Introductory Crop Physiology (1+1)

Brassinolides

Brassinolides (Brassinosteroids)

• 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.
• 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.
• 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.
• 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. 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.
• 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.
• 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; works with auxin to do so.
• It has an unclear role in cell division and cell wall regeneration.
• Promotion of vascular differentiation; BR signal transduction has been studied during vascular differentiation.
• Is necessary for pollen elongation for pollen tube formation.
• Acceleration of senescence in dying tissue cultured cells; delayed senescence in BR mutants supports that this action may be biologically relevant.
• Can provide some protection to plants during chilling and drought stress.
• Brassinosteroids activate signal transduction pathways, promote cell elongation and cell division.
• Brassinosteroids promotes differentiation of xylem tissue and perhaps other tissues too.
• Brassinosteroids can also retard leaf abscission abscission.
• Absence of brassinolides results in dwarf plants.
• BRs have been reported to counteract both abiotic and biotic stress in plants. 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.
  
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