Sulfite Reductase Protects Plants against Sulfite Toxicity    

Abstract

Plant sulfite reductase (SiR; Enzyme Commission 1.8.7.1) catalyzes the reduction of sulfite to sulfide in the reductive sulfate assimilation pathway. Comparison of SiR expression in tomato (Solanum lycopersicum ‘Rheinlands Ruhm’) and Arabidopsis (Arabidopsis thaliana) plants revealed that SiR is expressed in a different tissue-dependent manner that likely reflects dissimilarity in sulfur metabolism between the plant species. Using Arabidopsis and tomato SiR mutants with modified SiR expression, we show here that resistance to ectopically applied sulfur dioxide/sulfite is a function of SiR expression levels and that plants with reduced SiR expression exhibit higher sensitivity than the wild type, as manifested in pronounced leaf necrosis and chlorophyll bleaching. The sulfite-sensitive mutants accumulate applied sulfite and show a decline in glutathione levels. In contrast, mutants that overexpress SiR are more tolerant to sulfite toxicity, exhibiting little or no damage. Resistance to high sulfite application is manifested by fast sulfite disappearance and an increase in glutathione levels. The notion that SiR plays a role in the protection of plants against sulfite is supported by the rapid up-regulation of SiR transcript and activity within 30 min of sulfite injection into Arabidopsis and tomato leaves. Peroxisomal sulfite oxidase transcripts and activity levels are likewise promoted by sulfite application as compared with water injection controls. These results indicate that, in addition to participating in the sulfate assimilation reductive pathway, SiR also plays a role in protecting leaves against the toxicity of sulfite accumulation.

Impairment in Sulfite Reductase Leads to Early Leaf Senescence in Tomato Plants

“…Suppression by RNA interference was recently shown to down-regulate SiR transcript, protein, and activity (Yarmolinsky et al, 2013). Independent lines with impaired SiR expression due to RNA interference (SIR Ri), SIR Ri37, and SIR Ri40, displayed phenotypes of yellow spots on the older leaves and were employed in this study to unravel the connection between SiR impairment and accelerated chlorophyll degradation.…”

“…1; Supplemental Fig. S1), probably due to multiple insertions in its genome (Yarmolinsky et al, 2013).…”

“…One can expect that damage to the chloroplast in SIR Ri plants would be associated with a disturbance in sulfur metabolism that occurred in SiR-impaired plants (Khan et al, 2010;Yarmolinsky et al, 2013). To address this question, we measured major sulfur metabolites in SIR Ri and wild-type tomato plants.…”

“…Although SiR exhibits a semiconstitutive pattern of expression (Khan et al, 2010), this enzyme was induced by short-term sulfite application (Brychkova et al, 2007; Yarmolinsky et al, 2013) as well as long-term SO 2 fumigation . By employing tomato (Solanum lycopersicum) and Arabidopsis SiR mutants overexpressing or impaired in SiR activity, it was recently demonstrated that SiR not only functions as a component of the sulfate assimilation reductive pathway but also plays a role in protecting plants against sulfite toxicity (Yarmolinsky et al, 2013).…”

“…By employing tomato (Solanum lycopersicum) and Arabidopsis SiR mutants overexpressing or impaired in SiR activity, it was recently demonstrated that SiR not only functions as a component of the sulfate assimilation reductive pathway but also plays a role in protecting plants against sulfite toxicity (Yarmolinsky et al, 2013). Importantly, the plastidic 70-kD SiR enzyme acts as a binding protein in the plastidic DNA organization in addition to its functions in sulfite conversion (Cannon et al, 1999;Sato et al, 2001;Chi-Ham et al, 2002;Sekine et al, 2002Sekine et al, , 2007Kang et al, 2010).…”