Brassinosteroids and Salicylic Acid Mutually Enhance Endogenous Content and Signaling to Show a Synergistic Effect on Pathogen Resistance in Arabidopsis Thaliana

The crosstalk mechanism regulating content and signal transduction between brassinosteroids (BRs) and salicylic acid (SA) for plant defense was investigated in Arabidopsis. Compared to the wild type, an increased bacterial resistance was observed in bzr1-1D, a dominant mutant of the BR transcription factor BZR1. In bzr1-1D, SA biosynthetic gene ICS1 expression and endogenous SA content greatly increased upon Pst DC3000 infection, and the direct binding of BZR1 to the ICS1 promoter was confirmed through EMSA and ChIP. In bzr1-1D where NPR1 expression was almost absent, expression of PR genes was increased, and both BZR1 and PR5 expressions increased after SA treatment. EMSA and ChIP verified that BZR1 binds directly to the cis-element present in the PR5 promoter and a pull-down assay showed that TGAs, SA transcription factors upstream of PR genes, interact with BZR1 at the protein level. Crude enzyme assays demonstrated that BR C-6 oxidase activity, a CYP85A1 function, greatly increased during Pst DC3000 infection. In the tga1 tga4 double mutant lacking SA transcription factors TGA1 and TGA4, BR biosynthetic gene CYP85A1 expression was significantly reduced. EMSA and ChIP confirmed that both TGA1 and TGA4 bind to the cis-element present in the CYP85A1 promoter, and castasterone (CS), a bio-active BR, was significantly reduced in tga1 tga4. Taken together, the upregulation of ICS1 expression by BZR1 and CYP85A1 expression by TGA1/4 mutually enhanced endogenous level of BR and SA in Arabidopsis. Furthermore, TGAs and BZR1 interaction at the protein level induces SA-induced immunity through the upregulation of PR5 expression, increasing bacterial resistance in the plant. These results explain the mutual control mechanisms of the synergistic effects BR and SA have on plant defense and confirm BR’s effect on plant defense and growth promotion in A. thaliana.