Amyloplasts in a potato cell

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

A diagram showing the different types of plastid

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

  1. ^ a b c Wise 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.
  2. ^ a b Neuhaus 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. PMID 15012188.
  3. ^ 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.
  4. ^ Anstis PJ, Northcote DH (1973). "Development of chloroplasts from amyloplasts in potato tuber discs". New Phytologist. 72 (3): 449–463. doi:10.1111/j.1469-8137.1973.tb04394.x.
  5. ^ a b c Morita MT (2010-06-02). "Directional gravity sensing in gravitropism". Annual Review of Plant Biology. 61 (1): 705–20. doi:10.1146/annurev.arplant.043008.092042. PMID 19152486.
  6. ^ Tasaka M, Kato T, Fukaki H (March 1999). "The endodermis and shoot gravitropism". Trends in Plant Science. 4 (3): 103–7. doi:10.1016/S1360-1385(99)01376-X. PMID 10322541.
  7. ^ 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. PMID 15491921.
  8. ^ a b Kiss 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. PMID 11539189.
  9. ^ a b Toyota 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. PMID 24004104.
  10. ^ 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. JSTOR 23371730. PMID 24435121. S2CID 19797331.
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