Plastid

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. Plastids were discovered and named by Ernst Haeckel, but A. F. W. Schimper was the first to provide a clear definition. Plastids are the site of manufacture and storage of important chemical compounds used by the cells of autotrophic eukaryotes. They often contain pigments used in photosynthesis, and the types of pigments in a plastid determine the cell's color. They have a common evolutionary origin and possess a double-stranded DNA molecule that is circular, like that of prokaryotic cells.

Plagiomnium affine laminazellen.jpeg
Plant cells with visible chloroplasts.

In plants

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Leucoplasts in plant cells.

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.[2] 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.

Plastids types en

In plants, plastids may differentiate into several forms, depending upon which function they play in the cell. Undifferentiated plastids (proplastids) may develop into any of the following variants:[3]

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 75–250 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.

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 cells, long thin protuberances called stromules 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.[4] 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.[4][5]

In algae

In algae, the term leucoplast is used for all unpigmented plastids. Their function differs from the leucoplasts of plants. 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.

Glaucophyte algae contain muroplasts, which are similar to chloroplasts except that they have a peptidoglycan cell wall that is similar to that of prokaryotes. Red algae contain rhodoplasts, which are red chloroplasts that allow them to photosynthesize to a depth of up to 268 m.[3] The chloroplasts of plants differ from the rhodoplasts of red algae 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.[6]

Inheritance

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.[7]

DNA damage and repair

Plastid DNA of maize seedlings is subject to increased damage as the seedlings develop.[8] 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.[9] 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.[10]

Origin

Plastids are thought to have originated from endosymbiotic cyanobacteria. This symbiosis evolved around 1.5 billion years ago[11] and enabled eukaryotes to carry out oxygenic photosynthesis.[12] Three evolutionary lineages 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.

Complex plastids start by secondary endosymbiosis (where a eukaryotic organism engulfs another eukaryotic organism that contains a primary plastid resulting in its endosymbiotic fixation),[13] when a eukaryote engulfs a red or green alga and retains the algal plastid, which 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.

See also

References

  1. ^ Sato, N. (2006). "Origin and Evolution of Plastids: Genomic View on the Unification and Diversity of Plastids". In R.R. Wise; J.K. Hoober (eds.). The Structure and Function of Plastids. Advances in Photosynthesis and Respiration. 23. Springer Netherlands. pp. 75–102. doi:10.1007/978-1-4020-4061-0_4. ISBN 978-1-4020-4060-3.
  2. ^ 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
  3. ^ a b Wise, Robert R. (2006). "1. The Diversity of Plastid Form and Function" (PDF). Advances in Photosynthesis and Respiration. 23. Springer. pp. 3–26. doi:10.1007/978-1-4020-4061-0_1. ISBN 978-1-4020-4060-3.
  4. ^ a b "Plants Without Plastid Genomes". The Scientist. Retrieved 2015-09-26.
  5. ^ 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. PMID 16406301.
  6. ^ Viola R, Nyvall P, Pedersén M (July 2001). "The unique features of starch metabolism in red algae". Proceedings. Biological Sciences. 268 (1474): 1417–22. doi:10.1098/rspb.2001.1644. PMC 1088757. PMID 11429143.
  7. ^ Zhang Q (March 2010). "Why does biparental plastid inheritance revive in angiosperms?". Journal of Plant Research. 123 (2): 201–6. doi:10.1007/s10265-009-0291-z. PMID 20052516.
  8. ^ Kumar RA, Oldenburg DJ, Bendich AJ (December 2014). "Changes in DNA damage, molecular integrity, and copy number for plastid DNA and mitochondrial DNA during maize development". Journal of Experimental Botany. 65 (22): 6425–39. doi:10.1093/jxb/eru359. PMC 4246179. PMID 25261192.
  9. ^ Oldenburg DJ, Bendich AJ (2015). "DNA maintenance in plastids and mitochondria of plants". Frontiers in Plant Science. 6: 883. doi:10.3389/fpls.2015.00883. PMC 4624840. PMID 26579143.
  10. ^ Odahara M, Kishita Y, Sekine Y (August 2017). "MSH1 maintains organelle genome stability and genetically interacts with RECA and RECG in the moss Physcomitrella patens". The Plant Journal. 91 (3): 455–465. doi:10.1111/tpj.13573. PMID 28407383.
  11. ^ Ochoa de Alda JA, Esteban R, Diago ML, Houmard J (September 2014). "The plastid ancestor originated among one of the major cyanobacterial lineages". Nature Communications. 5: 4937. Bibcode:2014NatCo...5E4937O. doi:10.1038/ncomms5937. PMID 25222494.
  12. ^ Hedges SB, Blair JE, Venturi ML, Shoe JL (January 2004). "A molecular timescale of eukaryote evolution and the rise of complex multicellular life". BMC Evolutionary Biology. 4: 2. doi:10.1186/1471-2148-4-2. PMC 341452. PMID 15005799.
  13. ^ Chan CX, Bhattachary D (2010). "The Origin of Plastids". Nature Education. 3 (9): 84.

