Chromoplasts are brightly colored plastids found in flowers, fruits, leaves, and roots. They originate from chloroplasts and play an important role in the synthesis and storage of carotenoid pigments. As such, they are integral to cross-pollination and seed dispersal.
Like some of the other plastids, they vary in size and shape. With respect to structure, they are divided into several groups that include:
* The word "Chromo" is derived from Latin and Greek words meaning color.
* Chromoplasts do not have chlorophyll.
* Chromoplasts also differentiate from leucoplasts and proplastids.
They both contain pigments that distinguish them from the colorless leucoplasts. However, they also have a number of differences that make it possible to distinguish between them.
One of the main differences between the two plastids (chromoplasts and chloroplasts) is in their internal structure.
As mentioned, chloroplast differentiation can give rise to chromoplasts. During this process, one of the most significant changes involves the modification of the internal membrane in order to form structures capable of accumulating and storing carotenoids.
During the chloroplast-chromoplast transition, some of the structural changes readily observed include the degradation of chlorophyll and disruption of the thylakoid membrane.
At the same time, there is a noticeable increase in carotenoid-accumulating bodies (plastoglobules, microfibrillar structures, internal membranous structures, as well as crystalline structures). Therefore, the structural difference is one of the main differences between chloroplasts and chromoplasts.
* Intermediates between proplastids and chloroplasts are known as etioplasts. They are characterized by prothylakoids and numerous tubules in a semicrystalline structure. Unlike chloroplasts, etioplasts are incapable of photosynthesis.
Like chlorophyll, most of the carotenoids are found in leaves (b-carotene and xanthophylls) and produced in chloroplasts.
Within the light-harvesting complexes, they play an important role in photosynthesis. In fruits, on the other hand, carotenoids are synthesized within the chromoplasts.
In tomato, which is one of the best-studied fruits, carotenoid (lycopene) can be found within plastoglobules. The enzyme PSY is expressed during carotenoid synthesis in chromoplasts and has been shown to regulate the process.
There are also several types of enzymes and proteins found in different parts of plants. For instance, whereas the protein SIPSY1 is more abundant in the petals and fruits, SIPSY2 is commonly found in the petals, leaves, and sepals while SIPSY3 is abundant in the roots.
As well, the protein SISGR1 (STAY-GREEN protein) also regulates fruit maturation through their interaction with the enzymes (PSY1). For example, the interaction of the enzyme PSY1 and the STAY-GREEN protein within the nucleus causes the accumulation of lycopene through suppressed transcription.
* Found in different types of foods, carotenoids are isoprenoids (organic compounds with two or more hydrocarbons and five carbons in each unit) that can also exhibit vitamin A activity as retinoids.
Although carotenoids in chromoplasts are largely associated with attracting insects and birds (pollinators), they also have nutritional value. Based on a number of studies, optimal intake of these pigments is suspected to reduce the development of certain cancers as well as risks associated with skin, eye, and bone disorders.
For the most part, carotenoids are found in fruits and tubers (in chromoplasts) and vegetables (chloroplasts). Although in smaller quantities, carotenoids can also be found in several animal-derived foods including daily products, seafood, and egg yolk.
* In chromoplasts, carotenoids are synthesized through the isoprenoid pathway. The process starts with the conversion of pyruvate and glyceraldehyde 3-phosphate into 2-C methyl-D-erythritol 4-phosphate through condensation. 2-C methyl-D-erythritol 4-phosphate is then transformed into isopentenyl diphosphate and Dimethylallyl pyrophosphate through a series of reactions before being converted to phytoene which gives rise to different types of carotenoids.
Generally, chloroplasts are characterized by a rounded or ovoid body and contain structures known as thylakoid (site of light-independent reaction), lamella, granum (layered stacks of thylakoids), and a DNA ring. In the guard cells within the leaves, chloroplasts measure between 5 and 7 um in diameter and 1 to 2 um in thickness.
Although chromoplasts vary in size, they are generally smaller than chloroplasts. The interior part of these plastids has large molecular structures known as fibrils that contain carotenoid/lipid droplets. The outer membrane is made up of fibrillin proteins (35kDa).
As mentioned, chromoplasts are also divided into several classes based on structural differences.
Globular chromoplasts - Globular chromoplasts can be found in different parts of different types of plants. For instance, they can be found in the perianth of citrus fruit as well as capsicum of yellow fruit. They are simple chromoplasts and are characterized by lipoprotein particles known as plastoglobules (carotenoid pigments are contained in these particles).
Crystalline chromoplasts - Crystalline chromoplasts can be found in carrot roots and tomato fruit. As the name suggests, they accumulate crystals of carotenoid (lycopene and beta-carotene).
