Chloroplasts at work during plant innate immunity

The major role played by chloroplasts during light harvesting, energy production, redox homeostasis, and retrograde signalling processes has been extensively characterized. Beyond the obvious link between chloroplast functions in primary metabolism and as providers of photosynthesis-derived carbon sources and energy, a growing body of evidence supports a central role for chloroplasts as integrators of environmental signals and, more particularly, as key defence organelles. Here, we review the importance of these organelles as primary sites for the biosynthesis and transmission of pro-defence signals during plant immune responses. In addition, we highlight interorganellar communication as a crucial process for amplification of the immune response. Finally, molecular strategies used by microbes to manipulate, directly or indirectly, the production/function of defence-related signalling molecules and subvert chloroplast-based defences are also discussed.