Chloroplasts Orchestrate Multilayered Defences Against Plant Pathogens.

Chloroplasts are usually regarded as photosynthetic organelles that generate oxygen and carbohydrates. However, emerging studies have revealed their pivotal role in plant immunity. Chloroplasts play a crucial role in processing various external environmental stimuli and modulating plant immune responses. In this review, we elaborate on the contributions of chloroplasts to plant defence, emphasising their vital role in participating in the biosynthesis of cuticular wax, cell wall components and key hormones such as salicylic acid, jasmonic acid, abscisic acid and ethylene. We highlight that chloroplasts act as the sources of reactive oxygen species and calcium ion (Ca²⁺) bursts, which act as secondary messengers to amplify immune signalling and trigger plant immunity. We also discuss how chloroplasts are targeted by pathogens and explore the interactions between pathogens and chloroplasts. Emphasis was given to the complex interplay between chloroplast function and plant immunity and future research challenges were outlined.