Role of chloroplast protein CAS in HR-cell death in plant cells

Abstract

Hypersensitive response (HR) is critical to restrict local spread of pathogens in plant cells. Chloroplasts are photosynthetic organelles that are also involved in the synthesis of stress-induced hormones (JA, SA, ABA) and the generation of reactive oxygen species. In addition, recent works revealed that chloroplasts could quickly recognize the extracellular pathogens via Ca²⁺ signaling. However, the role of chloroplasts in the plant immune response remains elusive. CAS is a plant-specific Ca²⁺-binding protein that is localized in chloroplast thylakoid membranes. Previous studies demonstrated that CAS is essential for generation of cytosolic Ca²⁺ signal and subsequent stomatal closure induced by external Ca²⁺. We examined the role of CAS in plant defense responses in Arabidopsis and Nicotiana benthamiana. The bacterial growth (Pseudomonas syringae pv. tomato DC3000) was significantly suppressed in the CAS-1 knock-out mutants in Arabidopsis. In addition, the R protein-mediated HR-cell death was delayed in the CAS deficient plants of Arabidopsis and N. benthamiana. These findings suggest that chloroplast localized CAS plays a crucial regulatory role in the plant defense responses.