Plant Chloroplasts and Other Plastids

Abstract Plastids are a group of organelles that are characteristic of plant cells. They have derived in an endosymbiotic event from a cyanobacterial ancestor and still exhibit many prokaryotic features. Plastids are able to perform many specialised functions that are essential for plant growth and development, such as photosynthesis, nitrate and sulfate assimilation, the synthesis of amino acids and of fatty acids, storage of carbohydrates and lipids or the formation of colours in some fruits and flowers. To accomplish this, their membrane systems exert specialised transport functions, including the import and sorting of proteins and the exchange of metabolites in case of the two envelope membranes, as well as proton and electron transport in the case of the thylakoid membranes of chloroplasts. Moreover, plastids communicate with the nucleus by retrograde signalling to adjust the expression of nuclear genes according to the metabolic and developmental state of the organelle. Key Concepts: Plastids are of prokaryotic origin. Several forms of plastids exist with different functions and composition. Chloroplasts, the photosynthetically active plastids, are bounded like the other flowering plant plastids by a double membrane, but contain in addition the chlorophyll‐containing thylakoid membrane system, the site of photosynthesis. Plastids are essential for the assimilation of nitrate and sulfate, as well as for the synthesis of aromatic compounds. In contrast to animals, the major site of fatty acid biosynthesis in plants is within the plastid. Plastids import most of their several thousand different proteins because the corresponding genes are located in the nucleus. Various metabolites are imported in or exported from plastids through specific translocators within the envelope. Plastids communicate with the nucleus by as yet unknown messenger molecules.