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Brassinolide, the first brassinosteroid isolated and shown to have biological activity

Brassinosteroids (BRs) are a class of polyhydroxysteroids which have been recognized as a sixth class of plant hormones. These were first explored nearly forty years ago when Mitchell et al. reported promotion in stem elongation and cell division by the treatment of organic extracts of rapeseed (Brassica napus) pollen.[1]. Brassinolide was the first isolated brassinosteroid when it was shown that pollen from Brassica napus could promote stem elongation and cell divisions and the biologically active molecule was isolated.[2][3] The yield of brassinosteriods from 230 kg of Brassica napus pollen was only 10 mg. Since their discovery, over 70 BR compounds have been isolated from plants.[4]

The BR is biosynthesised from campesterol. The biosynthetic pathway was elucidated by Japanese researchers and later shown to be correct through the analysis of BR biosynthesis mutants in Arabidopsis, tomatoes and peas.[5] The sites for BR synthesis in plants have not been experimentally demonstrated. One well-supported hypothesis is that all tissues produce BRs, since BR biosynthetic and signal transduction genes are expressed in a wide range of plant organs, and short distance activity of the hormones also supports this.[6][7] Experiments have shown that long distance transport is possible and that flow is in an acropetal direction, but it is not known if this movement is biologically relevant.[6] Brassinosteroids are recognized at the cell membrane, although they are membrane soluble.

BRs have been shown to be involved in numerous plant processes:

  • Promotion of cell expansion and cell elongation;[6] works with auxin to do so.[8]
  • It has an unclear role in cell division and cell wall regeneration.[6]
  • Promotion of vascular differentiation; BR signal transduction has been studied during vascular differentiation.[9]
  • Is necessary for pollen elongation for pollen tube formation.[10]
  • Acceleration of senescence in dying tissue cultured cells; delayed senescence in BR mutants supports that this action may be biologically relevant.[6]
  • Can provide some protection to plants during chilling and drought stress.[6]

Extract from the plant Lychnis viscaria contains a relatively high amount of Brassinosteroids. Lychnis viscaria is said to increase the disease resistance of surrounding plants. In Germany extract from the plant is allowed for use as a "plant strengthening substance."

24-Epibrassinolide (EBL), a brassinosteroid isolated from Aegle marmelos Correa. (Rutaceae), was further evaluated for the antigenotoxicity against maleic hydrazide (MH) induced genotoxicity in Allium cepa chromosomal aberration assay. It was shown that the percentage of chromosomal aberrations induced by maleic hydrazide (0.01%) declined significantly with 24-epibrassinolide treatment.[11]

BRs have been reported to counteract both abiotic and biotic stress in plants.[12][13] Application of brassinosteroids to cucumbers was demonstrated to increase the metabolism and removal of pesticides, which could be beneficial for reducing the human ingestion of residual pesticides from non-organically grown vegetables.[14]

References

  1. ^ Mitchell JW, Mandava NB, Worley JF, Plimmer JR, Smith MV. 1970 "Nature" 281: 216-217.
  2. ^ Grove, M.D., Spencer, G.F., Rohwedder, W.K., Mandava, N., Worley, J.F., Warthen, J.D., Ste€ens, G.L., Flippen-Anderson, J.L. and Cook, J.C. 1979. Brassins:a new family of plant hormones from rape pollenNature 225:1065–66
  3. ^ Grove, Michael D.; Spencer, Gayland F.; Rohwedder, William K.; Mandava, Nagabhushanam; Worley, Joseph F.; Warthen, J. David; Steffens, George L.; Flippen-Anderson, Judith L. et al. (1979). "Brassinolide, a plant growth-promoting steroid isolated from Brassica napus pollen". Nature 281: 216–217. doi:10.1038/281216a0.  
  4. ^ Bajguz, A. 2007. Metabolism of brassinosteroids in plants. Plant Physiology and Biochemistry 45: 95-107
  5. ^ Fujioka S, Sakurai A. 1997. Biosynthesis and metabolism of brassinosteroids. Physiologia Plantarum 100:710–15
  6. ^ a b c d e f Clouse SD, Sasse JM. 1998. Brassinosteroids: Essential regulators of plant growth and development. Annu. Rev. Plant Physiol. Plant Mol. Biol. 49:427–51
  7. ^ Li JM, Chory J. 1997. A putative leucine rich repeat receptor kinase involved in brassinosteroid signal transduction. Cell 90:929–38
  8. ^ Nemhauser et al. 2004. Interdependency of Brassinosteroid and Auxin Signaling in Arabidopsis. PLoS Biology
  9. ^ Cano-Delgado A, 2004. BRL1 and BRL3 are novel brassinosteroid receptors that function in vascular differentiation in Arabidopsis. Development 131 :5341-5351
  10. ^ Hewitt FR, et al. 1985. Effect of brassinolide and other growth regulators on the germination and growth of pollen tubes of "Prunus avium" using a multiple hanging drop assay. Aust. J. Plant Physiol. 12:201–11
  11. ^ Sondhi, N., Bhardwaj, R., Kaur, S., Singh, B. and Kumar, N.(2008). Isolation of 24-epibrassinolide from leaves of "Aegle marmelos" and evaluation of its antigenotoxicity potential employing Allium cepa chromosomal aberration assay. "Plant Growth Regul." 54: 217-224
  12. ^ Sharma, P. and Bhardwaj, R. (2007) Effects of 24-Epibrassinolide on growth and metal uptake in "Brassica juncea" L. under copper metal stress. "Acta Physiologiae Plantarum." 29: 259-263
  13. ^ Sharma P, Bhardwaj R, Arora HK, Arora N (2008) Effects of 28-homobrassinolide on nickel uptake, protein content and antioxidative defence system in "Brassica juncea." "Biol. Plant." 52(4): 767-770
  14. ^ Xiao Jian Xia et al. 2009. Brassinosteroids Promote Metabolism of Pesticides in Cucumber. J. Agric. Food Chem., 57 (18): 8406–8413

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