Abstract
Small ubiquitin-like modifier proteases 1 and 2 (SUMO1/2) have been linked to the regulation of salicylic acid (SA)-mediated defence signalling in Arabidopsis thaliana. In order to define the role of the SUMO proteases OVERLY TOLERANT TO SALT1 and -2 (OTS1/2) in defence and to provide insight into SUMO1/2-mediated regulation of SA signalling, we examined the status of SA-mediated defences in ots1/2 mutants. The ots1 ots2 double mutant displayed enhanced resistance to virulent Pseudomonas syringae and higher levels of SA compared with wild-type (WT) plants. Furthermore, ots1 ots2 mutants exhibited upregulated expression of the SA biosynthesis gene ICS1 in addition to enhanced SA-responsive ICS1 expression beyond that of WT. SA stimulated OTS1/2 degradation and promoted accumulation of SUMO1/2 conjugates. These results indicate that OTS1 and -2 act in a feedback loop in SA signalling and that de novo OTS1/2 synthesis works antagonistically to SA-promoted degradation, adjusting the abundance of OTS1/2 to moderate SA signalling. Accumulation of SUMO1/2 conjugates coincides with SA-promoted OTS degradation and may play a positive role in SA-mediated signalling in addition to its repressive roles reported elsewhere.
Highlights
The small ubiquitin-like modifier (SUMO) takes its name from its similarity to the well-studied post-translational modifier ubiquitin and is conserved throughout all kingdoms of eukaryotes (Müller et al, 2001)
We showed that OTS1 and -2 negatively regulate salicylic acid (SA) biosynthesis and propose that de novo synthesis and SA-promoted degradation of OVERLY TOLERANT TO SALT1 and -2 (OTS1/2) antagonistically adjust the abundance of this negative regulator depending on the level of pathogen threat
Growth of the virulent bacterial plant pathogen Pst was 10 times lower in the ots1 ots2 double mutant compared with WT Columbia-0 plants (Fig. 1A), while WT plants transformed with constructs overexpressing OTS1 driven by the cauliflower mosaic virus 35S promoter (OTS1-HOx1 and -2) did not exhibit significantly different susceptibility to virulent Pst compared with non-transformants (Fig. 1A)
Summary
The small ubiquitin-like modifier (SUMO) takes its name from its similarity to the well-studied post-translational modifier ubiquitin and is conserved throughout all kingdoms of eukaryotes (Müller et al, 2001). SUMO may facilitate new protein– protein interactions through SUMO-interacting motifs (SIMs), and compete with other post-translational modifications such as ubiquitination and acetylation (Müller et al, 2001; Kerscher, 2007) In addition to their SUMO processing activities, SUMO proteases possess deconjugative activity capable of cleaving SUMO from target proteins, providing reversibility and buffering to the pathway (Mukhopadhyay and Dasso, 2007; Hickey et al, 2012). Tomato (Pst) has emerged from mutation of the SUMO E3 ligase SIZ1 and was further substantiated with knockdown of either SUMO1 or -2 in the mutant background of the other (SUMO1 or -2), which led to increases in SA, SA-O-β-glucoside (SAG), and Pst resistance (Lee et al, 2006; van den Burg et al, 2010) These findings have indicated that SUMO1/2 suppress activation of SA-mediated responses via the SIZ1 SUMO ligase (van den Burg and Takken, 2010). We provide evidence that accumulation of SUMO conjugates results from SA-promoted degradation of OTS1/2 and may play a signalling role
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