Type of plastid, double-enveloped organelles in plant cells
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.[1][2] Amyloplasts are found in roots and storage tissues, and they store and synthesize
starch for the plant through the
polymerization of
glucose.[1] Starch synthesis relies on the transportation of carbon from the
cytosol, the mechanism by which is currently under debate.[2][3]
Starch synthesis and storage also takes place in
chloroplasts, a type of pigmented plastid involved in
photosynthesis.[1] 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.[4]
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.[5] 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.[6][7] A plant lacking in
phosphoglucomutase (pgm), for example, is a starchless mutant plant, thus preventing the settling of the statoliths.[8] This mutant shows a significantly weaker gravitropic response as compared to a non-mutant plant.[8][9] A normal gravitropic response can be rescued with hypergravity.[9]
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.[5] However, if the root cap is regrown, the root's gravitropic response will recover.[10]
In stems, gravity is sensed in the
endodermal cells of the shoots.[5]
References
^
abcWise RR, Hoober JK (2006-01-01). "The Diversity of Plastid Form and Function".
The structure and function of plastids. Vol. 23. pp. 3–26. Retrieved 2018-11-28.
^
abNeuhaus HE, Emes MJ (June 2000). "Nonphotosynthetic Metabolism in Plastids". Annual Review of Plant Physiology and Plant Molecular Biology. 51: 111–140.
doi:
10.1146/annurev.arplant.51.1.111.
PMID15012188.
^Naeem M, Tetlow IJ, Emes MJ (March 2002). "Starch synthesis in amyloplasts purified from developing potato tubers". The Plant Journal. 11 (5): 1095–1103.
doi:
10.1046/j.1365-313x.1997.11051095.x.
^Morita MT, Tasaka M (December 2004). "Gravity sensing and signaling". Current Opinion in Plant Biology. 7 (6): 712–8.
doi:
10.1016/j.pbi.2004.09.001.
PMID15491921.
^
abKiss JZ, Wright JB, Caspar T (June 1996). "Gravitropism in roots of intermediate-starch mutants of Arabidopsis". Physiologia Plantarum. 97 (2): 237–44.
doi:
10.1034/j.1399-3054.1996.970205.x.
PMID11539189.
^
abToyota M, Ikeda N, Sawai-Toyota S, Kato T, Gilroy S, Tasaka M, Morita MT (November 2013). "Amyloplast displacement is necessary for gravisensing in Arabidopsis shoots as revealed by a centrifuge microscope". The Plant Journal. 76 (4): 648–60.
doi:
10.1111/tpj.12324.
PMID24004104.
^Wilkins H, Wain RL (January 1975). "The role of the root cap in the response of the primary roots of Zea mays L. seedlings to white light and to gravity". Planta. 123 (3): 217–22.
doi:
10.1007/BF00390700.
JSTOR23371730.
PMID24435121.
S2CID19797331.