Further reading

External links

Alveolate

The alveolates (meaning "with cavities") are a group of protists, considered a major clade and superphylum within Eukarya, and are also called Alveolata.

Amyloplast

Amyloplasts are a type of plastid, double-enveloped organelles in plant cells that are involved in various biological pathways. Amyloplasts are specifically a type of leucoplast, a subcategory for colorless, non-pigment-containing plastids. Amyloplasts are found in roots and storage tissues and store and synthesize starch for the plant through the polymerization of glucose. Starch synthesis relies on the transportation of carbon from the cytosol, the mechanism by which is currently under debate.Starch synthesis and storage also takes place in chloroplasts, a type of pigmented plastid involved in photosynthesis. Amyloplasts and chloroplasts are closely related, and amyloplasts can turn into chloroplasts; this is for instance observed when potato tubers are exposed to light and turn green.

Apicoplast

An apicoplast is a derived non-photosynthetic plastid found in most Apicomplexa, including Toxoplasma gondii, Plasmodium falciparum and other Plasmodium spp. (parasites causing malaria), but not in others such as Cryptosporidium. It originated from an alga (there is debate as to whether this was a green or red alga) through secondary endosymbiosis. The apicoplast is surrounded by four membranes within the outermost part of the endomembrane system. The apicoplast hosts important metabolic pathways like fatty acid synthesis, isoprenoid precursor synthesis and parts of the heme biosynthetic pathway

Archaeplastida

The Archaeplastida (or kingdom Plantae sensu lato) are a major group of autotrophic eukaryotes, comprising the red algae (Rhodophyta), the green algae, and the land plants, together with a small group of freshwater unicellular algae called glaucophytes. Unlike red and green algae, glaucophytes have never been involved in secondary endosymbiosis events. All of these organisms have chloroplasts that are surrounded by two membranes, suggesting that they were acquired directly from endosymbiotic cyanobacteria. In all other groups besides the amoeboid Paulinella chromatophora, the chloroplasts are surrounded by three or four membranes, suggesting they were acquired secondarily from red or green algae.

The cells of the Archaeplastida typically lack centrioles and have mitochondria with flat cristae. They usually have a cell wall including cellulose, and food is stored in the form of starch. However, these characteristics are also shared with other eukaryotes. The main evidence that the Archaeplastida form a monophyletic group comes from genetic studies, which indicate their plastids probably had a single origin. This evidence is disputed. Based on the evidence to date, it is not possible to confirm or refute alternative evolutionary scenarios to a single primary endosymbiosis. Photosynthetic organisms with plastids of different origin (such as brown algae) do not belong to the Archaeplastida.

