Amyloplast

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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 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[edit]

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 the plant's perception of gravity, triggering the asymmetrical distribution of auxin that causes the curvature in roots and stems.[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 roots, the asymmetrical distribution of auxin causes the root to grow along the gravity vector.

In stems, gravity is sensed in the endodermal cells of the shoots.[5] Unlike roots, the differential distribution of auxin causes stems to grow against the gravity vector.

References[edit]

  1. ^ a b c Wise, Robert (2006-01-01), "The Diversity of Plastid Form and Function", The structure and function of plastids, 23, pp. 3–26, retrieved 2018-11-28
  2. ^ a b Neuhaus, H. E.; Emes, M. J. (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. ISSN 1040-2519. PMID 15012188.
  3. ^ Naeem, M.; Tetlow, I.J.; Emes, M.J. (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. ISSN 0960-7412.
  4. ^ Anstis, P. J. P.; D. H. Northcote (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, Miyo Terao (2010-06-02). "Directional Gravity Sensing in Gravitropism". Annual Review of Plant Biology. 61 (1): 705–720. doi:10.1146/annurev.arplant.043008.092042. ISSN 1543-5008. PMID 19152486.
  6. ^ Tasaka, M.; Kato, T.; Fukaki, H. (1999-03-01). "The endodermis and shoot gravitropism". Trends in Plant Science. 4 (3): 103–107. ISSN 1360-1385. PMID 10322541.
  7. ^ Morita, Miyo Terao; Tasaka, Masao (December 2004). "Gravity sensing and signaling". Current Opinion in Plant Biology. 7 (6): 712–718. doi:10.1016/j.pbi.2004.09.001. ISSN 1369-5266. PMID 15491921.
  8. ^ a b Kiss, John Z.; Wright, Jonathan B.; Caspar, Timothy (August 1996). "Gravitropism in roots of intermediate-starch mutants of Arabidopsis". Physiologia Plantarum. 97 (2): 237–244. doi:10.1034/j.1399-3054.1996.970205.x. ISSN 0031-9317.
  9. ^ a b Toyota, Masatsugu; Ikeda, Norifumi; Sawai-Toyota, Satoe; Kato, Takehide; Gilroy, Simon; Tasaka, Masao; Morita, Miyo Terao (September 2013). "Amyloplast displacement is necessary for gravisensing in Arabidopsis shoots as revealed by a centrifuge microscope". The Plant Journal. 76 (4): 648–660. doi:10.1111/tpj.12324. ISSN 1365-313X. PMID 24004104.
  10. ^ Wilkins, Henry; Wain, R. L. (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): 217–222. JSTOR 23371730.