Fibrillar chromoplasts - Fibrillar chromoplasts can be found in plants like pepper. The plasmids are characterized by bundled microfibrillar structures in which the pigment is contained.
Tubular chromoplasts - Tubular chromoplasts can be found in capsicum fruits, roses, and hypanthium. As the name suggests, they are characterized by lipoprotein tubules in which the pigments are contained. In reticulo-tubular chromoplasts, these structures exist in the form of a twisted network of complex fibrils.
Membranous chromoplasts - Membranous chromoplasts can be found in flowers like tulips and daffodils as well as the petals of Citrus Sinensis. Pigments (carotene pigments) are contained in concentric membranes (about 20 in number)
The other difference between chromoplasts and chloroplasts is with regard to pigmentation.
As mentioned, the chloroplast-to-chromoplast transition is characterized by the degradation of chlorophyll and chlorophyll-containing thylakoid membranes. These structures are replaced with structures like protein fibrillin.
Chlorophyll, the primary pigment in chloroplasts plays an important role in photosynthesis. Specifically, chlorophyll serves to absorb light energy which is required to convert carbon dioxide and water into glucose.
In flowers, carotenoid pigments help attract pollinators thus promoting cross-pollination and seed dispersal.
* Chlorophyll is responsible for the green color of plants.
* Aside from chlorophyll, chloroplasts also contain a number of other pigments including B-carotene, zeaxanthin, violaxanthin, lutein, and neoxanthin. Like chlorophyll, these pigments also have a role to play in photosynthesis. For instance, beta carotenes found in chloroplast are involved in the transmission of light energy from chlorophyll. Moreover, they absorb energy from singlet oxygen thus protecting plant tissue. Xanthophylls, like Lutein, are also involved in light absorption which is essential for photosynthesis.
Chromoplasts, on the other hand, contain large amounts of carotenoids (not photosynthetically active). In different types of plants, the variation of carotenoid composition and concentration results in varying colors (red, orange, yellow).
For the most part, these plastids are found in the floral and fruit parts of the plant. In a few plants, however, they can also be found in the roots (e.g. in carrots).
* While chloroplasts are commonly found in the photosynthetic tissue of photosynthetic plants as well as algae, chromoplasts are not involved in photosynthesis and thus not abundant in photosynthetic tissue.
The other difference between chromoplasts and chloroplasts is with respect to transformation.
Chloroplasts can give rise to other plastids including chromoplasts, gerontoplasts, and leucoplasts (tannosome). These plastids can transform back into chloroplasts. This is an important characteristic that allows the chloroplast (and thus the plant) to adjust or prepare for various processes.
Chromoplasts can arise from proplastids and chloroplasts. Although they can change into chloroplast under certain conditions, it does not often give rise to other plastids.
Aside from structural differences, chromoplasts can also be divided based on where they are located in the plant.
Flower chromoplasts - As mentioned, flowers represent some of the parts in which chromoplasts can be found in abundance. Specifically, they can be found in abundance in the petals where they produce pigments to confer color to the flower and attract pollinators.
Here, it's worth noting that chloroplasts can be found in unopened buds within the young petals. However, the chloroplasts differentiate to form chromoplasts that produce and store pigments as the bud opens up.
* In some flowers, chloroplasts differentiate to produce leucoplasts. A good example of this is in Arabidopsis where the petals are white in color. In this case, immature petals contain chloroplasts.
However, as the petals mature, chloroplasts differentiate into leucoplasts which are colorless. They do not have the carotenoid pigment.
Though the chlorophyll and chloroplasts are degraded, failure to produce carotenoids causes part of the flower to remain white (the stalk is partially green due to the presence of chloroplasts).
Fruit chromoplasts - The presence of chromoplasts in fruits results in a wide array of colors ranging from yellow to red and orange. As is the case with flowers, chloroplasts differentiate into chromoplasts in fruits resulting in changes in color.
The change in color, from green to other colors, also indicates that the fruit is ripening. In various plants, studies have shown the process to result in the degradation of chlorophyll and chloroplasts as chromoplast membranes start forming.
In other studies, e.g. those involving mutant tomato plants, chlorophyll and thylakoid stacks were found to be unaffected even as the fruit ripened. However, chromoplasts were still formed and accumulated carotenoids as the fruit ripened.
Root chromoplasts - Although light is important for photosynthesis (within chloroplasts), it also plays an important role in the early phase of fruit ripening.
Generally, during the early phase of fruit ripening, the absorption of light by chloroplasts aids in the gradual synthesis of carotenoids in the peel of the fruit as well as the pulp.
In roots, however, very little light reaches the tissues. Moreover, exposure of roots to light activates chloroplast development which in turn causes the roots to become greener.