The archaeplastidans fall into two main evolutionary lines. The red algae are pigmented with chlorophyll a and phycobiliproteins, like most cyanobacteria, and accumulate starch outside the chloroplasts. The green algae and land plants – together known as Viridiplantae (Latin for "green plants") or Chloroplastida – are pigmented with chlorophylls a and b, but lack phycobiliproteins, and starch is accumulated inside the chloroplasts. The glaucophytes have typical cyanobacterial pigments, and are unusual in retaining a cell wall within their plastids (called cyanelles).Archaeplastida should not be confused with the older and obsolete name Archiplastideae, which refers to cyanobacteria and other groups of bacteria.

Chloroplast

Chloroplasts are organelles that conduct photosynthesis, where the photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in the energy-storage molecules ATP and NADPH while freeing oxygen from water in plant and algal cells. They then use the ATP and NADPH to make organic molecules from carbon dioxide in a process known as the Calvin cycle. Chloroplasts carry out a number of other functions, including fatty acid synthesis, much amino acid synthesis, and the immune response in plants. The number of chloroplasts per cell varies from one, in unicellular algae, up to 100 in plants like Arabidopsis and wheat.

A chloroplast is a type of organelle known as a plastid, characterized by its two membranes and a high concentration of chlorophyll. Other plastid types, such as the leucoplast and the chromoplast, contain little chlorophyll and do not carry out photosynthesis.

Chloroplasts are highly dynamic—they circulate and are moved around within plant cells, and occasionally pinch in two to reproduce. Their behavior is strongly influenced by environmental factors like light color and intensity. Chloroplasts, like mitochondria, contain their own DNA, which is thought to be inherited from their ancestor—a photosynthetic cyanobacterium that was engulfed by an early eukaryotic cell. Chloroplasts cannot be made by the plant cell and must be inherited by each daughter cell during cell division.

With one exception (the amoeboid Paulinella chromatophora), all chloroplasts can probably be traced back to a single endosymbiotic event, when a cyanobacterium was engulfed by the eukaryote. Despite this, chloroplasts can be found in an extremely wide set of organisms, some not even directly related to each other—a consequence of many secondary and even tertiary endosymbiotic events.

The word chloroplast is derived from the Greek words chloros (χλωρός), which means green, and plastes (πλάστης), which means "the one who forms".

Chloroplast DNA

Chloroplasts have their own DNA, often abbreviated as cpDNA. It is also known as the plastome when referring to genomes of other plastids. Its existence was first proven in 1962, and first sequenced in 1986—when two Japanese research teams sequenced the chloroplast DNA of liverwort and tobacco. Since then, hundreds of chloroplast DNAs from various species have been sequenced, but they are mostly those of land plants and green algae—glaucophytes, red algae, and other algae groups are extremely underrepresented, potentially introducing some bias in views of "typical" chloroplast DNA structure and content.

Chromera velia

Chromera velia, also known as a "chromerid", is a unicellular photosynthetic organism in the superphylum Alveolata. It is of interest in the study of apicomplexan parasites, specifically their evolution and accordingly, their unique vulnerabilities to drugs.The discovery of C. velia has sparked renewed interest in protist research, concerning both algae and parasites, as well as free-living unicells. Strict separation of botanical protists (algae) and zoological protists (protozoa) has been conventional but C. velia may be regarded as a good example of a bridge linking both categories.C. velia has typical features of alveolates, being phylogenetically related to Apicomplexa (a subgroup of alveolates), and contains a photosynthetic plastid (chloroplast) while the apicomplexans have a non-photosynthetic plastid called the apicoplast. C. velia is also related to another subgroup of alveolates the dinoflagellates of which most are photosynthetic.C. velia uses metabolites (reduced carbon) from its plastid as its primary energy source. The same is true of the algal cousin of C. velia, another chromerid Vitrella brassicaformis. Together these are phylogenetically the closest known autotrophic organisms to apicomplexans.Parasites in the apicomplexan genus Plasmodium are the causative agents of malaria. Studies of C. velia and V. brassicaformis are broadly useful for understanding the biochemistry, physiology and evolution of the malaria parasite, other apicomplexan parasites, and dinoflagellates.

