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brassin (brassinosteroid) Any of a group of steroid derivatives that occur at very low concentrations in plant tissues and may have hormone-like effects. Brassins have been found in pollen, leaves, stems, and flowers from various species. Extracts behave in a manner similar to auxins, for example stimulating elongation of hypocotyl and epicotyl tissue from seedlings when applied at concentrations as low as 10–7 mol dm–3. However, their precise role in plants remains unknown.