A Leaky Mutation in DWARF4 Reveals an Antagonistic Role of Brassinosteroid in the Inhibition of Root Growth by Jasmonate in Arabidopsis

The F-box protein CORONATINE INSENSITIVE1 (COl1) plays a central role in jasmonate (JA) signaling and is required for all JA responses in Arabidopsis (Arabidopsis thaliana). To dissect JA signal transduction, we isolated the partially suppressing (psc1) mutant, which partially suppressed coil insensitivity to JA inhibition of root growth. The pscl mutant partially restored JA sensitivity in coil-2 background and displayed JA hypersensitivity in wild-type COl1 background. Genetic mapping, sequence analysis, and complementation tests revealed that psc1 is a leaky mutation of DWARF4 (DWF4) that encodes a key enzyme in brassinosteroid (BR) biosynthesis. Physiological analysis showed that an application of exogenous BR eliminated the partial restoration of JA sensitivity by pscl in coi1-2 background and the JA hypersensitivity of psc1 in wild-type COl1 background. Exogenous BR also attenuated JA inhibition of root growth in the wild type. In addition, the expression of DWF4 was inhibited by JA, and this inhibition was dependent on COl1. These results indicate that (1) BR is involved in JA signaling and negatively regulates JA inhibition of root growth, and (2) the DWF4 is down-regulated by JA and is located downstream of COl1 in the JA-signaling pathway.