Extent and significance of non-covalent SUMO interactions in plant development.

Posttranslational modification of proteins by SUMO plays essential roles in plant growth and development. We, and others, have previously identified Arabidopsis proteins covalently modified by SUMO. In our recent report, we assessed the extent and significance of non-covalent SUMO interactions with plant proteins by using three Arabidopsis SUMO isoforms as baits in large-scale yeast two-hybrid screens. We identified six proteins that regulate the reversible methylation and demethylation of histones and DNA, and six proteins that we showed to be the plant homologs of SUMO-Targeted Ubiquitin E3 Ligases (STUbLs). This implicates SUMO in a variety of developmental programs including floral transition, genome imprinting, and transcriptional control of a large number of genes. Intriguingly, whereas only two STUbLs were identified in other organisms, the identification of six STUbLs in Arabidopsis is consistent with the more complex repertoire of genes regulating the SUMO system in plants. Some Arabidopsis STUbLs appear to have retained roles conserved throughout eukaryotes, whereas others may have evolved novel plant functions. AT-STUbL4, for example, contributes to the floral transition by reducing the levels of the floral repressor Cycling Dof Factor 2 (CDF2). I discuss our findings and the potential they provide to study the role of SUMO in plant development.