Brassinosteroid response of Brassica napus and functional characterization of BnBZL2

Brassinosteroids (BR) are important phytohormones that regulate plant growth, development, and seed yield. Brassica napus L. is one of the main eatable oil crops in China. Studying BR signaling and regulation of B. napus growth and development can provide clues to increase yield. We compared BR-related genes in B. napus and A. thaliana and analyzed their expression levels in different tissues (using ‘Huyou15’ as material). Results indicated that the key genes involved in BR synthesis and signal transduction pathways were highly expressed in the flower and young seed; seedling root growth was promoted under BR treatment at low concentration but was inhibited at high concentration; and hypocotyl elongation was inhibited under brassinozole (BRZ, a BR biosynthesis inhibitor) treatment in darkness. Furthermore, BR treatment reduced the expression levels of BR synthesis genes, whereas BRZ treatment increased their expression levels, suggesting that BR signaling feedback inhibited the expression of BR biosynthesis genes. Transient expression experiments in tobacco demonstrated that BnBZL2 (BZR1-like gene in B. napus) was localized in the cytoplasm and nucleus, and BR treatment increased the nuclear localization of BnBZL2. Western blotting revealed that BR treatment increased the ratio of dephosphorylated and phosphorylated BnBZL2. The vector of BnBZL2 with point mutations (BnBZL2^*, mimicking A. thaliana gain-of-function mutant bzr1-1D) was transformed into A. thaliana and the transgenic plants showed insensitivity to BRZ treatment in darkness, suggesting that BnBZL2^* increased BR signaling. Thus, the above results indicated that BR signaling and regulatory mechanisms were conserved in B. napus and A. thaliana.