Chromoplasts areplastids,heterogeneousorganelles responsible forpigment synthesis and storage in specificphotosyntheticeukaryotes.[1] It is thought (according tosymbiogenesis) that like all other plastids includingchloroplasts andleucoplasts they are descended fromsymbioticprokaryotes.[2]
Chromoplasts are found infruits,flowers,roots, and stressed and agingleaves, and are responsible for their distinctive colors. This is always associated with a massive increase in the accumulation ofcarotenoid pigments. The conversion ofchloroplasts to chromoplasts inripening is a classic example.
They are generally found in mature tissues and are derived from preexisting mature plastids. Fruits and flowers are the most common structures for the biosynthesis of carotenoids, although other reactions occur there as well including the synthesis of sugars, starches, lipids, aromatic compounds, vitamins, and hormones.[3] The DNA in chloroplasts and chromoplasts is identical.[2] One subtle difference in DNA was found after a liquid chromatography analysis of tomato chromoplasts was conducted, revealing increasedcytosine methylation.[3]
Chromoplasts synthesize and store pigments such as orangecarotene, yellowxanthophylls, and various other red pigments. As such, their color varies depending on what pigment they contain. The main evolutionary purpose of chromoplasts is probably to attractpollinators or eaters of colored fruits, which helpdisperse seeds. However, they are also found in roots such ascarrots andsweet potatoes. They allow the accumulation of large quantities of water-insoluble compounds in otherwise watery parts of plants.
Whenleaves change color in the autumn, it is due to the loss of greenchlorophyll, which unmasks preexisting carotenoids. In this case, relatively little new carotenoid is produced—the change inplastid pigments associated with leafsenescence is somewhat different from the active conversion to chromoplasts observed in fruit and flowers.
There are some species of flowering plants that contain little to no carotenoids. In such cases, there are plastids present within the petals that closely resemble chromoplasts and are sometimes visually indistinguishable.Anthocyanins andflavonoids located in the cell vacuoles are responsible for other colors of pigment.[1]
The term "chromoplast" is occasionally used to includeany plastid that has pigment, mostly to emphasize the difference between them and the various types ofleucoplasts, plastids that have no pigments. In this sense,chloroplasts are a specific type of chromoplast. Still, "chromoplast" is more often used to denote plastids with pigments other than chlorophyll.
Using alight microscope chromoplasts can be differentiated and are classified into four main types. The first type is composed of proteicstroma with granules. The second is composed of protein crystals andamorphous pigment granules. The third type is composed of protein and pigment crystals. The fourth type is a chromoplast which only contains crystals. An electron microscope reveals even more, allowing for the identification of substructures such as globules, crystals, membranes,fibrils andtubules. The substructures found in chromoplasts are not found in the matureplastid that it divided from.[2]
The presence, frequency and identification of substructures using an electron microscope has led to further classification, dividing chromoplasts into five main categories: Globular chromoplasts, crystalline chromoplasts, fibrillar chromoplasts, tubular chromoplasts and membranous chromoplasts.[2] It has also been found that different types of chromoplasts can coexist in the same organ.[3] Some examples of plants in the various categories includemangoes, which have globular chromoplasts, andcarrots which have crystalline chromoplasts.[4]
Although some chromoplasts are easily categorized, others have characteristics from multiple categories that make them hard to place.Tomatoes accumulate carotenoids, mainly lycopene crystalloids in membrane-shaped structures, which could place them in either the crystalline or membranous category.[3]
Plastids lining which pollinators visit a flower, as specific colors attract specific pollinators. White flowers tend to attractbeetles,bees are most often attracted to violet and blue flowers, andbutterflies are often attracted to warmer colors like yellows and oranges.[5]
Chromoplasts are not widely studied and are rarely the main focus of scientific research. They often play a role in research on the tomato plant (Solanum lycopersicum).Lycopene is responsible for the red color of a ripe fruit in the cultivatedtomato, while the yellow color of the flowers is due toxanthophyllsviolaxanthin andneoxanthin.[6]
Carotenoid biosynthesis occurs in both chromoplasts andchloroplasts. In the chromoplasts of tomato flowers, carotenoid synthesis is regulated by the genes Psyl, Pds, Lcy-b, and Cyc-b. These genes, in addition to others, are responsible for the formation of carotenoids in organs and structures. For example, the Lcy-e gene is highly expressed inleaves, which results in the production of the carotenoid lutein.[6]
White flowers are caused by a recessiveallele in tomato plants. They are less desirable in cultivated crops because they have a lower pollination rate. In one study, it was found that chromoplasts are still present in white flowers. The lack of yellow pigment in their petals and anthers is due to a mutation in the CrtR-b2 gene which disrupts the carotenoid biosynthesis pathway.[6]
The entire process of chromoplast formation is not yet completely understood on the molecular level. However, electron microscopy has revealed part of the transformation from chloroplast to chromoplast. The transformation starts with remodeling of the internal membrane system with thelysis of the intergranalthylakoids and thegrana. New membrane systems form in organized membrane complexes calledthylakoid plexus. The new membranes are the site of the formation of carotenoid crystals. These newly synthesized membranes do not come from the thylakoids, but rather from vesicles generated from the inner membrane of the plastid. The most obvious biochemical change would be the downregulation of photosynthetic gene expression which results in the loss ofchlorophyll and stopsphotosynthetic activity.[3]
Inoranges, the synthesis of carotenoids and the disappearance of chlorophyll causes the color of the fruit to change from green to yellow. The orange color is often added artificially—light yellow-orange is the natural color created by the actual chromoplasts.[7]
Valencia orangesCitris sinensis L are a cultivated orange grown extensively in the state of Florida. In the winter, Valencia oranges reach their optimum orange-rind color while reverting to a green color in the spring and summer. While it was originally thought that chromoplasts were the final stage of plastid development, in 1966 it was proved that chromoplasts can revert to chloroplasts, which causes the oranges to turn back to green.[7]
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