The plastid (Greek: πλαστός; plastós: formed, molded – plural plastids) is a membrane-bound organelle
found in the cell
s of plants
, and some other eukaryotic
organisms. They are considered to be intracelluar endosymbiotic Cyanobacteria
. Examples include chloroplast
s (used for photosynthesis
s (used for pigment synthesis and storage), and leucoplast
s (non-pigmented plastids that can sometimes differentiate).
The event 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
s about 90–140 million years ago, with the plastid being a sister 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
Plastids were discovered and named by Ernst Haeckel
, but A. F. W. Schimper
was the first to provide a clear definition. 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 autotroph
s. They possess a double-stranded DNA
molecule that is circular, like that of the circular chromosome
of prokaryotic cells
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
, 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
[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 meristem
atic 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:
s: typically green plastids used for photosynthesis
s are the precursors of chloroplasts
s: coloured plastids for pigment synthesis and storage
s: control the dismantling of the photosynthetic apparatus during plant senescence
s: colourless plastids for monoterpene
synthesis; leucoplasts sometimes differentiate into more specialized plastids:
s: for starch
storage and detecting gravity
s: for storing fat
s: for storing and modifying protein
s: for synthesizing and producing tannin
s 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 gene
s encoding ribosomal and transfer ribonucleic acid
s and tRNA
s) as well as proteins involved in photosynthesis and plastid gene transcription
. 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
Plastid DNA exists as large protein-DNA complexes associated with the inner envelope membrane
and called 'plastid nucleoids'. Each nucleoid particle may contain more than 10 copies of the plastid DNA. The proplastid contains a single nucleoid located in the centre of the plastid. The developing plastid has many nucleoids, localized at the periphery of the plastid, bound to the inner envelope membrane. During the development of proplastids to chloroplasts, and when plastids convert from one type to another, nucleoids change in morphology, size and location within the organelle. The remodelling of nucleoids is believed to occur by modifications to the composition and abundance of nucleoid proteins.
Many plastids, particularly those responsible for photosynthesis, possess numerous internal membrane layers.
In plant cell
s, long thin protuberances called stromule
s sometimes form and extend from the main plastid body into the cytosol
and interconnect several plastids. Proteins, and presumably smaller molecules, can move within stromules. Most cultured cells that are relatively large compared to other plant cells have very long and abundant stromules that extend to the cell periphery.
In 2014, evidence of possible plastid genome loss was found in ''Rafflesia
lagascae'', a non-photosynthetic parasitic
flowering plant, and in ''Polytomella
'', a genus of non-photosynthetic green algae
. Extensive searches for plastid genes in both ''Rafflesia'' and ''Polytomella'' yielded no results, however the conclusion that their plastomes are entirely missing is still controversial.
Some scientists argue that plastid genome loss is unlikely since even non-photosynthetic plastids contain genes necessary to complete various biosynthetic pathways
, such as heme biosynthesis.
In algae and protists
Plastid types in algae
* Chloroplasts: found in the green algae
and other organisms.
** Muroplasts: also known as cyanoplasts or cyanelles, the chloroplasts of glaucophyte
algae are similar to plant chloroplasts, except that they have a peptidoglycan
cell wall that is similar to that of prokaryote
** Rhodoplasts: the red chloroplasts found in red algae
, that allow them to photosynthesize to a depth of up to 268 m.
[ The chloroplasts of plants differ from the 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.]
** Secondary and tertiary chloroplasts: from endosymbiosis of green algae and red algae.
* Leucoplast: in algae, the term is used for all unpigmented plastids. Their function differs from the leucoplasts of plants.
* Apicoplast: the non-photosynthetic plastids of Apicomplexa derived from secondary endosymbiosis.
The plastid of photosynthetic ''Paulinella'' species is often referred to as the 'cyanelle' or chromatophore, and is used in photosynthesis; it had a much more recent endosymbiotic event about 90–140 million years ago, and is the only other known primary endosymbiosis event of cyanobacteria.
Etioplasts, amyloplasts and chromoplasts are plant-specific and do not occur in algae. Plastids in algae and hornworts may also differ from plant plastids in that they contain pyrenoids.
Most plants inherit the plastids from only one parent. In general, angiosperms inherit plastids from the female gamete, whereas many gymnosperms inherit plastids from the male pollen. Algae also inherit plastids from only one parent. The plastid DNA of the other parent is, thus, completely lost.
In normal intraspecific crossings (resulting in normal hybrids of one species), the inheritance of plastid DNA appears to be quite strictly 100% uniparental. In interspecific hybridisations, however, the inheritance of plastids appears to be more erratic. Although plastids inherit mainly maternally in interspecific hybridisations, there are many reports of hybrids of flowering plants that contain plastids of the father.
Approximately 20% of angiosperms, including alfalfa (''Medicago sativa''), normally show biparental inheritance of plastids.
DNA damage and repair
Plastid DNA of maize seedlings is subject to increased damage as the seedlings develop.
The DNA is damaged in oxidative environments created by photo-oxidative reactions and photosynthetic/respiratory electron transfer. Some DNA molecules are repaired while DNA with unrepaired damage appears to be degraded to non-functional fragments.
DNA repair proteins are encoded by the cell's nuclear genome but can be translocated to plastids where they maintain genome stability/integrity by repairing the plastid's DNA. As an example, in chloroplasts of the moss ''Physcomitrella patens'', a protein employed in DNA mismatch repair (Msh1) interacts with proteins employed in recombinational repair (RecA and RecG) to maintain plastid genome stability.
Plastids are thought to be endosymbiotic cyanobacteria. 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 green algae and plants, rhodoplasts in red algae, and muroplasts 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 phycobilisomes, the light harvesting complexes found in cyanobacteria, red algae and glaucophytes, but instead contain stroma and grana thylakoids. 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 ''Paulinella'' 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 was a sister to the "PS-clade" (of the cyanobacteria genera ''Prochlorococcus'' and ''Synechococcus''), 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 endosymbiosis in which a eukaryotic organism engulfed another eukaryotic organism that contained a primary plastid. When a eukaryote 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 heterokonts, haptophytes, cryptomonads, and most dinoflagellates (= rhodoplasts). Those that endosymbiosed a green alga include the euglenids and chlorarachniophytes (= chloroplasts). The Apicomplexa, a phylum of obligate parasitic protozoa including the causative agents of malaria (''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 as ''Cryptosporidium parvum'', which causes cryptosporidiosis). The 'apicoplast' is no longer capable of photosynthesis, but is an essential organelle, and a promising target for antiparasitic drug development.
Some dinoflagellates and sea slugs, 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 kleptoplasty, 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 a cyclic process several times. The proplatids are precursor of the more differentiated forms of plastids as shown in the diagram depicted.
Transplastomic plants for biocontainment (biological confinement of transgenes)
— Co-extra research project on coexistence and traceability of GM and non-GM supply chains
Tree of Life Eukaryotes