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A plastid is a membrane-bound organelle found in the cells of , , and some other organisms. Plastids are considered to be intracellular .

(2025). 9781402040603, Springer Netherlands.

Examples of plastids include (used for ); (used for synthesis and storage of pigments); (non-pigmented plastids, some of which can differentiate); and (non-photosynthetic plastids of derived from secondary endosymbiosis).

A permanent primary endosymbiosis event occurred about 1.5 billion years ago in the clade, , and with a , a symbiotic cyanobacteria related to the genus Gloeomargarita. Another primary endosymbiosis event occurred later, between 140 and 90 million years ago, in the photosynthetic plastids of the cyanobacteria genera and , or the "PS-clade". Secondary and tertiary endosymbiosis events have also occurred in a wide variety of organisms; and some organisms developed the capacity to sequester ingested plastidsa process known as .

A. F. W. SchimperSchimper, A.F.W. (1882) " Ueber die Gestalten der Stärkebildner und Farbkörper" Botanisches Centralblatt 12(5): 175–178. was the first to name, describe, and provide a clear definition of plastids, which possess a double-stranded DNA molecule that long has been thought of as circular in shape, like that of the circular chromosome of prokaryotic cellsbut now, perhaps not; (see "..a linear shape"). Plastids are sites for manufacturing and storing pigments and other important chemical compounds used by the cells of . Some contain biological pigments such as used in or which determine a cell's color. Plastids in organisms that have lost their photosynthetic properties are highly useful for manufacturing molecules like the . Picozoans Are Algae After All: Study | The Scientist Magazine®


In land plants

Chloroplasts, proplastids, and differentiation
In , the plastids that contain can perform , thereby creating internal chemical energy from external while capturing carbon from Earth's atmosphere and furnishing the atmosphere with life-giving oxygen. These are the chlorophyll-plastidsand they are named ; (see top graphic).

Other plastids can synthesize and , which may be used to produce energy or as raw material to synthesize other molecules. For example, plastid manufacture the components of the tissue system known as , including its , from itself is synthesized in the chloroplasts of the . Plastids function to store different components including , , and .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

All plastids are derived from proplastids, which are present in the regions of the plant. Proplastids and young chloroplasts typically divide by , 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, (see top graphic). They may develop into any of the following variants:

(2025). 9781402040603, Springer.
  • : typically green plastids that perform .
    • : precursors of chloroplasts.
  • : coloured plastids that synthesize and store pigments.
  • : plastids that control the dismantling of the photosynthetic apparatus during .
  • : colourless plastids that synthesize .
Leucoplasts differentiate into even more specialized plastids, such as:
  • the aleuroplasts;
    • : storing and detecting maintaining geotropism.
    • : storing .
    • : storing and modifying .
  • or : synthesizing and producing and .

Depending on their morphology and target function, plastids have the ability to differentiate or redifferentiate between these and other forms.


Plastomes and Chloroplast DNA/ RNA; plastid DNA and plastid nucleoids
Each plastid creates multiple copies of its own unique genome, or , (from 'plastid genome')which for a chlorophyll plastid (or chloroplast) is equivalent to a 'chloroplast genome', or a 'chloroplast DNA'.
(2025). 9780128134573 .
The number of genome copies produced per plastid is variable, ranging from 1000 or more in , encompassing only a few plastids, down to 100 or less in mature cells, encompassing numerous plastids.

A plastome typically contains a that encodes transfer ()s and ribosomal (). It also contains proteins involved in photosynthesis and plastid gene transcription and translation. But these proteins represent only a small fraction of the total protein set-up necessary to build and maintain any particular type of plastid. 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 and differention of plastids.

Many plastids, particularly those responsible for photosynthesis, possess numerous internal membrane layers. Plastid DNA exists as protein-DNA complexes associated as localized regions within the plastid's inner envelope membrane; and these complexes are called 'plastid '. Unlike the nucleus of a eukaryotic cell, a plastid nucleoid is not 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, the developing (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 chloroplastsand when plastids are differentiating from one type to anothernucleoids 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 normal long thin protuberances called sometimes formextending from the plastid body into the cell while interconnecting several plastids. Proteins and smaller molecules can move around and through the stromules. Comparatively, in the laboratory, most cultured cellswhich are large compared to normal plant cellsproduce very long and abundant stromules that extend to the cell periphery.

