The plastid (Greek: ; : formed, molded – plural plastids) is a membrane-bound organelle 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. 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). 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. 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. Even in organisms where the plastids have lost their photosynthetic properties, the plastid is kept because of its essential role in the production of molecules like the isoprenoids.
In land plants
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. 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: Chloroplasts: typically green plastids used for photosynthesis. Etioplasts are the precursors of chloroplasts Chromoplasts: coloured plastids for pigment synthesis and storage Gerontoplasts: control the dismantling of the photosynthetic apparatus during plant senescence Leucoplasts: colourless plastids for monoterpene synthesis; leucoplasts sometimes differentiate into more specialized plastids: Amyloplasts: for starch storage and detecting gravity (for geotropism) Elaioplasts: for storing fat Proteinoplasts: for storing and modifying protein Tannosomes: for synthesizing and producing tannins and polyphenols
Depending on their morphology and function, plastids have the ability to differentiate, or redifferentiate, between these and other forms.
Each plastid creates multiple copies of a circular 10–250 kilobase plastome. The number of genome copies per plastid is variable, ranging from more than 1000 in rapidly dividing cells, which, in general, contain few plastids, to 100 or fewer in mature cells, where plastid divisions have given rise to a large number of plastids. The plastome contains about 100 genes encoding ribosomal and transfer ribonucleic acids (rRNAs and tRNAs) as well as proteins involved in photosynthesis and plastid gene transcription and translation. However, these proteins only represent a small fraction of the total protein set-up necessary to build and maintain the structure and function of a particular type of plastid. Plant nuclear genes encode the vast majority of plastid proteins, and the expression of plastid genes and nuclear genes is tightly co-regulated to coordinate proper development of plastids in relation to cell differentiation.