Hydroxylation of the native brassinosteroids 24-epicastasterone and 24-epibrassinolide by the fungus Cunninghamella echinulata.

24-Epicastasterone and 24-epibrassinolide, two naturally occurring phytohormones of the brassinosteroid type, were transformed by the fungus Cunninghamella echinulata to give the corresponding 12 beta-hydroxylated compounds. The structures of these compounds were determined by spectroscopic methods, especially heterocorrelated two-dimensional nuclear magnetic resonance investigations. In the rice lamina inclination test the 12 beta-hydroxylation lowered the bioactivity at 0.1 ppm to 10% in comparison with that of the corresponding parent hormones. The described hydroxylations represent the first biotransformations of native brassinosteroids.