Small Ubiquitin-like Modifier Protein SUMO Enables Plants to Control Growth Independently of the Phytohormone Gibberellin

Plants survive adverse conditions by modulating their growth in response to a changing environment. Gibberellins (GAs) play a key role in these adaptive responses by stimulating the degradation of growth-repressing DELLA proteins. GA binding to its receptor GID1 enables association of GID1 with DELLAs. This leads to the ubiquitin-mediated proteasomal degradation of DELLAs and consequently growth promotion. We report that DELLA-dependent growth control can be regulated independently of GA. We demonstrate that when a proportion of DELLAs is conjugated to the Small Ubiquitin-like Modifier (SUMO) protein, the extent of conjugation increases during stress. We identify a SUMO-interacting motif in GID1 and demonstrate that SUMO-conjugated DELLA binds to this motif in a GA-independent manner. The consequent sequestration of GID1 by SUMO-conjugated DELLAs leads to an accumulation of non-SUMOylated DELLAs, resulting in beneficial growth restraint during stress. We conclude that plants have developed a GA-independent mechanism to control growth.