A. F. W. Schimper[6][a] was the first to name, describe, and provide a clear definition of plastids, which possess adouble-stranded DNA molecule that long has been thought of as circular in shape, like that of thecircular chromosome ofprokaryotic cells—but now, perhaps not; (see"..a linear shape"). Plastids are sites for manufacturing and storing pigments and other important chemical compounds used by the cells ofautotrophiceukaryotes. Some containbiological pigments such as used inphotosynthesis or which determine a cell's color. Plastids in organisms that have lost their photosynthetic properties are highly useful for manufacturing molecules like theisoprenoids.[8]
Inland plants, the plastids that containchlorophyll can performphotosynthesis, thereby creating internal chemical energy from externalsunlight energy while capturing carbon from Earth's atmosphere and furnishing the atmosphere with life-giving oxygen. These are thechlorophyll-plastids—and they are namedchloroplasts; (see top graphic).
All plastids are derived from proplastids, which are present in themeristematic regions of the plant. Proplastids and young chloroplasts typically divide bybinary fission, but more mature chloroplasts also have this capacity.
Plantproplastids (undifferentiated plastids) maydifferentiate into several forms, depending upon which function they perform in the cell, (see top graphic). They may develop into any of the following variants:[10]
Each plastid creates multiple copies of its own unique genome, orplastome, (from 'plastid genome')—which for a chlorophyll plastid (or chloroplast) is equivalent to a 'chloroplast genome', or a 'chloroplast DNA'.[11][12] The number of genome copies produced per plastid is variable, ranging from 1000 or more inrapidly dividing new cells, encompassing only a few plastids, down to 100 or less in mature cells, encompassing numerous plastids.
A plastome typically contains agenome that encodestransferribonucleic acids (tRNA)s andribosomalribonucleic acids (rRNAs). It also contains proteins involved in photosynthesis and plastid genetranscription andtranslation. But these proteins represent only a small fraction of the total protein set-up necessary to build and maintain any particular type of plastid.Nuclear genes (in the cell nucleus of a plant) encode the vast majority of plastid proteins; and the expression of nuclear and plastid genes is co-regulated to coordinate the development anddifferention of plastids.
Many plastids, particularly those responsible for photosynthesis, possess numerous internal membrane layers. Plastid DNA exists as protein-DNA complexes associated as localizedregions within the plastid's inner envelopemembrane; and these complexes are called 'plastidnucleoids'. Unlike the nucleus of a eukaryotic cell, a plastid nucleoid isnot surrounded by a nuclear membrane. The region of each nucleoid may contain more than 10 copies of the plastid DNA.
Where the proplastid (undifferentiated plastid) contains a single nucleoid region located near the centre of the proplastid, thedeveloping (or differentiating) plastid has many nucleoids localized at the periphery of the plastid and bound to the inner envelope membrane. During the development/ differentiation of proplastids to chloroplasts—and when plastids are differentiating from one type to another—nucleoids change in morphology, size, and location within the organelle. The remodelling of plastid nucleoids is believed to occur by modifications to the abundance of and the composition of nucleoid proteins.
In normalplant cells long thin protuberances calledstromules sometimes form—extending from the plastid body into the cellcytosol while interconnecting several plastids. Proteins and smaller molecules can move around and through the stromules. Comparatively, in the laboratory, most cultured cells—which are large compared to normal plant cells—produce very long and abundant stromules that extend to the cell periphery.
In 2014, evidence was found of the possible loss of plastid genome inRafflesia lagascae, a non-photosyntheticparasitic flowering plant, and inPolytomella, a genus of non-photosyntheticgreen algae. Extensive searches for plastid genes in bothtaxons yielded no results, but concluding that their plastomes are entirely missing is still disputed.[13] Some scientists argue that plastid genome loss is unlikely since even these non-photosynthetic plastids contain genes necessary to complete variousbiosynthetic pathways including heme biosynthesis.[13][14]
Even with any loss of plastid genome inRafflesiaceae, the plastids still occur there as "shells" without DNA content,[15] which is reminiscent ofhydrogenosomes in various organisms.
Rhodoplasts: the red plastids found inred algae, which allows them to photosynthesize down to marine depths of 268 m.[10] The chloroplasts of plants differ from rhodoplasts in their ability to synthesize starch, which is stored in the form of granules within the plastids. In red algae,floridean starch is synthesized and stored outside the plastids in the cytosol.[16]
Leucoplast: inalgae, the term is used for all unpigmented plastids. Their function differs from the leucoplasts of plants.
Apicoplast: the non-photosynthetic plastids ofApicomplexa derived from secondary endosymbiosis.
The plastid of photosyntheticPaulinella species is often referred to as the 'cyanelle' or chromatophore, and is used in photosynthesis.[17][18] It had a much more recent endosymbiotic event, in the range of 140–90 million years ago, which is the only other known primary endosymbiosis event of cyanobacteria.[19][20]
In reproducing, most plants inherit their plastids from only one parent. In general,angiosperms inherit plastids from the femalegamete, where manygymnosperms inherit plastids from the malepollen. Algae also inherit plastids from just one parent. Thus the plastid DNA of the other parent is completely lost.
