SUMOylation of PHYTOCHROME INTERACTING FACTOR 3 promotes photomorphogenesis in Arabidopsis thaliana.

-In Arabidopsis thaliana phytochrome B (phyB) is the dominant receptor of photomorphogenic development under red light. phyB interacts with a set of downstream regulatory proteins, including PHYTOCHROME INTERACTING FACTOR 3 (PIF3). The interaction of PIF3 and the photo-activated phyB leads to the rapid phosphorylation and degradation of PIF3 and also to the degradation of phyB, which events are required for proper photomorphogenesis. -Here we report that PIF3 is SUMOylated on the Lys13 (K13) residue and we could detect this posttranslational modification in a heterologous experimental system and also in planta. -We also found, that the SUMO acceptor site mutant PIF3(K13R) binds stronger to the target promoters than its SUMOylated, wild-type counterpart. Seedlings expressing PIF3(K13R) show elongated hypocotyl response, elevated photoprotection and higher transcriptional induction of red light responsive genes compared with plantlets expressing wild-type PIF3. -These observations are supported by the lower level of phyB in those plants which possess only PIF3(K13R) indicating that SUMOylation of PIF3 alters photomorphogenesis also via the regulation of phyB amount. Conclusively, whereas SUMOylation is generally connected to different stress responses it also fine tunes light signalling by reducing the biological activity of PIF3 thus promoting photomorphogenesis.