Gerontoplasts are a type of plastid formed during leaf senescence (deterioration with age). Like chromoplasts, gerontoplasts are derived from chloroplasts.
Although they are often described as aged chloroplasts, these plastids are thought to play an important role in nutrient recycling as well as the development of other organelles.
* Change in leaf color is also considered a good indicator of gerontoplast formation.
As mentioned, gerontoplasts are sometimes referred to as old/aged chloroplasts. Therefore, before looking at these plastids in detail, it's important to understand the origin and functions of chloroplasts.
Chloroplasts originate from cytoplasmic bodies known as proplastids.
Essentially, proplastids are undifferentiated plasmids located in the meristematic tissue. Proplastids also support cell development by producing a number of components including nucleotide precursors, lipids, fatty acids, and amino acids.
During the transition from proplastid to chloroplast, various molecules are synthesized and assembled in the membrane and intracellular environment.
The thylakoid membrane found in chloroplasts is formed through the budding of vesicles from the inner membrane of proplastids. About 90 percent of the proteins required for the development of the plasmid are imported as polypeptide chains (they are synthesized in the cytosol by cytosolic ribosomes).
Generally, development continues until the plastid reaches the stationary state with respect to structure and function (where they no longer develop and are mature enough to perform their functions).
Measuring between 5 and 7 um in diameter, mature chloroplasts are rounded, biconvex, or ovoid in shape. In addition to the thylakoid membrane (an internal membrane system), they also have a double membrane (chloroplast envelope with inner and outer membrane).
Thylakoids (flattened discs) are formed by the thylakoid membrane and arranged into stacks known as grana.
Because of this organization, chloroplasts are divided into several compartments that include:
The intermembrane space - This is the space between the chloroplast envelope (space between the inner and outer membrane).
The stroma - The stroma is the space between the inner membrane and the thylakoid membrane.
Thylakoid lumen - This is the space (an aqueous phase) within the thylakoids.
Some of the other components of a chloroplast include:
* Chlorophyll, a green pigment, and the majority of carotenoids are found within the thylakoid membrane.
Though chloroplasts have been associated with several other functions (e.g. sensory and regulatory functions), they are primarily involved in photosynthesis (a process through which light energy is converted to chemical energy).
* Chlorophyll absorbs red and blue light. However, it reflects green light away which is why many plants appear green in color (leaves at least).
In chloroplasts, the pigment chlorophyll is contained in the photosystems of the thylakoid.
Chlorophyll plays a vital role in photosynthesis given that it serves to absorb light photons (fundamental particles of light). Once captured, the light energy excites electrons in the photosystem (photosystem II) to a higher state.
In this state, the electrons flow through the membrane (thylakoid membrane) to photosystem I (PS1) causing the membrane to become negatively charged. This triggers protein pumps on the membrane to move hydrogen into the thylakoid.
At the same time, water molecules are broken down by water-splitting enzymes and electrons from these molecules enter the photosystem (PS11). As they continue to be activated, these electrons continue to flow along the membrane towards PS1. Water is therefore important for plants because it helps provide electrons required for the process while releasing oxygen.
As is the case with PS11, PSI, which also contains chlorophyll, absorbs light energy which excites electrons in the electron chain. These electrons provide the energy required to bind NADP+ and H+ from the stroma creating NADPH.
On the other hand, the hydrogen ions that had accumulated into the thylakoid start to diffuse into the stroma where they promote the binding of Adenosine diphosphate (ADP) to Phosphate (P) to form ATP.
This entire process takes place in a stage known as light-dependent reactions. As the name suggests, it takes place in the presence of sunlight.
In the second phase of photosynthesis (light-independent reactions or Calvin cycle which occurs in the stroma) carbon dioxide is attached to Ribulose 1, 5-bisphosphate (RuBP) in a process known as carbon dioxide fixation to form a six-carbon molecule.
The molecule is then broken down into two, three-carbon molecules (3-Phosphoglycerate). In the reduction stage, the two molecules are converted to glyceraldehyde 3-phosphate under the influence of ATP and NADPH. From here, glyceraldehyde 3-phosphate (G3P) is then transported to the cytoplasm where it is used to form glucose and other sugar molecules.
* Through photosynthesis, chloroplasts are not only involved in the production of chemical energy but also sugars that are consumed by animals (and used by the plant) as well as oxygen which is required for respiration and aerobic metabolism.
In plants, senescence is characterized by the dismantling or disassembly of cellular organelles.
Though chloroplast has been shown to exhibit the first signs of senescence, it survives longer as compared to the other organelles.
The dismantling of chloroplast does not occur all at once. Rather, different components are degraded and the process can stop and revert depending on a variety of conditions. For this reason, the stage of leaf senescence can be used to tell the stage of chloroplast dismantling and gerontoplast development.
The decline in photosynthetic reactions is one of the factors that can trigger and initiate the transformation of chloroplast to gerontoplast. Generally, the chlorophyll and thylakoid proteins are some of the first components that are disassembled.
Carotenoids, on the other hand, remain relatively stable causing the leaves to appear yellowing in color. The plastid (chloroplast) also gradually loses protein as gerontoplast develops. The degradation of proteins is carried out by proteases enzymes.
This process signifies a decline in macromolecule synthesis within the plastid. However, some studies have also identified the synthesis of senescence-related proteins.
* The degradation of proteins (through cleavage of peptide bonds) has also been observed in the transformation of proplastid to chloroplast.
* Chlorophyllase is involved in the breakdown of chlorophyll during gerontoplasts formation.
As the chloroplast transforms into gerontoplast, the lamellar is broken down resulting in the unstacking of the grana. When viewed under the microscope, this has been shown to result in the swelling of the intrathylakoid space.
