Plastid
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 (formerly "eoplasts",
eo-: dawn, early), which are present in the meristematic regions of the plant. Proplastids and young chloroplasts commonly divide, but more mature chloroplasts also have this capacity.
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 1000 in rapidly
dividing cells, which generally contain few plastids, to 100 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.
Most plants inherit the plastids from only one parent.
Angiosperms generally inherit plastids from the mother, while many
gymnosperms inherit plastids from the father.
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.
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.
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 by secondary
endosymbiosis, when a
eukaryote engulfs a red or green alga and retains the algal plastid, which is typically surrounded by more than two membranes, and reduced in its 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 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.
*
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*Birky, C. W. 2001. The inheritance of genes in mitochondria and chloroplasts: laws, mechanismsand models. Annual Review of Genetics 35: 125-148.
