Reevaluation of the role of chloroplasts in the calsium signaling in guard cells

保护细胞叶绿体在气孔功能中的作用一直存在争议。通常认为，在气孔关闭之前，保护细胞内胞质游离Ca^<2+>浓度（Ca^<2+>）_<cyt>)会短暂升高。CAS （Ca^<2+>传感受体）是一种植物特异性的Ca^<2+>结合蛋白。我们发现CAS是一个完整的类囊体膜蛋白，并且n端酸性Ca^<2+>-结合区可能暴露在膜的基质侧。CAS对外部Ca^<2+>诱导的气孔关闭至关重要。这些结果证明叶绿体在拟南芥胞外Ca^<2+>([Ca^<2+>]_<ext>)诱导的[Ca^<2+>]_<cyt>瞬态的产生和气孔关闭中起关键作用。此外，我们发现细胞外非生物和病原体相关的胁迫信号迅速传递到叶绿体，并以胁迫依赖的方式引起基质Ca^<2+>浓度的快速和短暂增加。此外，我们还证明了CAS还与病原体激发子诱导的基质Ca2+瞬态的产生和/或从光到暗的转变有关。此外，我们还证明了CAS调节多种植物先天免疫反应，如flg22诱导的气孔关闭、病原体相关基因的表达、ROS的产生以及抗性(R)基因引发的拟南芥超敏细胞死亡。值得注意的是，cas1没有表现出明显的生长表型。这些结果可能暗示了叶绿体在调节植物先天免疫反应中的重要作用，而不是以前认为的那样。

The role of guard cell chloroplasts in stomatal function is controversial. It is usually assumed that stomatal closure is preceded by a transient increase in cytosolic free Ca^<2+> concentration (Ca^<2+>)_<cyt>) in the guard cells. CAS (Ca^<2+> sensing receptor) is a plant-specific putative Ca^<2+>-binding protein. We foundthat CAS is an integral thylakoid membrane protein, and the N-terminus acidic Ca^<2+>-binding region is likely exposed to the stromal side of the membrane. CAS is essential for stomatal closure induced by external Ca^<2+>. These results provide the evidence that chloroplasts play a critical role in the generation of extracellular Ca^<2+>([Ca^<2+>]_<ext>)-induced[Ca^<2+>]_<cyt> transients and stomatal closure in Arabidopsis.In addition, we founs that extracellular abiotic and pathogen-related stress signals are relayed quickly to chloroplasts, and evoke rapid and transient increases in stromal Ca^<2+> concentration in a stress-dependent manner. Furthermore we demonstrated that CAS is also related to the generation of stromal Ca2+ transients induced by pathogen elicitors and/or light to dark transition. Furthermore, we demonstrated that CAS regulates a multitude of plant innate immune responses, such as flg22-induced stomatal closure, expression of pathogen-related genes, ROS production and a resistance (R) gene-triggered hypersensitive cell death in Arabidopsis. It should be noted that cas-1 shows no significant growth phenotype. These results may have implications for a far more important role of chloroplasts in the regulation of plant innate immune response than was thought.