Calcium and plant organelles

Abstract. The role of intracellular organelles in the regulation of cytosolic Ca²⁺ levels and whether changes in these levels affect organelle metabolism is considered. We have assessed the biochemical properties of the Ca²⁺ transporting systems in mitochondrial, chloroplast and microsomal fractions. It is proposed that although all of these organelles can transport Ca²⁺ to varying extents it would appear that in some tissues at least mitochondria do not play a significant role in the maintenance of cytosolic Ca²⁺. The most important Ca²⁺ transporting systems are probably the ATP dependent Ca²⁺ extrusion across the plasma membrane and Ca²⁺ uptake by endoplasmic reticulum, as well as light driven Ca²⁺ uptake by chloroplasts. Changes in cytoplasmic Ca²⁺] do appear to regulate the activity of NAD kinase in chloroplasts, the mitochondrial external NADH dehydrogenase and intra-mitochondrial glutamate dehydrogenase, all of which play a key role in plant cell metabolism. Since some of these enzymes are affected by primary stimuli such as light or hormones, it is concluded that Ca²⁺ may act as a second messenger mediating some of the primary responses.