Brassinosteroids(BRs) are important plant hormones that act synergistically with auxin to regulate a variety of plant developmental and physiological processes.In the past decade,genetic and biochemical studies have revealed a linear signaling pathway that relies on protein phosphorylation to transmit the BR signal into the nucleus,altering expression of hundreds of genes to promote plant growth.We conducted an activation-tagging based suppressor screen to look for Arabidopsis genes that,when overexpressed by inserted 35S enhancer elements,could suppress the dwarf phenotype of a weak BR receptor mutant bri1-301.This screen identified a total of six dominant activation-tagged bril suppressors(atbs-Ds).Using a plasmid rescue approach,we discovered that the bri1-301 suppression effect in four atbs-D mutants(atbs3-D to atbs6-D) was caused by overexpression of a YUCCA gene thought to be involved in tryptophandependent auxin biosynthesis.Interestingly,the three activation-tagged YUCCA genes belong to the YUCCA IIA subfamily that includes two other members out of 11 known Arabidopsis YUCCA genes.In addition,our molecular studies revealed a T-DNA insertion near a basic helix-loop-helix gene in atbs1-D and a T-DNA insertion in a region carrying a BR biosynthetic gene in atbs2-D.Further studies of these atbs-D mutants could lead to better understanding of the BR signaling process and the BR-auxin interaction.