OTS1/2-Dependent DeSUMOylation Boosts Salt Tolerance

Abstract

Dubbed "ubiquitin's mysterious cousin" (Müller et al., 2001Go), 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., 2003Go), pathogen defense, abscisic acid signaling, and flower induction (reviewed in Novatchkova et al., 2004Go). 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., 2001Go). Conti et al. (pages 2894–2908) provide evidence that two SUMO proteases, OVERLY TOLERANT TO SALT1 (OTS1) and OTS2, act redundantly to regulate the salt stress response in Arabidopsis. In a loss-of-function analysis using T-DNA insertion knockouts, the authors demonstrated that the ots1 ots2 double mutant is hypersensitive to salt, whereas the ots1 and ots2 single mutants exhibit the same degree of salt-dependent reduction in root growth as the wild type. Immunoblot analysis of proteins isolated from wild-type Arabidopsis seedlings exposed to a range of salt concentrations revealed a dose-dependent increase in SUMOylation of cellular proteins. Although both the single (ots1 or ots2) and double (ots1 ots2) mutants had higher levels of SUMOylated proteins than the wild type under normal growth conditions, only the double mutant had unusually high levels of SUMOylation when exposed to salt stress. At high salinity, plants overexpressing OTS1 (see figure ) faired much better than both wild-type plants and plants ectopically expressing the active site mutant ots1 (C526S) and had reduced levels of SUMOylated proteins. Thus, OTS1 protease activity appears to impart salt tolerance by reducing levels of SUMOylation.