The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle[1] found in the cells of plants, algae, and some other eukaryotic organisms. They are considered to be intracellular endosymbiotic cyanobacteria. Examples include chloroplasts (used for photosynthesis), chromoplasts (used for pigment synthesis and storage), and leucoplasts (non-pigmented plastids that can sometimes differentiate).
The event which led to permanent endosymbiosis in the Archaeplastida clade (of land plants, red algae, and green algae) probably occurred with a cyanobiont (a symbiotic cyanobacteria) related to the genus Gloeomargarita, around 1.5 billion years ago.[2][3] A later primary endosymbiosis event occurred in photosynthetic Paulinella amoeboids about 90–140 million years ago. This plastid belongs to the "PS-clade" (of the cyanobacteria genera Prochlorococcus and Synechococcus).[4][5] Secondary and tertiary endosymbiosis has also occurred, in a wide variety of organisms; additionally, some organisms sequester ingested plastids in a process that is known as kleptoplasty.
A. F. W. Schimper was the first to name and provide a clear definition of plastids.[6][a] They often contain pigments used in photosynthesis, and the types of pigments in a plastid determine the cell's color. They are also the site of manufacture and storage of important chemical compounds used by the cells of autotrophic eukaryotes. They possess a double-stranded DNA molecule that is circular, like that of the circular chromosome of prokaryotic cells.
In land plants, plastids that contain chlorophyll can carry out photosynthesis and are called chloroplasts. Plastids can also store products like starch and can synthesize fatty acids and terpenes, which can be used for producing energy and as raw material for the synthesis of other molecules. For example, the components of the plant cuticle and its epicuticular wax are synthesized by the epidermal cells from palmitic acid, which is synthesized in the chloroplasts of the mesophyll tissue.[8] All plastids are derived from proplastids, which are present in the meristematic regions of the plant. Proplastids and young chloroplasts commonly divide by binary fission, but more mature chloroplasts also have this capacity.
Plant proplastids (undifferentiated plastids) may differentiate into several forms, depending upon which function they perform in the cell. They may develop into any of the following variants:[9]
Depending on their morphology and function, plastids have the ability to differentiate, or redifferentiate, between these and other forms.