Brassinolide is a

plant hormone Plant hormone (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pa ...

. The first isolated brassinosteroid, it was discovered when it was shown that pollen from rapeseed (''Brassica napus'') could promote stem elongation and cell division. The biologically active component was isolated and named brassinolide.

References

Plant hormones Steroids Epsilon-lactones {{steroid-stub