The arabidopsis gulliver2/phyB mutant exhibits reduced sensitivity to brassinazole

Brassinosteroids (BRs) are essential plant hormones with roles in normal growth and development. Based on the severe light-independent hypocotyl shortening phenotype of BR mutants, it has been proposed that BRs are involved in cell elongation in darkness. To better appreciate the role of BRs in light-dependent plant development, we investigated Arabidopsis gulliver2 (gul2) mutants, which were isolated from a resistance screen against the BR biosynthetic inhibitor, brassinazole (Brz), in the presence of light. Through mapbased cloning and candidate gene sequencing, we revealed that gul2 mutants have defects in PHYTOCHROME B (PhyB). A double mutant of phyB and the BR biosynthetic mutant dwf4-1 had a dwarf phenotype that resembled that of dwf4-1, suggesting that BR biosynthesis is essential for the elongated phenotype of phyB/gul2. In addition, because preferential growth of hypocotyls in darkness is stimulated by transcription factors such as basic helix-loop-helix (bHLH) PHYTOCHROME INTERACTING FACTOR (PIF) and these proteins are more stable in darkness, we tested whether BRs affected the stability of PIFs. We found that the level of PIFs was affected neither by epi-BL nor by Brz. Therefore, we conclude that the phyB/gul2-mediated growth of hypocotyls requires the proper functioning of BR biosynthesis, and that the interplay of light and BR signaling pathways in hypocotyl growth is mediated by other mechanisms than the regulation of PIF stability.