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

保卫细胞叶绿体在气孔功能中的作用是有争议的。通常认为气孔关闭之前保卫细胞胞质游离Ca^&lt;2+&gt;浓度（Ca^&lt;2+&gt;）的瞬时增加<cyt>。CAS（Ca^2+ sensing receptor）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，N端酸性Ca^2+结合区可能暴露在膜的基质侧。CAS是外源Ca^&lt;2+&gt;诱导气孔关闭所必需的。这些结果表明，叶绿体在拟南芥细胞外Ca^2+（[Ca^2+]_<ext>）诱导的[Ca ^2 +]_<cyt>瞬变和气孔关闭中起着重要作用，同时我们还发现，细胞外非生物和病原相关的胁迫信号能迅速传递到叶绿体，并以胁迫依赖的方式引起基质Ca^2+浓度的迅速和短暂的升高。此外，我们证明，CAS也与病原体诱导剂和/或光暗转换诱导的基质Ca 2+瞬变的产生有关。此外，我们证明CAS调节多种植物先天免疫反应，如flg 22诱导的气孔关闭，病原体相关基因的表达，ROS产生和抗性（R）基因触发的拟南芥过敏性细胞死亡。应该注意的是，cas-1没有表现出显著的生长表型。这些结果可能意味着叶绿体在调节植物先天免疫反应中的作用比想象的要重要得多。

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.