Regulation of brassinosteroid signaling through Arabidopsis transcription factor TCP8 by a conserved post-translational mechanism

The ability of plants to respond to changing environmental conditions is dependent on their capacities to produce fast, transient, and reversible changes to stress-responsive gene expression profiles. Post-translation modification of transcription factors has been identified as a contributing mechanism to these responses but has yet to be explored mechanistically in the TCPs, a plant-specific TF family known to be broadly involved in phytohormone signaling governing biotic and abiotic stress responses. Here, we use a recombinant expression system and proteomic approach to identify the specific residues at which AtTCP8 is covalently modified by the small peptide SUMO and demonstrate that loss of key lysine residues abolishes modification of the TF. We validate these observations by demonstrating the failure of a sumoylation-site-deficient AtTCP8 mutant to complement a known tcp8 BR signaling deficiency phenotype natively in Arabidopsis and reduced transcriptional activity of the protein at a known regulatory target promoter. Further, we show that class I TCP orthologs in the model non-vascular plant Physcomitrium patens are sumoylated as well in our system, a novel finding for TFs in this organism. These data support a conserved model for post-translational regulation by SUMO of phytohormone-responsive gene expression between vascular and non-vascular plants.