- Sulfite reductase (SiR) is an essential enzyme of the sulfate assimilation reductive pathway, which catalyzes the reduction of sulfite to sulfide. Here we show that tomato plants (Solanum lycopersicum L.) with impaired SiR expression due to RNA interference (SIR Ri) developed early leaf senescence. The visual chlorophyll degradation in leaves of SIR Ri mutants was accompanied by a reduction of maximal quantum yield, as well as accumulation of H2O2 and of malondialdehyde (MDA), a product of lipid peroxidation. Interestingly, mRNA transcripts and proteins involved in chlorophyll breakdown in the chloroplasts were found to be enhanced in the mutants, while transcripts and their plastidic proteins, functioning in photosystem II, were reduced in these mutants compared with wild-type leaves. As a consequence of SiR impairment the levels of sulfite, sulfate and thiosulfate were higher and glutathione levels were lower compared to wild-type. Unexpectedly, in a futile attempt to compensate for the low glutathione, the activity of adenosine-5'-phosphosulfate reductase was enhanced leading to further sulfite accumulation in SiR Ri plants. Increased sulfite oxidation to sulfate and incorporation of sulfite into sulfoquinovosyl diacylglycerols were not sufficient to maintain low basal sulfite levels, resulting in accumulative leaf damage in mutant leaves. Our results indicate that in addition to its biosynthetic role SiR plays an important role in prevention of premature senescence. The higher sulfite is likely the main reason for the initiation of chlorophyll degradation while the lower glutathione as well as the higher H2O2 and MDA additionally contribute to premature senescence in mutant leaves.