Plastids

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Plastids

All eukaroytic cells are divided into separate compartments, each surrounded by an independent membrane system. These compartments are called organelles, and they include the nucleus, mitochondria , vacuoles , Golgi bodies, endoplasmic reticulum , and microbodies. In addition to these organelles, plant cells contain a compartment that is unique to them. This is the plastid.

General Description of Plastids

Plastid is a term applied to an organelle that is exclusive to plant cells. Most of the compounds important to a plant, and to human diet, start out in the plastid. It is the place in the cell where carbohydrates, fats, and amino acids are made. As the name suggests, the plastid is plastic (i.e., changeable) in both appearance and function, and the different types of plastids can change from one type to another. The signals that trigger these changes can come from within the plant itself (e.g., developmental changes such as fruit ripening or leaf senescence) or from the surrounding environment (e.g., changes in day length or light quality). Despite this plasticity, all plastids have the following features in common: They are 5 to 10 microns in diameter and approximately 3 microns thick, are all surrounded by a double membrane termed the envelope that encloses a water-soluble phase, the stroma, and they all contain deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

The presence of DNA is one indication that plastids used to exist as free-living organisms. Plastids would have once contained all of the genes necessary for their growth and development. Although plastid DNA (the plastid genome ) still encodes many essential plastid components, most of the genetic information now resides in the nucleus. During evolution most of the DNA became integrated into the nucleus so that the host cell controlled the genes needed for division and development of plastids. This important evolutionary step has consequently enabled the host cell to control most features of plastid structure and function. Thus, distinct types of plastids are found in different cells and tissues of the plant.

Eoplasts

Eoplasts (eo, meaning "early") represent the first stage of plastid development. They are spherical and lack any obvious internal membranes of the kind seen in chloroplasts . They occur in young, dividing cells of a plant (i.e., the meristematic cells) and are functionally immature. Their presence in egg cells prior to pollination means that they are transferred through generations via maternal inheritance. This means that eoplasts can only be made from preexisting eoplasts inherited via the egg. Eoplasts are able to divide along with the cell so that their number is maintained during plant growth. Increases in eoplast numbers occur when they continue to divide after cell division is complete. This is accompanied by cell differentiation, where the cell becomes specialized, and the eoplast matures into one of the functional types of plastid described next.

Chloroplasts

Chloroplasts are plastids found in photosynthetic tissue. This includes leaves, but also green stems, tendrils, and even fruit. Unripe tomato fruit, for example, contains chloroplasts as long as the tissue is green. On ripening, the chloroplasts change into chromoplasts and accumulate the pigments responsible for the red coloration of ripe fruit. Chloroplasts are distinguished from all other types of plastids by the presence of a complex organization of the internal membranes that form thylakoids. These form stacks of parallel membranes (called granal stacks) that contain the light-harvesting complexes involved in capturing light energy for use in photosynthesis. The chloroplast is the location of the photosynthetic processes occurring within the tissue. As well as the light-harvesting reactions, the enzymes responsible for carrying out the fixation of carbon dioxide, in a process called the Calvin-Benson cycle, are also located here. A mature cell of a cereal leaf, such as wheat, can contain up to two hundred chloroplasts.

Chloroplasts are formed from the eoplasts present in very young leaf cells. Another route, although less common in nature, is for them to form from etioplasts, but this happens only if the leaves have been kept out of the light for several days and then transferred back into sunlight.

Etioplasts

Etioplasts are a special type of plastid that only occurs in leaf tissue that has been kept in the dark for several days. This dark treatment causes the leaves to lose their green color, becoming pale yellow and losing their ability to photosynthesize. These leaves are described as being etiolated, hence the term etioplast. Etioplasts are characterized by containing semicrystalline structures called prolamellar bodies made up of complex arrays of membranes. When etiolated leaves are exposed to the light, the leaves turn green and the etioplasts change into chloroplasts within a very short time. The membranes of the prolamellar body are converted into the thylakoid membranes, and chlorophyll is formed together with all of the enzymes needed for photosynthesis. All of these processes are reversible. When green leaves are put back into continued darkness for several days, the chloroplasts revert once more to etioplasts.

Chromoplasts

Chromoplasts (Greek chromo, meaning "color") are colored plastids found in flower petals and sepals , fruit, and in some roots, such as carrots. They are colored because they contain pigments. These are the carotenoids, and they produce a range of coloration including yellow, orange, and red. The purpose of this coloration is to attract pollinators and, in the case of edible fruits, animals that will aid fruit and seed dispersal. Sometimes the color may act as a warning signal to tell insects and animals that the plant is poisonous. As noted above, chromoplasts may be formed from chloroplasts as green fruit ripens and matures. Alternatively, they may be formed from the conversion of amyloplasts or by development of eoplasts.

Leucoplasts

Leucoplasts are colorless, nonphotosynthetic plastids found in nongreen plant tissue such as roots, seeds, and storage organs (e.g., potato tubers). Their main function is to store energy-rich compounds, and types of leucoplasts include amyloplasts and elaioplasts. Amyloplasts store starch and elaioplasts contain oils and fats. In roots, amyloplasts serve two important functions. Their high starch content makes them relatively dense, and this is thought to be important in helping the root to respond to gravity (geotropism). Root amyloplasts are also very important in that they contain many of the enzymes needed for converting inorganic nitrogen taken up from the soil (as nitrate and ammonium) into organic forms, such as amino acids and proteins. Starch is a major food product and as a consequence, a lot of current research is aimed at understanding how amyloplasts work and what controls the rate of starch formation in these plastids. Similarly, the formation of oils (e.g., in oil seed rape seeds) in elaioplasts is being studied in many research and industrial laboratories throughout the world.

see also Cells; Chloroplasts; Endosymbiosis.

Alyson K. Tobin