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First published online October 10, 2008; 10.1105/tpc.108.201011

The Plant Cell 20:2542

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IN BRIEF

OTS1/2-Dependent DeSUMOylation Boosts Salt Tolerance

Kathleen L. Farquharson

Science Editor

kfarquharson{at}aspb.org

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.


Figure 1
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  		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., 2004Go). 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.

Footnotes

www.plantcell.org/cgi/doi/10.1105/tpc.108.201011

REFERENCES

Conti, L., Price, G., O'Donnell, E., Schwessinger, B., Dominy, P., and Sadanandom, A. (2008). Small ubiquitin-like modifier proteases OVERLY TOLERANT TO SALT1 and -2 regulate salt stress responses in Arabidopsis. Plant Cell 20: 2894–2908.

Kurepa, J., Walker, J.M., Smalle, J., Gosink, M.M., Davis, S.J., Durham, T.L., Sung, D.-Y., and Vierstra, R.D. (2003). The small ubiquitin-like modifier (SUMO) protein modification system in Arabidopsis. J. Biol. Chem. 278: 6862–6872.[Abstract/Free Full Text]

Müller, S., Hoege, C., Pyrowolakis, G., and Jentsch, S. (2001). Sumo, ubiquitin's mysterious cousin. Nat. Rev. Mol. Cell Biol. 2: 202–210.[CrossRef][Web of Science][Medline]

Novatchkova, M., Budhiraja, R., Coupland, G., Eisenhaber, F., and Bachmair, A. (2004). SUMO conjugation in plants. Planta 220: 1–8.[CrossRef][Web of Science][Medline]


Related articles in Plant Cell:

Small Ubiquitin-Like Modifier Proteases OVERLY TOLERANT TO SALT1 and -2 Regulate Salt Stress Responses in Arabidopsis
Lucio Conti, Gillian Price, Elizabeth O'Donnell, Benjamin Schwessinger, Peter Dominy, and Ari Sadanandom
Plant Cell 2008 20: 2894-2908. [Abstract] [Full Text]  




This Article
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