Plastids are organelles found in plant and algal cells, although they are also found in some marine animals. These organelles arose during evolution after an endosymbiosis process, when a bacterium with photosynthetic abilities, similar to current cyanobacteria, was engulfed by an eukaryote cell, and became an endosymbiont instead of being digested . During evolution, most genes of the engulfed cyanobacteria were transferred to the host nucleus, and endosymbiont became plastids, which developed a diverse set of functions: photosynthesis, amino acid and lipid synthesis, storage of lipids, carbohydrates and proteins, providing color to different parts of the body plant, sensing gravity, regulation of stomata behavior, and many others.
Plastids are limited by a double membrane and an intermembrane space (Figure 1). Inside, they contain other membrane limited compartments like thylakoids of chloroplasts and tubules of chromoplasts. Like mitochondria, plastids also contain DNA and the molecular machinery to grow and divide, both processes under the control of the cell nuclear genes. Every plastid in every cell comes from another existing plastid, and they are transmitted to the next plant generation included in gametes during fertilization. In this way, all the plastids of a plant are descendant of the embryo plastids, known as proplastids. Proplastids are also found in meristematic cells of adult plants, where they divide before the cell division to ensure a proper number of proplastids in the two new cells. When cell differentiation begins, proplastids also initiate their own differentiation program, so that different types of mature plastids are generated according to the cell type. They can be leucoplastids (elaioplasts, amiloplastids, and proteoplastids), chloroplasts and chromoplasts. Chloroplasts may also differentiate into other types of plastids and other plastids may differentiate into chloroplasts. It is a bidirectional differentiation path (Figure 1).
Proplastids are small plastids, about 1 µm in diameter, and less complex at the structural level than other plastids of the plant. They are colorless, can change their morphology and may content a variable amount of tubular-like internal membranous compartments, as well as starch depots. These features are shared by two types of proplastids: germinal and nodule proplastids. Germinal proplastids are found in plant embryos (in seeds) and meristematic cells. By division and differentiation, they give rise to the rest of the plastids of the plant. They may also carry out some metabolic functions such as gibberellic acid synthesis, which is very important for meristematic metabolism. Nodule proplastids, as the name suggests, are found in the nodules of the roots and are involved in the fixation of nitrogen.
Etioplasts, another type of plastids, are found in stems, but not in roots. They are a long-lasting intermediate stage in the way of differentiation from proplastids to chloroplasts under very low intensity of light or darkness. Etioplasts restart the differentiation toward chloroplasts as soon as the light is intense enough.
Leucoplasts are colorless plastids (without pigments), that function as storaging organelles. Leucoplasts comprise amyloplasts, elaioplasts (or oleoplasts), and proteinoplasts. They store starch, lipids and proteins, respectively.
In plant cells, amyloplasts synthesize starch (Figure 2). All the stored starch in a cell can be found in plastids as starch granules. Amyloplasts are specialized in this role and contain large depots of starch. Apart from storing starch, mayloplasts are gravity sensors in root cells. Starch granules are denser than water so that amyloplasts fall to the bottom of the cell, and this is what cells need to detect the gravity vector. Amyloplasts are also involved in the metabolism of nitrogen.
Elaioplasts are small size plastids containing oil and lipids forming fat drops. In plant cells, there are two synthetic pathways for lipids. The eukaryotic pathway depends on the smooth endoplasmic reticulum, whereas the so-called prokaryotic pathway involves elaioplastids. These two pathways produce different types of lipids. Elaioplast are also involved in pollen maturation. Some plants are able to store lipids in other organelles known as elaiosomes, which are derived from endoplasmic reticulum.
Proteinoplasts contain a high concentration of proteins, so high that proteins form crystals or very dense amorphous material. However, it is not clear yet if there is a type of plastids exclusively dedicated to protein storaging in plant cells.
Chromoplasts contain carotenoid pigments that give the red, orange and yellow colors to the plant structure where they are present (Figure 2. These plastids are abundant in flowers, fruits, old leaves, and some roots. It is thought that one of their main function is to attract animals for pollination or for spreading the seeds. Chromoplasts are metabolically active, though they contain fewer DNA copies than chloroplasts.
Chromoplasts contain lipid drops with carotenoids and complex molecules known as fibrils, which also contain a core of carotenoids. Chromoplasts are differentiated from chloroplasts, as well as from proplastids. During differentiation from chloroplasts, the photosynthetic machinery and thylakoids are degraded and carotenoids are synthesized along with compartments where they are going to be included. These compartments, known as plastoglobules, are lipid drops with abundant triacylglycerides located in the stroma of the plastid. Plastoglobules may be also found in other plastids. Carotenoids, mainly xanthophylls, are also stored in other places of the plastid stroma, so concentrated that they can form filaments or crystals. Chromoplasts develop an internal layered membrane system in the outer part of the stroma. These new membranes arise from the invagination of the inner membrane and do not from the old degraded thylakoids. Carotenoids like lutein, beta-carotenoid, and others, may be also associated to these new membranes. Occasionally, the internal membranes show a reticular arrangement.
During chromoplast maturation, the concentration of pigments may be so high that they can form crystal, like beta-carotenoids in the carrot root, or lycopene in tomato. Carotenoids can also be grouped in tubular structures. Chromoplasts may contain other components such as starch granules and protein aggregates.
Although chromoplasts have been considered an advanced stage of the chloroplast development, it has been observed that they are able to become chloroplasts again. Some root and fruit tissues may become green again. For example, lemon fruits which are left on the tree change the color from yellow to green, and carrot roots turn green when they are exposed to the light.
Chloroplasts are the most abundant and studied plastids. They will be study in the next page.
Another types of plastids
During aging and death periods, cells contain gerontoplasts, which are differentiated from chloropasts. Muroplasts are found in glaucocystophyta algae, and contain a vestigial wall of peptidoglycan located between the inner and the outer membrane of the organelle. Type S and T plastids have been found In the sieve cells of the phloem, and may respond to wounds. Rhodoplasts are photosynthetic plastids found in red algae, and contain type a chlorophyll, but not b or c chlorophylls. Rhodoplasts contain thylakoids not arranged in stacks, and also molecular aggregates containing red pigments known as phycobilisomes, which are able to capture the light wavelengths that reach long distances under the sea water. Finally, some animals eat algae and chloroplasts are incorporated into the animal cells instead of being digested. These plastids are able to photosynthesize during months, feeding in this way the animal (for example, Elysia chlorotica). These plastids are known as cleptoplasts. Furthermore, some parasites, like Plasmodium, may contain plastids known as apicoplasts.
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