Regulation of brassinosteroid responses by phytochrome B in rice

Plant growth and development are coordinately controlled by environmental signals and internal factors. Light signals, mediated by phytochromes, regulate photomorphogenesis by interacting with endogenous programmes that involve multiple phytohormones. Brassinosteroids (BRs) are a group of growth-promoting phytohormones with a crucial role in the light-dependent development of plants. However, the interaction between light-signalling pathways and BR signalling is not well understood. Here, we examined the responses of lamina joint inclination and coleoptile elongation to exogenous brassinolide (BL) under light or dark conditions. Both responses were more pronounced under darkness, implying that BR signalling is inhibited by light. To elucidate which phytochrome is involved in this interaction, we isolated rice phytochrome-deficient mutants (osphyA, osphyB and osphyC) from a T-DNA insertional population. Whereas the osphyA and osphyC knockout mutants did not differ from the wild-type plants in their BL responses, osphyB mutants were more sensitive. In addition, RT-PCR analysis revealed enhanced expression of BR-inducible genes and decreased transcript levels of BR-biosynthetic genes in osphyB plants. These results suggest that Phytochrome B acts as a negative regulator of BL-regulated growth and development processes in rice.