Regulation of castasterone level in primary roots of maize, Zea mays

Abstract

Gas chromatography-mass spectrometry analysis revealed that primary roots of maize contain 28-norcastasterone (28-norCS) and its biosynthetic precursors, cholesterol, and cholestanol, which suggests that the C-27-brassinosteroid (C-27-BR) biosynthetic pathway to generate 28-norCS is operative in the roots. A cell-free enzyme solution prepared from maize roots successfully mediated C24-methylation of 28-norCS to produce castasterone (CS) with the aid of S-adenosyl-L-methionine, which indicates that CS can be generated through C-27-BR biosynthesis, as well as C-28-BR biosynthesis, in maize roots. Enzymatic conversion study using the cell-free enzyme solution demonstrated that CS is converted into 26-norCS in the enzyme solution. Exogenously applied 28-norCS and 26-norCS showed less activity than CS in the activation of gravitropic curvature and inhibition of root elongation. Taken together, a steady-state level of CS, the active BR in maize roots, seems to be strictly controlled by complicated processes such as C-28- and C-27-BR biosynthesis and biodegradation by C26-demethylation to exert its biological activity.