Plant Endosymbiotic Organellar Calcium Signaling under Biotic and Abiotic Stresses

Mitochondria and chloroplasts in eukaryotic cells are endosymbiotic organelles derived from an α-proteobacterium and a cyanobacterium, respectively. The former are responsible for oxidative respiration, while the latter are the sites of photosynthesis within photosynthetic eukaryotic cells. The catabolic and anabolic processes in these organelles are regulated in response to a fluctuating environment. Ca2+ plays important roles in signal transduction pathways mediating a wide variety of physiological responses in eukaryotic cells. An increase in cytosolic Ca2+ concentrations leads to the activation of Ca2+ sensor and/or Ca2+-binding proteins, followed by modulation of the activity of metabolic enzymes and transcription patterns. Similar Ca2+ signaling is expected to occur not only in the cytosol but also in endosymbiotic organelles such as mitochondria and chloroplasts. Increasing evidence suggests that mitochondria and chloroplasts are associated with the intracellular Ca2+ signaling network in plant cells. Organellar Ca2+ signaling may play important roles in the regulation of cellular processes in response to biotic and abiotic (environmental) stresses. This paper summarizes current information on organellar Ca2+ signaling in plant cells.