The Primary Signaling Outputs Of Brassinosteroids Are Regulated By Abscisic Acid Signaling

Phytohormones play essential roles in coordinately regulating a large array of developmental processes.Studies have revealed that brassinosteroids and abscisic acid interact to regulate hundreds of genes'expression,governing many biological processes.However,whether their interaction is through modification or intersection of their primary signaling cascades or by independent or parallel pathways remains a big mystery.Using biochemical and molecular markers of brassinosteroid signaling and abscisic acid biosynthetic mutants,we demonstrated that exogenous abscisic acid rapidly inhibits brassinosteroid signaling outputs as indicated by the phosphorylation status of BES1 and brassinosteroid-responsive gene expression.Experiments using a bri1 null-allele,bri1-116,and analysis of subcellular localization of BKI1-YFP further revealed that the brassinosteroid receptor complex is not required for abscisic acid to act on brassinosteroid signaling outputs.However,when the BR downstream signaling component BIN2 is inhibited by LiCl,abscisic acid failed to inhibit brassinosteroid signaling outputs.Furthermore,using a set of abscisic acid insensitive mutants,we found that regulation of abscisic acid on brassinosteroid primary signaling pathway is dependent on the abscisic acid early signaling components ABI1 and ABI2.We propose that the signaling cascades of abscisic acid and brassinosteroid primarily crosstalk after brassinosteroid perception,but before their transcriptional activation.This model provides a reasonable explanation for why a large proportion of brassinosteroid-responsive genes are also regulated by abscisic acid,and provides novel insight into the molecular mechanisms by which brassinosteroids could interact with abscisic acid.