Molecular Structure and Biological Activity of Brassinolide and Related Brassinosteroids

Brassinosteroids (BRs) are steroidal growth-promoting plant hormones. Some reviews on BRs have appeared. Brassinolide is the first BR isolated from rape pollen and elicits strong growth-promoting activity. The bean second internode bioassay using pinto bean is quite a historical one and was successfully used as a guide for the isolation of brasinolide. In bean second internode bioassay, BRs with 6-ketone are generally more than 100 times less active than lactone-type BRs. Isomeric lactone of brassinolide are inactive, The acetates of highly active BRs are only marginally active. L. Kohout and M. Strnad examined some BRs with a cholestane side chain. Their biological activities are more than three orders of magnitude less active than 24-epibrassinolide and the maximum response observed was far less than that of 24-epibrassinolide. Cells aligned in the adaxial side of the joint between leaf blade and sheath is swollen by BRs, causing lamina bending. This reaction is quite specific for BRs.