Chromoplast

Chromoplasts are plastids, heterogeneous organelles responsible for pigment synthesis and storage in specific photosynthetic eukaryotes. It is thought that like all other plastids including chloroplasts and leucoplasts they are descended from symbiotic prokaryotes.

Elaioplast

Elaioplasts are one of the three possible forms of leucoplasts, sometimes broadly referred to as such. The main function of elaioplasts are synthesis and storage of fatty acids, terpenes, and other lipids, and they can be found in the embryonic leaves of oilseeds, citrus fruits, as well as the anthers of many flowering plants.

Gerontoplast

A gerontoplast is a plastid that develops from a chloroplast during the senescing of plant foliage. Gerontoplast development is generally seen to be the process of grana being unstacked, loss of thylakoid membranes, and large accumulation of plastoglobuli.

Green algae

The green algae (singular: green alga) are a large, informal grouping of algae consisting of the Chlorophyta and Charophyta/Streptophyta, which are now placed in separate divisions, as well as the potentially more basal Mesostigmatophyceae, Chlorokybophyceae and Spirotaenia.The land plants, or embryophytes, are thought to have emerged from the charophytes. Therefore, cladistically, embryophytes belong to green algae as well. However, because the embryophytes are traditionally classified as neither algae nor green algae, green algae are a paraphyletic group. Since the realization that the embryophytes emerged from within the green algae, some authors are starting to include them. The clade that includes both green algae and embryophytes is monophyletic and is referred to as the clade Viridiplantae and as the kingdom Plantae. The green algae include unicellular and colonial flagellates, most with two flagella per cell, as well as various colonial, coccoid and filamentous forms, and macroscopic, multicellular seaweeds. There are about 8,000 species of green algae. Many species live most of their lives as single cells, while other species form coenobia (colonies), long filaments, or highly differentiated macroscopic seaweeds.

A few other organisms rely on green algae to conduct photosynthesis for them. The chloroplasts in euglenids and chlorarachniophytes were acquired from ingested green algae, and in the latter retain a nucleomorph (vestigial nucleus). Green algae are also found symbiotically in the ciliate Paramecium, and in Hydra viridissima and in flatworms. Some species of green algae, particularly of genera Trebouxia of the class Trebouxiophyceae and Trentepohlia (class Ulvophyceae), can be found in symbiotic associations with fungi to form lichens. In general the fungal species that partner in lichens cannot live on their own, while the algal species is often found living in nature without the fungus. Trentepohlia is a filamentous green alga that can live independently on humid soil, rocks or tree bark or form the photosymbiont in lichens of the family Graphidaceae. Also the macroalga Prasiola calophylla (Trebouxiophyceae) is terrestrial, and

Prasiola crispa, which live in the supralittoral zone, is terrestrial and can in the Antarctic form large carpets on humid soil, especially near bird colonies.

Guillardia

Guillardia is a genus of flagellate cryptomonad algae belonging to the family Geminigeraceae, containing a secondary plastid within a reduced cytoplasmic compartment that contains a vestigial nucleomorph. There is only one characterised member of this genus, Guillardia theta.

Kleptoplasty

Kleptoplasty or kleptoplastidy is a symbiotic phenomenon whereby plastids, notably chloroplasts from algae, are sequestered by host organisms. The word is derived from Kleptes (κλέπτης) which is Greek for thief. The alga is eaten normally and partially digested, leaving the plastid intact. The plastids are maintained within the host, temporarily continuing photosynthesis and benefiting the predator. The term was coined in 1990 to describe chloroplast symbiosis.

Maturase K

Maturase K (matK) is a plant plastidial gene, although higher plants have it moved into the nuclear genome. The protein it encodes is an organelle intron maturase, a protein that splices Group II introns. It is essential for in vivo splicing of Group II introns. Amongst other maturases, this protein retains only a well conserved domain X and remnants of a reverse transcriptase domain.Universal matK primers can be used for DNA barcoding of angiosperms.