In a few plants, (e.g. carrots), studies have revealed an abundance of chromoplasts and accumulated carotenoids. When the roots are exposed to light, as is the case with other roots, the level of carotenoid decreases, and chloroplasts start to develop.
* In many plants, leaves are green in color due to the presence of chloroplasts (chloroplasts contain chlorophyll which confers the green color to leaves). However, leaves can start losing their green color as a result of the process known as chlorosis.
In this case, chloroplasts do not differentiate into chromoplasts. Rather, they give rise to gerontoplasts. Although chlorophyll is degraded, carotenoids in the plastid are partially retained.
As compared to chromoplasts, additional carotenoid is not synthesized. For this reason, the new color of the plant is the result of carotenoids retained as the chloroplast transitioned into gerontoplasts.
Although chloroplasts transition to give rise to chromoplasts in different parts of the plant, the opposite can also occur. This is often described as regreening. Regreening can be caused by a variety of factors and can occur at different parts of the plant (e.g. in fruits and roots).
Essentially, this process is characterized by increased chlorophyll content as well as carotenoid reduction. For instance, as a result of increased nitrogen concentration in the pericarp tissue of citrus fruit, the chloroplast-chromoplast transition is significantly slowed. At the same time, chlorophyll content increases causing the fruit to become greener.
In various plants, the regreening process is not only characterized by increased chlorophyll content but also by the formation of new thylakoids.
For the most part, the new thylakoids are formed from vesicles located in the plastid (chromoplasts) as well the invagination of the inner membrane. Increased formation of thylakoids is also associated with the formation of new grana and photosynthetic capabilities of the plastid.
As mentioned, chromoplasts are primarily involved in the production and storage of carotenoids.
Though there are different types of carotenoids in these plastids (lycopene, neoxanthin, lutein, and violaxanthin), some can be transformed into others through a series of complex reactions.
This, however, is dependent on a number of factors including the season and environmental conditions. Regardless, carotenoids are the most abundant content of chromoplasts and gradually increase in some parts of the plant (e.g. in the fruits as they ripen).
Aside from carotenoids, these plastids also produce a number of other substances required for various activities.
Lipids - Lipids are an important part of the transition process (chloroplast to chromoplast) and are required for restructuring.
Lipoproteins are required for the development of substructures like plastoglobules in which carotenoid is accumulated. During the transition process, studies have shown various proteins and enzymes to actively metabolize lipids and synthesize fatty acids. Lipids promote the reorganization of the internal membrane thus preparing the plastid for pigment storage.
Carbohydrates - Accumulation of carotenoids has been associated with increased sugar levels in chromoplasts. This was particularly reported in the tobacco plants where the transition of amyloplast to chromoplasts exhibited a positive correlation with starch catabolism and nectar sugar.
According to a number of researchers, sugars like sucrose are also suspected to play a regulatory role in the differentiation process.
For instance, whereas sucrose has been shown to promote the transition of chloroplast to chromoplasts, it also prohibits the reversal process (regreening). Consequently, the sugar also regulates the accumulation of carotenoids in the plastid.
In a study investigating the impact of sucrose depletion, findings revealed increased expression of the gene PSY1 which caused delayed accumulation of the pigment in tomato fruit.
* Sugar level in the plastids does not appear to affect the rate at which chlorophyll is degraded.
In fruits, there are three main factors that influence the differentiation of chromoplasts from other plastids.
Nutrients - As mentioned, carbohydrates and nitrogen have been shown to influence the rate at which plastids differentiate to form chromoplasts and accumulate carotenoids. Nutrition is therefore one of the most important factors when it comes to the formation of chromoplasts.
Though carbohydrates and nitrogen have individual effects on chromoplasts, a high carbon-to-nitrogen ratio has also been shown to positively promote this differentiation. Specifically, the high ratio contributes to the breakdown of chloroplast structures which in turn promotes chromoplast development.
Light - Light energy is a vital component of photosynthesis in chloroplasts. It's also an important factor in the biogenesis of chloroplasts. In its absence, however, studies have shown the plastid to gradually lose chloroplastic thylakoid membranes.
As mentioned, light also plays an important role in the differentiation of chromoplasts in citrus peel. During the initial phase of fruit coloration (as it ripens), the light helps in the development of chromoplasts as well as the accumulation of carotenoids.
Temperature - Temperature is also an important factor in chromoplast differentiation. Whereas lower temperatures influence carotenoid biosynthesis, relatively higher temperatures have been shown to decrease carotenoid content.
Therefore, in sensitive fruits like tomatoes, it's important to determine the ideal temperature range in order to ensure proper color characteristics among other qualities of a good yield.
* The number and size of chromoplasts is the biggest factor that determines the amount of carotenoids accumulated in a given plant.
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