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.

Contents Summary901I.Introduction901II.Biochemistry and structure of plant SBTs902III.Phylogeny of plant SBTs and family organization903IV.Physiological roles of plant SBTs905V.Conclusions and outlook911Acknowledgements912References912 Summary Subtilases (SBTs) are serine peptidases that are found in all three domains of life. As compared with homologs in other Eucarya, plant SBTs are more closely related to archaeal and bacterial SBTs, with which they share many biochemical and structural features. However, in the course of evolution, functional diversification led to the acquisition of novel, plant‐specific functions, resulting in the present‐day complexity of the plant SBT family. SBTs are much more numerous in plants than in any other organism, and include enzymes involved in general proteolysis as well as highly specific processing proteases. Most SBTs are targeted to the cell wall, where they contribute to the control of growth and development by regulating the properties of the cell wall and the activity of extracellular signaling molecules. Plant SBTs affect all stages of the life cycle as they contribute to embryogenesis, seed development and germination, cuticle formation and epidermal patterning, vascular development, programmed cell death, organ abscission, senescence, and plant responses to their biotic and abiotic environments. In this article we provide a comprehensive picture of SBT structure and function in plants.

Successful sexual reproduction often relies on the ability of plants to recognize self- or genetically-related pollen and prevent pollen tube growth soon after germination in order to avoid self-fertilization. Angiosperms have developed different reproductive barriers, one of the most extended being self-incompatibility (SI). With SI, pistils are able to reject self or genetically-related pollen thus promoting genetic variability. There are basically two distinct systems of SI: gametophytic (GSI) and sporophytic (SSI) based on their different molecular and genetic control mechanisms. In both types of SI, programmed cell death (PCD) has been found to play an important role in the rejection of self-incompatible pollen. Although reactive oxygen species (ROS) were initially recognized as toxic metabolic products, in recent years, a new role for ROS has become apparent: the control and regulation of biological processes such as growth, development, response to biotic and abiotic environmental stimuli, and PCD. Together with ROS, nitric oxide (NO) has become recognized as a key regulator of PCD. PCD is an important mechanism for the controlled elimination of targeted cells in both animals and plants. The major focus of this review is to discuss how ROS and NO control male-female cross-talk during fertilization in order to trigger PCD in self-incompatible pollen, providing a highly effective way to prevent self-fertilization.