In plants, developmental programmes are regulated by brassinosteroids, which activate transcriptional effectors that drive plant growth. This study shows that in Arabidopisis thaliana, brassinosteroid signalling leads to the accumulation of the transcription factor CES, which has been implicated in brassinosteroid responses, in nuclear bodies and that this is dependent on CES sumoylation in a novel sumoylation motif. Furthermore, phosphorylation at specific residues in that motif antagonized sumoylation and CES nuclear body formation; abolishing these phosphorylation sites promoted CES sumoylation and, moreover, increased its stability and transcriptional activity. Thus, the authors propose that brassinosteroid signalling controls CES protein fate, subnuclear localization and activity.