The important role of chloroplasts in plant immunity.

In nature, plants are under attack by a range of pathogens. To cope with these pathogens, a sophisticated immune system has evolved in plants, including plant cell surface localized pattern recognition receptor (PRR)-initiated pattern-triggered immunity (PTI) and intracellular nucleotide-binding and leucine-rich repeat receptor (NLR)-activated effector-triggered immunity (ETI). In recent years, increasing evidence has demonstrated that organelles, such as the chloroplast, play crucial roles in the complete activation of plant immunity. Therefore, in this review, we focus on the chloroplast and summarize its role in regulating the activation of immune events, including calcium (Ca) influx, reactive oxygen species (ROS) accumulation, phytohormone biosynthesis, and the expression of defense-related genes. Moreover, information exchange between the chloroplast and the nucleus is highly 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 the complete activation of plant immunity involving chloroplasts.