Plant hormone; natural steroid containing a seven-membered B-ring lactone, that promotes both cell elongation and cell division. Over ten brassinosteroids have been isolated and characterized from sources such as pollen, seedling, leaf. Isoln, structure and activity of brassinolide from rape pollen, Brassica napus L.: M. D. Grove et al., Nature 281, 216 (1979). Stereoselective synthesis: S. Fung, J. B. Siddall, J. Am. Chem. Soc. 102, 6580 (1980). Synthesis of two stereoisomers: M. J. Thompson et al., J. Org. Chem. 44, 5002 (1979). Improved synthesis: T. Kametani et al., J. Org. Chem. 53, 1982 (1988). Structure-activity relationship of brassinosteroids: S. Takatsuto et al., Phytochemistry 22, 2437 (1983); interaction with cytokinin: C. Schlagnhaufer et al., Physiol. Plant. 60, 347 (1984); bioassay: K. Wada et al., Agric. Biol. Chem. 48, 719 (1984).