OTS1/2-Dependent DeSUMOylation Boosts Salt Tolerance

Dubbed “ubiquitin's mysterious cousin” (Müller et al., 2001), the small ubiquitin-like modifier (SUMO) reversibly tags proteins to regulate their activity, location, or stability. SUMOylation (SUMO conjugation) is thought to be essential for the survival of all multicellular organisms and, in plants, has been implicated in the abiotic stress response (Kurepa et al., 2003), pathogen defense, abscisic acid signaling, and flower induction (reviewed in Novatchkova et al., 2004). SUMO proteases maintain an equilibrium between the SUMOylated (SUMO conjugated) and deSUMOylated (SUMO deconjugated) state of proteins by cleaving SUMO–substrate bonds and are key players in SUMO-mediated signaling in yeast and mammals (Müller et al., 2001). The ots1 ots2 double mutant is hypersensitive to salt (top), whereas lines overexpressing OTS1 (bottom) are more tolerant to salt stress than wild-type plants. Bars = 1 cm. Next, the scientists investigated the effect of salt stress on SUMO protease levels. While salinity did not affect mRNA levels of OTS1, protein levels of TAP-tagged OTS1 constitutively expressed in an ots1 background rapidly declined in the presence of salt. Preincubation with proteasome inhibitor prevented loss of OTS1 during salt stress. The authors propose that the natural salt sensitivity seen in Arabidopsis is partially due to the accumulation of SUMO conjugates that occurs when OTS1 and OTS2 proteases are degraded. Finally, GFP-tagged OTS1 and YFP-tagged OTS2 were both shown to localize to the nucleus when transiently expressed in tobacco cells. Interestingly, most SUMO substrates in animals, yeast, and plants also localize to the nucleus (Novatchkova et al., 2004). Identifying the SUMO-conjugated substrates of OTS1 and OTS2 and determining how salinity triggers proteasome-dependent degradation of OTS1 are likely to reveal much about the molecular mechanisms underlying salt tolerance in plants.