Functional identification of MdSIZ1 as a SUMO E3 ligase in apple

• The MdSIZ1 is a SUMO (small ubiquitin-related modifier) E3 ligase in apple and plays an important role in responding to stress conditions.
• The SUMOylation of MdCIbHLH1 is dependent on MdSIZ1 in vitro.
• MdSIZ1 mediated SUMOylation in response to stress conditions in vivo.
• The ectopic expression of MdSIZ1 in Arabidopsis siz1-2 mutant plants partially complemented the morphological mutant phenotypes and enhanced the levels of SUMO conjugation.

SUMOylation, the conjugation of target proteins with SUMO (small ubiquitin-related modifier), is a type of post-translational modification in eukaryotes and involves the sequential action of activation (E1), conjugation (E2) and ligation (E3) enzymes. In Arabidopsis, the AtSIZ1 protein is a SUMO E3 ligase that promotes the conjugation of SUMO proteins to target substrates. Here, we isolated and identified a SUMO E3 ligase, MdSIZ1, in apple, which was similar to AtSIZ1. SUMOylation analysis showed that MdSIZ1 had SUMO E3 ligase activity in vitro and in vivo. SUMO conjugation was increased by high temperatures, low temperatures, and abscisic acid (ABA). The ectopic expression of MdSIZ1 in Arabidopsis siz1-2 mutant plants partially complemented the morphological mutant phenotype and enhanced the levels of SUMO conjugation. Taken together, these results suggest that MdSIZ1-mediated SUMO conjugation of target proteins is an important process that regulates the adaptation of apple plants to various environmental stresses.