The Importance of Myo-inositol in Plants

Myo-inositol (inositol) is a sugar-like carbohydrate produced by most plants. It has a taste which has been assayed at half the sweetness of table sugar (sucrose). Myo-inositol (inositol) is an added ingredient in various ROOTs products because it is important for normal plant growth and development. Myo-inositol is important for phosphate storage, cell wall biosynthesis, the production of stress related molecules, cell-to-cell communication, storage and transport of plant hormones.

Phosphate Storage

Myo-inositol is involved in the storage of phosphate in seeds. Phosphate is vital plant nutrient. Plants pack important nutrients into their seeds to support the early growth of the new seedling. Seeds store phosphorous as inositol hexaphosphate (IP6). IP6 molecules are deposited in the seeds in specialized cellular organelles called globoids. During seed germination, IP6 is broken back down into phosphorous and inositol by the enzyme phytase.6 IP6 may also assist in the production of plant proteins by facilitating the extraction of genetic information from the nucleus (translocation of messenger RNA).13

Plant Cell Wall Biosynthesis

Inositol is also an essential molecule for the production of the plant cell wall. Plant cell walls are made primarily of sugar chains called polysaccharides. An oxidized form of inositol is the most common and important sugar involved in polysaccharide production for cell walls.7 In addition to cell wall biosynthesis, these inositol-derived sugars provide components for other vital pathways involved in storage, transport, and development.6

Myo-inositol Affects Salt Tolerance

Inositol and related molecules contribute to plant protection against salt stress2,7. The accumulation of inositol and inositol derivatives in response to salinity has been studied extensively in the ice plant Mesmbryanthemum crystallinum. Inositol and related molecules are suggested to function in salt tolerance in two major ways: (1) to protect cellular structures from reactive oxydizers like as hydrogen peroxide, and (2) to control water pressure inside cells. Under salt stress conditions, the ice plant undergoes shock and wilts for a short period of time, followed by production and accumulation of inositol. These inositol molecules accumulate as wilting subsides. As water pressure is restored, inositol is detected in the phloem sap.9 Sodium (Na+) and inositol then accumulate in the leaves. This suggests that the mechanism for salt tolerance in ice plant involves controlling the proper direction of sodium transport out of the roots.9 It has been suggested that altering the biosynthesis of inositol and inositol derivatives may confer salt tolerance to crop species2.

Cell-to-cell Communication

Plants, like animals, require the ability to respond to their environment. Organisms use chemical signaling pathways for relaying information among cells. Myo-inositol is an essential component of a signaling pathway in plants called the PI signaling pathway.8 This pathway has been shown to participate in a variety of plant responses including the ability of roots to grow downward in response to gravity,10 and pressure changes in leaf pores that control wilting.3

Storage and Transport of Plant Hormones

Myo-inositol also serves to store and transport an important group of plant hormones, called auxins. Plant hormones control growth of plant tissues. Therefore, it is important for plants to provide a mechanism to transport hormones in a non-reactive form to target tissues or cells. Myo-inositol has been hypothesized to play a central role in the control of the plant hormone auxin1. Myo-inositol links up with auxins forming hormone conjugates that are temporarily inactive. These inactive conjugates allow for safe storage and/or transport auxins, and may regulate the availability of active auxins for physiological responses. Corn enzymes capable of conjugating myo-inositol to auxin have been isolated, as well as an enzyme that may be involved in the release of auxin from its inositol conjugate.4Although not itself a hormone, myo-inositol may help to ensure that auxin is not functional at the wrong time or place.

Literature Cited

  1. Bandurski, R.S. (1979) Chemistry and physiology of myo-inositol esters of indole 3-acetic acid. In: Wells W.W., Eisenberg, F. Jr . Cyclitols and Phosphoinositides. Academic Press, London, New York, 35-54.
  2. Bohnert, H.J., Nelson, D.E. and Jensen, R.G. (1995) Adaptations to environmental stresses. The Plant Cell. 7:1099-1111.
  3. Cote, G.G. and Crain, R.C. (1993) Biochemistry of phosphoinositides. Annual Review of Plant Physiology. 44:333-356.
  4. Kowalczyk, S. and Bandurski, R. (1991) Enzymatic synthesis of 1-O-(indol-3-ylacetyl)-beta-D-glucose: purification of the enzyme from Zea mays, and preparation of antibodies to the enzyme. Biochemistry Journal. 279:509-14.
  5. Loewus, F. (1990) Structure and occurrence of inositols in plants. In: Inositol Metabolism in Plants. D.J. Morre, W.F. Boss and F.A. Loewus Wiley-Liss, Inc., New York, 1-11.
  6. Loewus, F. and Loewus, M.W. (1983) Myo-inositol: Its biosynthesis and metabolism. Annual Review of Plant Physiology. 34:137-161.
  7. Loewus, F. and Murthy, P. (2000) Myo-inositol metabolism in plants. Plant Science. 150:1-19. (Elsevier)
  8. Munnik, T., Irvine, R., and Musgrave, A. (1998) Phospholipid signaling in plants. Biochemica Biosphysica Acta. 1389:222-272.
  9. Nelson, D.E., Rammesmayer, G. and Bohnert, H. (1998) Regulation of cell specific inositol metabolism and transport in plant salinity tolerance. The Plant Cell. 10:753-764
  10. Perera, I., Heilmann, I., Boss, W. (1999) Transient and sustained increases in inositol 1,4,5-trisphosphate precede the differential growth response in gravistimulated maize pulvini. Proceeding of the National Academy of Science. 96:5838-43.
  11. Shears, S. (1996) Inositol pentakis- and hexakisphosphate metabolism adds versatility to the actions of inositol polyphosphates: Novel effects on ion channels and protein traffic. Subcelluar Biochemistry. 26:187-226.
  12. Styer, J. C. 2000. Regulating Inositol Biosynthesis in Plants: Myo-inositol Phosphate Synthase and Myo-inositol Monophosphatase. Master Thesis, Virginia Polytechnic Institute. 78p.
  13. York, J., Odom, A., Murphy, R., Ives, E., and Wente, S. (1999) A phospholipase C dependent inositol polyphosphatase kinase pathway required for efficient messenger RNA export. Science. 285:96-100.