Inhibition of 4-Hydroxyphenylpyruvate Dioxygenase expression by brassinosteroid reduces carotenoid accumulation in Arabidopsis.

Unlike the indispensable function of the steroid hormone brassinosteroid (BR) in regulating growth and development of plants, the metabolism of secondary metabolites regulated by BR is not well known. Here we show that BR reduces carotenoid accumulation in Arabidopsis seedlings. BR-deficient or BR-insensitive mutants accumulated a higher level of carotenoids than the wild-type plant whereas BR treatment reduced carotenoid contents. We demonstrated that BR transcriptionally suppresses 4-hydroxyphenylpyruvate dioxygenase (HPPD) gene involved in carotenogenesis via plastoquinone production. We found that the expression of HPPD displays an oscillation pattern that is expressed more strongly in dark than in the light conditions. Moreover, BR appeared to inhibit more strongly HPPD expression in darkness than in light, leading to suppression of a diurnal oscillation of HPPD expression. BR-responsive transcription factor BZR1 directly bound to HPPD promoter and the HPPD suppression by BR was increased by the bzr1-1D gain-of-function mutation. Interestingly, dark-induced HPPD expression did not cause carotenoid accumulation due to downregulation of other carotenoid biosynthetic genes in the dark. Our results suggest that BR regulates different physiological responses in dark and light through inhibition of HPPD expression.