Abstract

Chloroplasts have a critical role in plant immunity as a site for the production for salicylic acid and jasmonic acid, important mediators of plant immunity. However, the molecular link between chloroplasts and the cytoplasmic-nuclear immune system remains largely unknown. Here we show that pathogen-associated molecular pattern (PAMP) signals are quickly relayed to chloroplasts and evoke specific Ca2+ signatures in the stroma. We further demonstrate that a chloroplast-localized protein, named calcium-sensing receptor (CAS), is involved in stromal Ca2+ transients and responsible for both PAMP-induced basal resistance and R gene-mediated hypersensitive cell death. CAS acts upstream of salicylic acid accumulation. Transcriptome analysis demonstrates that CAS is involved in PAMP-induced expression of defence genes and suppression of chloroplast gene expression possibly through 1O2-mediated retrograde signalling, allowing chloroplast-mediated transcriptional reprogramming during plant immune responses. The present study reveals a previously unknown chloroplast-mediated signalling pathway linking chloroplasts to cytoplasmic-nuclear immune responses. Mediators of plant immunity, such as salicylic acid, are produced in the chloroplasts, but the molecular link between chloroplasts and the immune system is unclear. This work finds that pathogen-associated molecular pattern signals are relayed to chloroplasts and evoke Ca2+signatures in the stroma.