Amyloplast
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, a subcategory for colorless, non-pigment-containing plastids.<ref name=":0">Template:Cite book</ref><ref name=":3">Template:Cite journal</ref> Amyloplasts are found in roots and storage tissues, and they store and synthesize starch for the plant through the polymerization of glucose.<ref name=":0" /> Starch synthesis relies on the transportation of carbon from the cytosol, the mechanism by which is currently under debate.<ref name=":3" /><ref>Template:Cite journal</ref>
Starch synthesis and storage also takes place in chloroplasts, a type of pigmented plastid involved in photosynthesis.<ref name=":0" /> 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.<ref>Template:Cite journal</ref>
Role in gravity sensing
Amyloplasts are thought to play a vital role in gravitropism. Statoliths, a specialized starch-accumulating amyloplast, are denser than cytoplasm, and are able to settle to the bottom of the gravity-sensing cell, called a statocyte.<ref name=":4">Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> A plant lacking in phosphoglucomutase, for example, is a starchless mutant plant, thus preventing the settling of the statoliths.<ref name=":1">Template:Cite journal</ref> This mutant shows a significantly weaker gravitropic response as compared to a non-mutant plant.<ref name=":1" /><ref name=":2">Template:Cite journal</ref> A normal gravitropic response can be rescued with hypergravity.<ref name=":2" /> In roots, gravity is sensed in 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.<ref name=":4" /> However, if the root cap is regrown, the root's gravitropic response will recover.<ref>Template:Cite journal</ref> In stems, gravity is sensed in the endodermal cells of the shoots.<ref name=":4" />