Amyloplasts are a type of plastid known for storing food materials. Plastids are a double-membraned structure found in plant cells, algae and some eukaryotes. The name was given by A.F.W. Schimper. Plastids may contain pigments to provide colour to the cells and are further divided into chromoplast, chloroplast and leucoplast. Amyloplasts fall into the category of leucoplasts. Plastids play a vital role in the plants’ ability to perform photosynthesis and store substances like metabolites, starch, etc.
Plastids
Plastids contain two membranes; hence, it is called double-membraned organelle. It is known to contain its own DNA. It can be found in both plant cells and algae. Plastids are found to be developed from cyanobacteria, so it is regarded as endosymbiont. Plastids are known for their role in photosynthesis and storing food materials like starch.
In some cases, plastid can be inherited from a single parent. Angiosperms are known to inherit plastids from female gamete, and gymnosperms inherit plastids from the pollen grains. Plastids are further classified into three types: Chloroplast, chromoplast and leucoplast.
Chloroplast
It is a double-membraned cell organelle. It is present in the mesophyll cells of the leaves of the plant. They contain chlorophyll responsible for trapping sunlight and performing photosynthesis in plants. A section of the chloroplast that transforms into a different organelle when the leaf does not perform photosynthesis is known as gerontoplast. Synthesised from etioplast and having its own DNA, it consists of an outer membrane, an inner membrane and a thylakoid membrane. The chloroplast consists of two different regions known as grana and stroma. Grana are stacks of thylakoids, and it consists of chlorophyll. The stroma is the matrix where the grana are present; it contains all the chloroplast’s enzymes, DNA, etc., required. Chloroplast absorbs light with the help of chlorophyll pigment and converts the light energy into chemical energy by producing NADPH, ATP and oxygen.
Chromoplast
Chromoplasts are a type of plastid that synthesises and stores pigments. They contain carotenoid pigments, which provide different colours to the plant. The primary role of imparting colours to the plant is to attract insects, bees and animals for pollination.
Leucoplast
These organelles do not contain pigments – they do not even have any colour. It is generally found in parts of plants that do not perform photosynthesis, like the roots. They can convert amino acids and fatty acids and are also known for storing starch, lipid and proteins for plant needs. Leucoplasts are further divided into three types: proteinoplast, elaioplast and amyloplast.
- Proteinoplast helps store proteins in the seeds of the plant for future use of the plant.
- Elaioplast is known for storing fats and oils required by the plant.
- Amyloplast is known to synthesise and store starch.
Role of Amyloplast
Amyloplast is a plastid that contains a double membrane found in plant cells. It is found in the non-photosynthesising parts of the plants, like the roots. It is involved in different metabolic pathways in a plant. It is known for synthesising starch through polymerisation or joining glucose molecules. The synthesis of starch depends on the transfer of carbon from the cytosol. It is also known for storing these starch molecules. Amyloplasts are closely related to chloroplast as the chloroplast can also synthesise and store starch. For example, the potato tubers that normally function as amyloplast when exposed to light turn green and function like that of a chloroplast.
Amyloplasts also play an essential part in gravitropism – the growth of a plant in response to gravity. A specialised amyloplast known as statoliths is known to store more starch and tends to become denser than the cytoplasm, moving towards the bottom of osteocytes (gravity-sensing cells). This mechanism leads to uneven distribution of phytohormone auxin responsible for plant growth. A phosphoglucomutase mutant plant cannot perform this process, showing decreased gravitropism.
Conclusion
Amyloplast is a double-membraned plastid found in the non-photosynthesising parts of the plants like the roots. It is involved in different metabolic pathways in a plant. It is known for synthesising starch through polymerisation or joining glucose molecules and plays an essential role in gravitropism.