Activation-Tagged Suppressors of a Weak Brassinosteroid Receptor Mutant

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 ex- pression 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 phe- notype of a weak BR receptor mutant bril-301. This screen identified a total of six dominant activation-tagged bril sup- pressors （atbs-Ds）. Using a plasmid rescue approach, we discovered that the bril-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 tryptophan- dependent 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 atbsl-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.