Calcium signal transduction network in plant innate immunity

线粒体和叶绿体不仅是真核细胞的能量来源，而且还在调节细胞功能方面发挥着额外的重要作用。例如，哺乳动物线粒体有助于启动细胞凋亡、疾病和衰老。植物叶绿体还在植物免疫中发挥重要作用，包括水杨酸(SA)的生物合成和活性氧(ROS)的产生，这些都是植物免疫反应中的关键信号分子。有趣的是，先前的研究表明，叶绿体含有参与植物免疫反应的蛋白质。此外，一些病原体效应物以叶绿体为靶标来抑制叶绿体衍生的防御信号。因此，预计叶绿体在植物先天免疫中发挥作用。然而，叶绿体如何感知病原体信号和控制免疫信号网络仍不清楚。这项研究通过叶绿体…阐明了以前未知的叶绿体和先天免疫之间的串扰。线粒体Ca~(2+)-gt；动态平衡通过转换Ca~(2+)诱导的细胞色素C(caspase辅因子)的释放在细胞凋亡中起关键作用。然而，对叶绿体中Ca~(2+)的动态平衡知之甚少。在这里，我们报告了病原体相关分子模式(PAMP)信号诱导基质钙离子浓度的快速增加。药理学研究表明，胞质Ca~(2+)&gt；信号被传递到叶绿体，并诱导基质Ca~(2+)&gt；瞬变。我们进一步证明了叶绿体定位的可能的钙结合蛋白CAS参与了基质钙信号的产生。这些研究表明了一种新的机制，即在免疫过程开始时将PAMP信号传递到叶绿体中。PAMP诱导的基质钙信号可能控制或激活天然免疫反应。因此，我们研究了CAS在植物先天免疫中的作用。值得注意的是，CAS缺陷植物(CAS-1)在广泛的先天免疫反应中受到损害，包括PAMP诱导的基础防御和R基因介导的过敏性细胞死亡。由于CAS缺陷突变系表现出正常的表型和光合作用能力，因此CAS-1植物的免疫功能受损不太可能是由有限的细胞资源引起的。此外，对植物激素动态的综合分析表明，CAS是PAMP诱导的SA积累所必需的，SA是植物免疫反应中的关键信号分子。这些结果表明，叶绿体蛋白CAS通过SA依赖的信号对植物的天然免疫起到积极的调节作用，CAS很可能在转录水平上调节SA的生物合成。因此，预计在免疫反应过程中，叶绿体释放逆行信号来控制防御基因的表达。PAMP可诱导多种防御相关基因在细胞核内表达。利用全基因组转录组分析表明，在SA生物合成开始之前，CAS对于防御基因的全球诱导是必不可少的，这表明CAS参与了叶绿体到细胞核的逆行信号转导，并介导了植物先天免疫的叶绿体控制。叶绿体衍生的~1O_2~-信号与胁迫防御相关基因的表达调控有关。在这里，我们发现在CAS缺乏植物中大多数下调的基因都富含^1O_2^-应答基因，这为CAS在^1O_2^信号控制核编码防御基因中的作用提供了证据。本研究综合阐述了先前未知的协调叶绿体功能和核编码防御基因表达的机制。在感染后，PAMP信号通过细胞质Ca^&lt；2+&gt；诱导的基质Ca^&lt；2+&gt；信号通路传递到叶绿体。这项研究进一步表明，^1O_2^-逆行信号是叶绿体控制植物天然免疫的潜在机制。叶绿体蛋白CAS在这两个过程中都起着关键作用。叶绿体依赖的免疫途径的识别可能为植物的病害控制提供新的策略。较少

Mitochondria and chloroplasts are not only energy source for eukaryotic cells, but also play additional important roles to modulate cellular functions. For example, mammal mitochondria contribute to the initiation of apoptosis, diseases and aging. Plant chloroplast also serves several important functions in plant immunity, including biosynthesis of salicylic acid (SA) and the generation of reactive oxygen species (ROS), key signaling molecules in plant immune response. Interestingly, previous studies have shown that chloroplasts contain proteins involved in plant immune responses. Furthermore, several pathogen effectors target chloroplasts to suppress chloroplast-derived defense signals. Therefore, it is anticipated that chloroplasts play a role in plant innate immunity. However, how chloroplasts sense pathogen signals and control immune signaling network remains unclear. This study illustrates a previously unknown crosstalk between chloroplasts and innate immunity through chloroplast … More Ca^<2+> signaling in plants.Mitochondrial Ca^<2+> homeostasis plays a key role in apoptosis by switching Ca^<2+>-induced release of cytochrome C (caspase cofactor). However, very little is known about Ca^<2+> homeostasis in chloroplasts. Here, we report that pathogen associated molecular pattern (PAMP) signals induce a rapid increase in the stromal Ca^<2+> concentration. Pharmacological studies demonstrate that the cytoplasmic Ca^<2+> signal is transferred to the chloroplasts and induces the stromal Ca^<2+> transient. We further demonstrate that chloroplast-localized putative Ca^<2+> binding protein, CAS is involved in the generation of stromal Ca^<2+> signals. These studies suggest a novel mechanism relaying PAMP signals into chloroplasts at the beginning of the immune process.The PAMP-induced stromal Ca^<2+> signaling may control or activate the innate immune response. Thus, we examined the role of CAS in plant innate immunity. Remarkably, CAS deficient plants (cas-1) are compromised in broad range of innate immune responses including PAMP-induced basal defense and R-gene mediated hypersensitive cell death. It is unlikely that the compromised immunity of cas-1 plants is caused by limited cellular resources, since CAS deficient mutant lines exhibit normal phenotype and photosynthetically competent. Furthermore, a comprehensive analysis of plant hormone dynamics demonstrated that CAS is specifically required for PAMP-induced accumulation of SA, a key signal molecule in plant immune responses. These results suggest that chloroplast protein CAS acts as a positive regulator of plant innate immunity through SA-dependent signaling.CAS regulates SA biosynthesis most likely at transcriptional level. It is therefore anticipated that chloroplasts release retrograde signals that control defense gene expression during immune responses. PAMP induces expression of numerous defense related genes in nucleus. Using a genome wide transcriptome analysis we show that CAS is indispensable for the global induction of defense gene expression before SA biosynthesis starts, suggesting that CAS is involved in the chloroplast-to-nucleus retrograde signaling and mediates chloroplast control of plant innate immunity. Chloroplast-derived ^1O_2^- signaling has been implicated with the control of stress defense-related gene expression. Here we show that most genes downregulated in CAS deficient plants were enriched for ^1O_2^- responsive genes, providing evidence for a role of CAS in ^1O_2^- signaling to control nuclear-encoded defense genes.Collectively this study illustrates previously unknown mechanisms that coordinate chloroplast functions with nuclear-encoded defense gene expression. Upon infection, PAMP signals are relayed to chloroplasts through the cytoplasmic Ca^<2+>-induced stromal Ca^<2+> signaling pathway. This study further implicates a role of ^1O_2^- retrograde signaling as underlying mechanisms of chloroplast control of plant innate immunity. Chloroplast protein CAS plays a critical role in both processes. Identification of the chloroplast-dependent immune pathway may provide a novel strategy for the disease control of plants. Less