Brassinosteroid

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Brassinosteroids (BR) are a group of steroidal plant hormones. Brassinolide was the first of these steroid compounds discovered in 1973, when it was shown that pollen from Brassica napus could promote stem elongation and cell divisions and that the biologically active molecule was a steroid that the authors called Brassins.^[1] Since their discovery over 70 BR compounds have been isolated from plants^[2]

The BR is biosynthesised from campesterol, the biosynthetic pathway was elucidated by Japanese researchers and later show to be correct through the analysis of BR biosynthesis mutants in Arabidopsis, tomato and peas.^[3] The sites for BR synthesis in planta 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.^[4][5] Experiments have shown that long distance transport is possible and flows in an acropetal direction, but it is not known if this movement is biologically relevant.^[4]

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

• Promotes cell expansion and cell elongation;^[4] works with auxin to do so^[6]
• It has an unclear role in cell division and cell wall regeneration^[4]
• Promotes vascular differentiation; BR signal transduction has been studied during vascular differentiation^[7]
• Is necessary for pollen elongation for pollen tube formation^[8]
• Accelerates senescence in dying tissue cultured cells; delayed senescence in BR mutants supports that this action may be biologically relevant^[4]
• Can provide some protection to plants during chilling and drought stress^[4]

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