Calcium signal transduction network in plant innate immunity

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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