The important role of chloroplasts in plant immunity

In nature, plants are under attack by a range of pathogens. To cope with these pathogens, plants have evolved a sophisticated immune system, including pattern-triggered immunity (PTI) initiated by pattern recognition receptors on the cell surface and effector-triggered immunity (ETI) activated by intracellular nucleotide-binding and leucine-rich repeat receptors. In recent years, increasing evidence has demonstrated that organelles such as the chloroplast play crucial roles in complete activation of plant immunity. In this review, we focus on the chloroplast and summarize its role in regulating the activation of immune events, including influx of calcium (Ca²⁺), accumulation of reactive oxygen species (ROS), biosynthesis of phytohormones, and expression of defense-related genes. Because information exchange between the chloroplast and the nucleus is very important during plant immunity, we also highlight the importance of chloroplast-nucleus communication via stromules in plant immunity. This review reveals the function of the chloroplast in maintaining the trade-off between plant growth and immunity, and expands our understanding of how chloroplasts enable complete activation of plant immunity.