Amyloplasts are a type of plastid, double-enveloped organelles in plant cells that are involved in various biological pathways. Amyloplasts are specifically a type of

leucoplast Leucoplasts (λευκός leukós "white", πλαστός plastós "formed, molded") are a category of plastid and as such are organelles found in plant cells. They are non-pigmented, in contrast to other plastids such as the chloroplast. Lacki ...

, a subcategory for colorless, non-pigment-containing plastids. Amyloplasts are found in roots and storage tissues, and they store and synthesize starch for the plant through the

polymer A polymer (; Greek '' poly-'', "many" + ''-mer'', "part") is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...

ization of

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...

. Starch synthesis relies on the transportation of carbon from the

cytosol The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells ( intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondri ...

, the mechanism by which is currently under debate. Starch synthesis and storage also takes place in chloroplasts, a type of pigmented plastid involved in

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

. 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.

Role in gravity sensing

Amyloplasts are thought to play a vital role in

gravitropism Gravitropism (also known as geotropism) is a coordinated process of differential growth by a plant in response to gravity pulling on it. It also occurs in fungi. Gravity can be either "artificial gravity" or natural gravity. It is a general featu ...

. Statoliths, a specialized starch-accumulating amyloplast, are denser than

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...

, and are able to settle to the bottom of the gravity-sensing cell, called a

statocyte Statocytes are gravity-sensing ( gravitropic) cells in higher plants. They contain amyloplasts-statoliths – starch-filled amyloplastic organelles – which sediment at the lowest part of the cells. Statocytes are present in the elongating region o ...

. This settling is a vital mechanism in plant's perception of gravity, triggering the asymmetrical distribution of auxin that causes the curvature and growth of stems against the gravity vector, as well as growth of roots along the gravity vector. A plant lacking in

phosphoglucomutase Phosphoglucomutase () is an enzyme that transfers a phosphate group on an α-D-glucose monomer from the 1 to the 6 position in the forward direction or the 6 to the 1 position in the reverse direction. More precisely, it facilitates the interconve ...

(pgm), for example, is a starchless mutant plant, thus preventing the settling of the statoliths. This mutant shows a significantly weaker gravitropic response as compared to a non-mutant plant. A normal gravitropic response can be rescued with hypergravity. In roots, gravity is sensed in the

root cap The root cap is a type of tissue at the tip of a plant root. It is also called calyptra. Root caps contain statocytes which are involved in gravity perception in plants. If the cap is carefully removed the root will grow randomly. The root cap ...

, a section of tissue at the very tip of the root. Upon removal of the root cap, the root loses its ability to sense gravity. However, if the root cap is regrown, the root's gravitropic response will recover. In stems, gravity is sensed in the endodermal cells of the shoots.

References

{{Cellular structures Organelles Plant cells Plant physiology Cell anatomy