Amyloplasts are non-
pigmented organelles found in some
plant cells. They are responsible for the synthesis and storage of
starch granules, through the
polymerization of
glucose.
[1] Amyloplasts also convert this starch back into sugar when the plant needs energy. Large numbers of amyloplasts can be found in fruit and in underground storage
tissues of some plants, such as in
potato tubers.
Amyloplasts are plastids, specifically leucoplasts. Plastids are a specialized class of cellular organelles that carry their own genome and are believed to be descendants of cyanobacteria (blue-green algae) which formed a symbiotic relationship with the eukaryotic cell.
Starch synthesis and storage also takes place in chloroplasts, a type of pigmented plastid involved in photosynthesis. Amyloplasts and chloroplasts are closely related, and amyloplasts can turn into chloroplasts; this is for instance observed when potato tubers are exposed to light and turn green.[2]
In the root cap (a tissue at the tip of the root) some specialized amyloplasts are involved in the perception of gravity by the plant (gravitropism). These specialized amyloplasts are denser than the cytoplasm and can sediment according to the gravity vector. They are found in a special subset of cells of the root cap (a tissue at the tip of the root) called statocytes. Statoliths are enmeshed in a web of actin and it is thought that their sedimentation transmits the gravitropic signal by activating mechanosensing channels. The gravitropic signal then leads to reorientation of auxin efflux carriers and subsequent redistribution of auxin streams in root cap and root as a whole. The changed relations in concentration of auxin leads to differerential growth of the root tissues. Taken together, the root is then turning, following the gravity stimuli. They are also found in the endodermic layer of the inflorescence stem. The redistribution of auxin causes the shoot to turn in a direction opposite that of the gravity stimuli.[citation needed]
