 is not required for stem-elongation, indicating pathway specific requirements for phosphorylation-based Ca2+-sensitivity priming. We additionally provide evidence for a conserved function for Ser318 phosphorylation in related group IV CDPKs which holds promise for biotechnological applications by generating CDPK alleles that enhance resistance to microbial pathogens without consequences to yield.

中文翻译：

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钙依赖性蛋白激酶CPK28的磷酸化依赖性亚功能化

钙（Ca2 +）依赖性蛋白激酶（CDPK或CPK）是具有独特功能的Ca2 +传感器/激酶效应蛋白家族，在植物中具有多种功能。在拟南芥中，CPK28通过促进关键的免疫信号受体样细胞质激酶BOTRYTIS诱导的激酶1（BIK1）的降解来促进免疫稳态，并在从营养到生殖的阶段过渡中起作用。CPK28如何控制这些看似完全不同的途径尚不清楚。在这里，我们确定了CPK28（Ser318）的激酶域中的单个磷酸化位点，该位点在免疫稳态和茎伸长中的功能不同。我们显示CPK28在Ser318上进行分子内和分子间自磷酸化，并且可以通过BIK1在该残基上进行反磷酸化。对其他几个磷酸化位点的分析表明，Ser318磷酸化是启动CPK28激活Ca2 +生理浓度下Ca2 +激活所特有的，可能是通过固有的荧光实验表明，通过稳定Ca2 +结合的活性状态来实现的。总之，我们的数据表明在低细胞内[Ca2 +]激活CPK28时需要Ser318的磷酸化，以防止在没有感染的情况下引发免疫反应。相比之下，Ser318的磷酸化不需要进行茎伸长，这表明基于磷酸化的Ca2 +-敏感性引发的途径特定要求。