This phase of chloroplast disassembly is suspected to be a response to the loss of chlorophyll b and LHCII (the two are involved in the stacking of grana).
The breakdown of thylakoid membranes is accompanied by the formation of plastoglobuli. According to some researchers, these globuli may contain some of the lipids retained during the degradation of the lamellar. It is thought that these lipids are important for the formation of thylakoids as gerontoplast transform back to chloroplasts.
In the later stages of gerontoplast development, studies have also identified a significant reduction of polysomes as well as a loss of electron density in the chloroplast stroma. In the process, the plastid shrinks in size and loses buoyancy density. The plastid envelope, however, remains intact.
The transformation process is also characterized by changes in the pattern of gene expression. Whereas some of the genes are up-regulated (e.g. SAGs), others are down-regulated (e.g. photosynthetic genes).
Generally, most of the up-regulated genes have been shown to play a role in the degradation of various macromolecules.
During the degradation of chloroplasts and the development of gerontoplasts, a number of important processes take place. These processes constitute gerontoplast functions.
Mobilization of Nitrogen - As the chloroplast is disassembled, a number of enzymes (proteases) are involved in the breakdown of proteins. The resulting nitrogen is then converted to asparagine and glutamine.
In this form, the nitrogen can then be transported to other parts of the plant through the phloem and xylem. Here, it's worth noting that nitrogen is one of the most important components required for plant development.
It's involved in the development of chlorophyll and various cellular structures. Therefore, the conversion of nitrogen into glutamine and asparagine in gerontoplasts is important given that these molecules can be stored and used as nitrogen sources later.
Provision of respiratory substrates - Respiration refers to a process through which energy is released from sugars. In plants, this is an important process that not only provided the energy required for photosynthesis, but also for the development of various cellular structures.
In addition to senescing, studies have shown that leaves located underneath canopies also experience shading stress which negatively affects photosynthesis. As a result, sugar production is affected which in turn affects plant development.
During gerontoplasts development (as a result of chloroplast transformation), a number of enzymes have been shown to break down amino acids, lipids, and carbohydrates in these plastids in order to supply components that are required for respiration.
In gerontoplasts, for instance, one of the most common processes among such plants is the breakdown of lipids and gradual conversion to sugars (this process is known as gluconeogenesis). This has been shown to be an important adaptive strategy during senescing that prevents sugar starvation.
On the other hand, the expression of din genes (codes for enzymes like B-g1ucosidase) promotes the breakdown of proteins during senescence which also allows the plant to survive with declined photosynthesis.
Mobilization and storage of nutrients - As mentioned, gerontoplasts are involved in the mobilization of nitrogen during senescence. They are also involved in the mobilization and storage of other minerals and nutrients (e.g. carbon and metallic ions).
During senescence, studies have also revealed the conversion of cysteine to glutathione. This is an important process that helps minimize damage caused by oxygen-free radicals. Glutathione is also involved in the transport of sulfur from the leaves to other parts of the plant.
Protection against damage induced by free radicals - The production of free radicals is common during senescence. This has been shown to induce the expression of genes that help minimize the toxic effects of these radicals.
Ferritins are some of the proteins expressed during senescence. As the name suggests, the protein is involved in binding iron so that it can be stored in a form that can be used by the plant.
In Fenton reaction, free iron can catalyze the production of hydroxyl radical as well as other toxic oxygen species that can cause damage to senescing cells. The production of proteins like ferritins during senescence is then crucial in protecting plants against such activities.
Aside from protecting the cells from such damage, the production of proteins like ferritin is also important for the transportation of iron atoms which can be used in other parts of the plant.
* As environmental conditions worsen, resources from gerontoplasts are transported to the seeds or perennial tissues where they can be stored and used for germination when conditions improve.
Like leucoplasts, gerontoplasts can also transform back to chloroplasts.
This is made possible by the fact that like other active plastids, it contains active genetic material capable of synthesizing proteins.
Moreover, the envelope (membrane) retains characteristics that make selective transport possible. These features are often used as evidence that the plastid is not dead as initially thought.
During leaf senescence, studies have shown the plastid to import nuclear-coded proteins more effectively compared to chloroplasts found in young tissue.
As expected, the transformation of gerontoplast to chloroplast is characterized by the regreening of leaves. However, even as the plastid transforms to chloroplast, it retains some of its own characteristics including the presence of plastoglobuli. This is used to differentiate between chloroplasts that originated from proplastids and those that originated from chloroplasts.
* Regreening is rare in other parts of the plant (e.g. fruits and sepals).
* The change from gerontoplasts to chloroplasts is not associated with any mitotic division. For this reason, there is no change in the number of chloroplasts. This has been used as evidence that some chloroplasts originate from gerontoplasts that survive.
During the transformation (gerontoplast to chloroplast), the plastid recovers various important structures including the lamellar systems, components of the stroma, and chlorophyll.
The other gerontoplast pigments are also retained in the chloroplast. This is also evidence that despite the dismantling of chloroplast during senescence, the plastid maintains genetic potential and the ability to interact with the nuclear genome as seen in other plastids. It is these capabilities that make it possible for the plastid to revert back to chloroplast.
* Though a number of factors have been found to influence the transformation of gerontoplasts to chloroplasts, the level of cytokines is perhaps one of the most important factors. The hormone is also vital for the transformation of proplastids to chloroplasts.
More plant biology related articles:
Biswal, U., Biswal, B. and Raval, M. (2003). Chloroplast Biogenesis From Proplastid to Gerontoplast.
Cooper, G. and Sunderland, M. (2000). Chloroplasts and Other Plastids: The Cell: A Molecular Approach. 2nd edition.
Schmidt, H. and Naturforsch, Z. (1987). The Structure and Function of Grana-Free Thylakoid Membranes in Gerontoplasts of Senescent Leaves of Vicia faba L.