Genome-wide Expression and Network Analyses of Mutants in Key Brassinosteroid Signaling Genes

Brassinosteroid (BR) signaling regulates plant growth and development in concert with other signaling pathways. Although many genes have been identified that play a role in BR signaling, the biological and functional consequences of disrupting those key BR genes still require detailed investigation. Here we performed phenotypic and transcriptomic comparisons of A. thaliana lines carrying a loss-of-function mutation in BRI1 gene, bri1–5, that exhibits a dwarf phenotype and its three activation-tag suppressor lines that were able to partially revert the bri1–5 mutant phenotype to a WS2 phenotype, namely bri1–5/bri1–1D, bri1–5/brs1–1D, and bri1–5/bak1–1D. From the three investigated bri1–5 suppressors, bri1–5/bak1–1D was the most effective suppressor at the transcriptional level. All three bri1–5 suppressors showed altered expression of the genes in the abscisic acid (ABA signaling) pathway, indicating that ABA likely contributes to the partial recovery of the wild-type phenotype in these bri1–5 suppressors. Network analysis revealed crosstalk between BR and other phytohormone signaling pathways, suggesting that interference with one hormone signaling pathway affects other hormone signaling pathways. In addition, differential expression analysis suggested the existence of a strong negative feedback from BR signaling on BR biosynthesis and also predicted that BRS1, rather than being directly involved in signaling, might be responsible for providing an optimal environment for the interaction between BRI1 and its ligand. Our study provides insights into the molecular mechanisms and functions of key brassinosteroid (BR) signaling genes, especially BRS1.