The covalent addition of Small Ubiquitin-Related Modifier (SUMO) to various intracellular proteins is an essential regulatory step in most eukaryotes. Due to its necessity and the large number of putative targets, SUMO is thought to be second only to ubiquitin (Ub) among Ub-fold proteins in terms of regulatory influence. Whereas, ubiquitylation (i.e., the attachment of Ub) is generally associated with protein degradation, SUMOylation appears to have more diverse consequences, including the regulation of transcription, chromatin structure/accessibility, nuclear import, and various protein-protein interactions, and even appears to block the action of Ub by competing for the same binding sites on targets. Paramount to understanding SUMO function(s) is knowing the complete catalog of SUMO targets. In the following addendum we review our recent publication describing the proteomic identification of SUMO substrates in the model plant, Arabidopsis thaliana, and expand our analyses with regard to the changes in SUMOylation patterns that are induced by heat stress. Collectively, our data indicate that SUMOylation is highly dynamic with evidence that SUMO addition globally modifies transcription and chromatin accessibility, especially during stress.