In 2014, evidence was found of the possible loss of plastid genome in lagascae, a non-photosynthetic flowering plant, and in , a genus of non-photosynthetic . Extensive searches for plastid genes in both yielded no results, but concluding that their plastomes are entirely missing is still disputed. Some scientists argue that plastid genome loss is unlikely since even these non-photosynthetic plastids contain genes necessary to complete various including heme biosynthesis.

Even with any loss of plastid genome in , the plastids still occur there as "shells" without DNA content, which is reminiscent of in various organisms.


In algae and protists
Plastid types in and include:

  • : found in (plants) and other organisms that derived their genomes from green algae.
  • Muroplasts: also known as cyanoplasts or cyanelles, the plastids of algae are similar to plant chloroplasts, excepting they have a that is similar to that of .
  • Rhodoplasts: the red plastids found in , which allows them to photosynthesize down to marine depths of 268 m. 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, is synthesized and stored outside the plastids in the cytosol.
  • Secondary and tertiary plastids: from endosymbiosis of and .
  • : in , the term is used for all unpigmented plastids. Their function differs from the leucoplasts of plants.
  • : the non-photosynthetic plastids of derived from secondary endosymbiosis.

The plastid of photosynthetic species is often referred to as the 'cyanelle' or chromatophore, and is used in photosynthesis. 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.

, and are plant-specific and do not occur in algae. Plastids in algae and may also differ from plant plastids in that they contain .


Inheritance
In reproducing, most plants inherit their plastids from only one parent. In general, inherit plastids from the female , where many inherit plastids from the male . Algae also inherit plastids from just one parent. Thus the plastid DNA of the other parent is completely lost.

In normal intraspecific crossingsresulting in normal hybrids of one speciesthe 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, including ( Medicago sativa), normally show biparental inheriting of plastids.


DNA damage and repair
The plastid of seedlings is subjected to increasing damage as the seedlings develop. The DNA damage is due to oxidative environments created by photo-oxidative reactions and / respiratory electron transfer. Some DNA molecules are but DNA with unrepaired damage is apparently degraded to non-functional fragments.

proteins are encoded by the cell's and then translocated to plastids where they maintain stability/ integrity by repairing the plastid's DNA. For example, in of the moss Physcomitrella patens, a protein employed in DNA mismatch repair (Msh1) interacts with proteins employed in recombinational repair ( and RecG) to maintain plastid genome stability.


Origin
Plastids are thought to be descended from . The primary endosymbiotic event of the Archaeplastida is hypothesized to have occurred around 1.5 billion years ago and enabled eukaryotes to carry out oxygenic photosynthesis. Three evolutionary lineages in the Archaeplastida have since emerged in which the plastids are named differently: chloroplasts in and/or plants, in , and in the glaucophytes. The plastids differ both in their pigmentation and in their ultrastructure. For example, chloroplasts in plants and green algae have lost all , the light harvesting complexes found in cyanobacteria, red algae and glaucophytes, but instead contain stroma and grana . 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 photosynthetic 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. The plastid belongs to the "PS-clade" (of the cyanobacteria genera and ), which is a different sister clade to the plastids belonging to the Archaeplastida.

In contrast to primary plastids derived from primary endosymbiosis of a prokaryoctyic cyanobacteria, complex plastids originated by secondary in which a eukaryotic organism engulfed another eukaryotic organism that contained a primary plastid. When a 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 the , , , and most (= rhodoplasts). Those that endosymbiosed a green alga include the and chlorarachniophytes (= chloroplasts). The , a phylum of obligate parasitic including the causative agents of ( spp.), ( Toxoplasma gondii), and many other human or animal diseases also harbor a complex plastid (although this organelle has been lost in some apicomplexans, such as Cryptosporidium parvum, which causes cryptosporidiosis). The '' is no longer capable of photosynthesis, but is an essential organelle, and a promising for antiparasitic drug development.

Some and , in particular of the genus Elysia, 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 as , from the Greek, kleptes (), thief.


Plastid development cycle
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.


See also

Notes

Further reading


External links

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