Mesodinium chamaeleon

Mesodinium chamaeleon is a ciliate of the genus Mesodinium. It is known for being able to consume and maintain algae endosymbiotically for days before digesting the algae. It has the ability to eat red and green algae, and afterwards using the chlorophyll granules from the algae to generate energy, turning itself from being a heterotroph into an autotroph. The species was discovered in January 2012 outside the coast of Nivå, Denmark by professor Øjvind Moestrup.

In contrast to certain other species of the genus, Mesodinium chamaeleon can be maintained in culture for short periods only. It captures and ingests flagellates including cryptomonads. The prey is ingested very rapidly into a food vacuole without the cryptomonad flagella being shed and the trichocysts being discharged. The individual food vacuoles subsequently serve as photosynthetic units, each containing the cryptomonad chloroplast, a nucleus, and some mitochondria. The ingested cells are eventually digested. This type of symbiosis differs from other plastid-bearing Mesodinium spp. in retaining ingested cryptomonad cells almost intact. The food strategy of the new species appears to be intermediate between heterotrophic species, such as Mesodinium pulex and Mesodinium pupula, and species with red cryptomonad endosymbionts, such as Mesodinium rubrum.

Mutant

In biology and especially genetics, a mutant is an organism or a new genetic character arising or resulting from an instance of mutation, which is generally an alteration of the DNA sequence of the genome or chromosome of an organism. The term mutant is also applied to a virus with an alteration in its nucleotide sequence whose genome is RNA, rather than DNA. In multicellular eukaryotes, a DNA sequence may be altered in an individual somatic cell that then gives rise to a mutant somatic cell lineage as happens in cancer progression. Also in eukaryotes, alteration of a mitochondrial or plastid DNA sequence may give rise to a mutant lineage that is inherited separately from mutant genotypes in the nuclear genome. The natural occurrence of genetic mutations is integral to the process of evolution. The study of mutants is an integral part of biology; by understanding the effect that a mutation in a gene has, it is possible to establish the normal function of that gene.

Nucleomorph

Nucleomorphs are small, vestigial eukaryotic nuclei found between the inner and outer pairs of membranes in certain plastids. They are thought to be vestiges of primitive red and green algal nuclei that were engulfed by a larger eukaryote. Because the nucleomorph lies between two sets of membranes, nucleomorphs support the endosymbiotic theory and are evidence that the plastids containing them are complex plastids. Having two sets of membranes indicate that the plastid, a prokaryote, was engulfed by a eukaryote, an alga, which was then engulfed by another eukaryote, the host cell, making the plastid an example of secondary endosymbiosis.

Symbiogenesis

Symbiogenesis, or endosymbiotic theory, is an evolutionary theory of the origin of eukaryotic cells from prokaryotic organisms, first articulated in 1905 and 1910 by the Russian botanist Konstantin Mereschkowski, and advanced and substantiated with microbiological evidence by Lynn Margulis in 1967. It holds that the organelles distinguishing eukaryote cells evolved through symbiosis of individual single-celled prokaryotes (bacteria and archaea).

The theory holds that mitochondria, plastids such as chloroplasts, and possibly other organelles of eukaryotic cells represent formerly free-living prokaryotes taken one inside the other in endosymbiosis. In more detail, mitochondria appear to be related to Rickettsiales proteobacteria, and chloroplasts to nitrogen-fixing filamentous cyanobacteria.

Among the many lines of evidence supporting symbiogenesis are that new mitochondria and plastids are formed only through binary fission, and that cells cannot create new ones otherwise; that the transport proteins called porins are found in the outer membranes of mitochondria, chloroplasts and bacterial cell membranes; that cardiolipin is found only in the inner mitochondrial membrane and bacterial cell membranes; and that some mitochondria and plastids contain single circular DNA molecules similar to the chromosomes of bacteria.

Target peptide

A target peptide is a short (3-70 amino acids long) peptide chain that directs the transport of a protein to a specific region in the cell, including the nucleus, mitochondria, endoplasmic reticulum (ER), chloroplast, apoplast, peroxisome and plasma membrane. Some target peptides are cleaved from the protein by signal peptidases after the proteins are transported.

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