Plastid
From Freepedia
Plastids are major organelles found only in plants and algae. Plastids are responsible for photosynthesis, storage of products like starch and for the synthesis of many classes of molecules such as fatty acids and terpenes which are needed as cellular building blocks and/or for the function of the plant. Depending on their morphology and function, plastids are commonly classified as chloroplasts, leucoplasts, amyloplasts or chromoplasts. However, these different forms are not fixed, and plastids have the ability to differentiate, or redifferentiate, between these forms. All plastids are derived from proplastids, which are present in the meristematic regions of the plant. Proplastids and young chloroplasts commonly divide, but more mature chloroplasts also have this capacity.
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Plastids in plants
In plants, plastids may differentiate into several forms, depending upon which function they need to play in the cell. Undifferentiated plastids (proplastids) may develop into any of the following plastids:
- Amyloplasts: for starch storage
- Chloroplasts: for photosynthesis
- Etioplasts: chloroplasts that have not been exposed to light
- Elaioplasts: for storing fat
- Chromoplasts: for pigment synthesis and storage
- Leucoplasts: for monoterpene synthesis
Each plastid contains multiple copies of the circular 75-250 kilo bases plastid genome. The number of genome copies per plastid is flexible, ranging from more than 100 in rapidly dividing cells, which generally contain few plastids, to 20 or fewer in mature cells, where plastid divisions has given rise to a large number of plastids. The plastid genome 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. Nuclear genes encode the vast majority of plastid proteins, and the expression of plastid genes and nuclear genes is tightly co-regulated to allow 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.
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.
Plastids in algae
In algae, the term leucoplast (leukoplast) is used for all unpigmented plastids. Their function differ from the leukoplasts in plants. Etioplast, amyloplast and chromoplast are plant-specific and do not occur in algae. Algal plastids may also differ from plant plastids in that they contain pyrenoids.
Origin of plastids
Plastids are thought to have originated from endosymbiotic cyanobacteria. Due to a split-up into three evolutionary lineages, the plastids are named differently: chloroplasts in green algae and plants, rhodoplasts in red algae and cyanelles in the glaucophytes. The plastids differ by their pigmentation, but also in ultrastructure. The chloroplasts e.g. have lost all phycobilisomes, the light harvesting complexes found in cyanobacteria, red algae and glaucophytes, but - only in plants and in closely related green algae - contain stroma and grana thylakoids. The glaucocystophycean plastid - in contrast to the chloroplasts and the rhodoplasts - is still surrounded by a remains of the cyanobacterial cell wall. All these primary plastids are surrounded by two membranes.
Complex plastids originate from a secondary endosymbiosis (i.e. a eukaryote engulfed a red or green alga and reduced it to a plastid) and are surrounded by more than two membranes. Algae with complex plastids derived from a secondary endosymbiosis event with a red alga are the heterokonts, haptophytes, cryptomonads, and most dinoflagellates (= rhodoplasts). Those with endosymbioses with green algae are the euglenids and the chlorarachniophytes (= chloroplasts). The Apicomplexa (a phylum of obligate parasitic protozoans, which includes Plasmodium spp., causing malaria; Toxoplasma gondii, causing toxoplasmosis; Cryptosporidium parvum, causing cryptosporidiosis; Neospora spp.; and Eimeria spp.) also have complex plastids that are no longer capable of photosynthesis. These plastids (also known as apicoplasts) are extremely reduced and it is not yet clear whether they derived from red or green algae.
Some dinoflagellates 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. These captured plastids are known as kleptoplastids.
Sources
- A Novel View of Chloroplast Structure: contains fluorescence images of chloroplasts and stromules as well as an easy to read chapter.
- Continuous expression in tobacco leaves of a Brassica napus PEND homologue blocks differentiation of plastids and development of palisade cells Wycliffe et al., 2005. The Plant Journal Volume 44 Issue 1 Page 1. PMID: 16167891
