Developing seedlings are well equipped to alter their growth in response to external factors in order to maximize their chances of survival. SUPPRESSOR OF PHYTOCHROME B4-#3 (SOB3) and other members of the AT-HOOK MOTIF CONTAINING NUCLEAR LOCALIZED (AHL) family of transcription factors modulate the development of Arabidopsis (Arabidopsis thaliana) by repressing hypocotyl elongation in young seedlings growing in light. However, the molecular mechanism behind how AHLs influence seedling development is largely unknown. We have identified genes associated with auxin-mediated hypocotyl elongation as downstream targets of SOB3. We found that YUCCA8 (YUC8) as well as members of the SMALL AUXIN UP-REGULATED RNA19 (SAUR19) subfamily were down-regulated in the short-hypocotyl, gain-of-function SOB3-D mutant and up-regulated in the dominant-negative, tall-hypocotyl sob3-6 mutant. SOB3-D and sob3-6 hypocotyls also exhibited altered sensitivity to the polar auxin transport inhibitor N-1-napthylphthalamic acid, suggesting a critical connection between auxin and the modulation of seedling elongation by SOB3. Finally, we found that overexpression of GREEN FLUORESCENT PROTEIN-SAUR19 in the SOB3-D line partially rescued defects in hypocotyl elongation, and SOB3 bound directly to the promoters of YUC8 and SAUR19 subfamily members. Taken together, these data indicate that SOB3 modulates hypocotyl elongation in young seedlings by directly repressing the transcription of genes associated with auxin signaling.

SUMMARY Interactions between signaling pathways help guide plant development. In this study, we found that brassinosteroid (BR) signaling converges with SUPPRESSOR OF PHYTOCHROME B4-#3 (SOB3) to influence both the transcription of genes involved in cell elongation and hypocotyl growth. Specifically, SOB3 mutant hypocotyl phenotypes, which are readily apparent when the seedlings are grown in dim white light, were attenuated by treatment with either brassinolide (BL) or the BR biosynthesis inhibitor brassinazole (BRZ). Hypocotyls of SOB3 mutant seedlings grown in white light with a higher fluence rate also exhibited altered sensitivities to BL, further suggesting a connection to BR signaling. However, the impact of BL treatment on SOB3 mutants grown in moderate-intensity white light was reduced when polar auxin transport was inhibited. BL treatment enhanced transcript accumulation for all six members of the SMALL AUXIN UP RNA19 (SAUR19) subfamily, which promote cell expansion, are repressed by SOB3 and light, and are induced by auxin. Conversely, BRZ inhibited the expression of SAUR19 and its homologs. Expression of these SAURs was also enhanced in lines expressing a constitutively active form of the BR signaling component BZR1, further indicating that the transcription of SAUR19 subfamily members are influenced by this hormone signaling pathway. Taken together, these results indicate that SOB3 and BR signaling converge to influence the transcription of hypocotyl growth-promoting SAUR19 subfamily members.