In normal intraspecific crossings—resulting in normal hybrids of one species—the inheriting of plastid DNA appears to be strictly uniparental; i.e., from the female. In interspecific hybridisations, however, the inheriting is apparently more erratic. Although plastids are inherited mainly from the female in interspecific hybridisations, there are many reports of hybrids of flowering plants producing plastids from the male.Approximately 20% of angiosperms, includingalfalfa (Medicago sativa), normally show biparental inheriting of plastids.[22]
DNA repair proteins are encoded by the cell'snuclear genome and then translocated to plastids where they maintaingenome stability/ integrity by repairing the plastid's DNA.[24] For example, inchloroplasts of the mossPhyscomitrella patens, a protein employed in DNA mismatch repair (Msh1) interacts with proteins employed in recombinational repair (RecA and RecG) to maintain plastid genome stability.[25]
Plastids are thought to be descended fromendosymbioticcyanobacteria. The primary endosymbiotic event of the Archaeplastida is hypothesized to have occurred around 1.5 billion years ago[26] and enabled eukaryotes to carry outoxygenic photosynthesis.[27] Three evolutionary lineages in the Archaeplastida have since emerged in which the plastids are named differently: chloroplasts ingreen algae and/or plants,rhodoplasts inred algae, andmuroplasts in the glaucophytes. The plastids differ both in their pigmentation and in their ultrastructure. For example, chloroplasts in plants and green algae have lost allphycobilisomes, thelight harvesting complexes found in cyanobacteria, red algae and glaucophytes, but instead containstroma and granathylakoids. The glaucocystophycean plastid—in contrast to chloroplasts and rhodoplasts—is still surrounded by the remains of the cyanobacterial cell wall. All these primary plastids are surrounded by two membranes.
The plastid of photosyntheticPaulinella species is often referred to as the 'cyanelle' or chromatophore, and had a much more recent endosymbiotic event about 90–140 million years ago; it is the only known primary endosymbiosis event of cyanobacteria outside of the Archaeplastida.[17][18] The plastid belongs to the "PS-clade" (of the cyanobacteria generaProchlorococcus andSynechococcus), which is a different sister clade to the plastids belonging to the Archaeplastida.[4][5]
In contrast to primary plastids derived from primary endosymbiosis of a prokaryoctyic cyanobacteria, complex plastids originated by secondaryendosymbiosis in which a eukaryotic organism engulfed another eukaryotic organism that contained a primary plastid.[28] When aeukaryote engulfs a red or a green alga and retains the algal plastid, that plastid is typically surrounded by more than two membranes. In some cases these plastids may be reduced in their metabolic and/or photosynthetic capacity. Algae with complex plastids derived by secondary endosymbiosis of a red alga include theheterokonts,haptophytes,cryptomonads, and mostdinoflagellates (= rhodoplasts). Those that endosymbiosed a green alga include theeuglenids andchlorarachniophytes (= chloroplasts). TheApicomplexa, a phylum ofobligate parasiticalveolates including the causative agents ofmalaria (Plasmodium spp.),toxoplasmosis (Toxoplasma gondii), and many other human or animal diseases also harbor a complex plastid (although this organelle has been lost in some apicomplexans, such asCryptosporidium parvum, which causescryptosporidiosis). The 'apicoplast' is no longer capable of photosynthesis, but is an essential organelle, and a promisingtarget forantiparasitic drug development.
Somedinoflagellates andsea slugs, in particular of the genusElysia, take up algae as food and keep the plastid of the digested alga to profit from the photosynthesis; after a while, the plastids are also digested. This process is known askleptoplasty, from the Greek,kleptes (κλέπτης), thief.
An illustration of the stages of inter-conversion in plastids
In 1977 J.M Whatley proposed a plastid development cycle which said that plastid development is not always unidirectional but is instead a complicated cyclic process. Proplastids are the precursor of the more differentiated forms of plastids, as shown in the diagram to the right.[29]
^SometimesErnst Haeckel is credited to coin the term plastid, but his "plastid" includes nucleated cells and anucleated "cytodes"[7] and thus totally different concept from the plastid in modern literature.
^Sato N (2007). "Origin and Evolution of Plastids: Genomic View on the Unification and Diversity of Plastids". In Wise RR, Hoober JK (eds.).The Structure and Function of Plastids. Advances in Photosynthesis and Respiration. Vol. 23. Springer Netherlands. pp. 75–102.doi:10.1007/978-1-4020-4061-0_4.ISBN978-1-4020-4060-3.
^Kolattukudy, P.E. (1996) "Biosynthetic pathways of cutin and waxes, and their sensitivity to environmental stresses", pp. 83–108 in:Plant Cuticles. G. Kerstiens (ed.), BIOS Scientific publishers Ltd., Oxford
^abWise, Robert R. (2006). "The Diversity of Plastid Form and Function".The Structure and Function of Plastids. Advances in Photosynthesis and Respiration. Vol. 23. Springer. pp. 3–26.doi:10.1007/978-1-4020-4061-0_1.ISBN978-1-4020-4060-3.
^Barbrook AC, Howe CJ, Purton S (February 2006). "Why are plastid genomes retained in non-photosynthetic organisms?".Trends in Plant Science.11 (2):101–8.doi:10.1016/j.tplants.2005.12.004.PMID16406301.
^Robison, T. A., Oh, Z. G., Lafferty, D., Xu, X., Villarreal, J. C. A., Gunn, L. H., Li, F.-W. (3 January 2025). "Hornworts reveal a spatial model for pyrenoid-based CO2-concentrating mechanisms in land plants".Nature Plants.11. Nature Publishing Group:63–73.doi:10.1038/s41477-024-01871-0.ISSN2055-0278.
