OTS1-dependent deSUMOylation increases tolerance to high copper levels in Arabidopsis.

The conjugation of SUMO (small ubiquitin-like modifier) to protein substrates is a reversible process (SUMOylation/deSUMOylation) that regulates plant development and stress responses. The essential metal copper (Cu) is required for normal plant growth, but excess amounts are toxic. The SUMO E3 ligase, SIZ1, and SIZ1-mediated SUMOylation function in plant tolerance to excess Cu. It is unknown whether deSUMOylation also contributes to Cu tolerance in plants. Here, we report that OTS1, a protease that cleaves SUMO from its substrate proteins, participates in Cu tolerance in Arabidopsis thaliana (Arabidopsis). OTS1 loss-of-function mutants (ots1-2 and ots1-3) displayed increased sensitivity to excess Cu. Redox homeostasis and the balance between SUMOylation and deSUMOylation were disrupted in the ots1-3 mutant under excess Cu conditions. The ots1-3 mutant accumulated higher levels of Cu in both shoots and roots compared to wild type. Specific Cu-related metal transporter genes were upregulated due to the loss-of-function of OTS1, which might explain the high Cu levels in ots1-3. These results suggest that the SUMOylation/deSUMOylation machinery is activated in response to excess Cu, and modulates Cu homeostasis and tolerance by regulating both Cu uptake and detoxification. Together, our findings provide insight into the biological function and regulatory role of SUMOylation/deSUMOylation in plant tolerance to Cu.

Stability and localization of 14-3-3 proteins are involved in salt tolerance in Arabidopsis.

KEY MESSAGE: Salt stress induces the degradation of 14-3-3 proteins, and affects the localization of 14-3-3 λ. Both the modulation of 14-3-3 protein stability and the subcellular localization of these proteins are involved in salt tolerance in plants. Salt tolerance in plants is regulated by multiple signaling pathways, including the salt overly sensitive (SOS) pathway, of which the SOS2 protein is a key component. SOS2 is activated under salt stress to enhance salt tolerance in plants. We previously identified 14-3-3 λ and κ as important regulators of salt tolerance. Both proteins interact with SOS2 to inhibit its kinase activity under normal growth conditions. In response to salt stress, 14-3-3 proteins dissociate from SOS2, releasing its activity and activating the SOS pathway to confer salt tolerance (Zhou et al. Plant Cell 26:1166-1182, 2014). Here we report that salt stress promotes the degradation of 14-3-3 λ and κ, at least in part via the actions of SOS3-like calcium binding protein 8/calcineurin-B-like10, and also decreases the plasma membrane (PM) localization of 14-3-3 λ. Salt stress also partially represses the interaction of SOS2 and 14-3-3 λ at the PM, but activates PM-localized SOS2. Together, these results suggest that, in plants, both the modulation of 14-3-3 stability and the subcellular localization of these proteins in response to salt stress are important for SOS2 activation and salt tolerance. These data provide new insights into the biological roles of 14-3-3 proteins in modulating salt tolerance.