A NOVEL THIOREDOXIN-LIKE PROTEIN LOCATED IN THE CHLOROPLAST IS INDUCED BY WATER-DEFICIT IN SOLANUM-TUBEROSUM L. PLANTS

By analysing two-dimensional patterns of chloroplastic proteins from S olanum tuberosum, the authors observed the accumulation of a 32-kDa po lypeptide in the stroma of plants subjected to water deficit. N-termin us and internal peptides of the protein, named CDSP 32 for chloroplast ic drought-induced stress protein, showed no obvious homology with kno wn sequences. Using a serum raised against the protein N-terminus, a c DNA encoding CDSP 32 was cloned by screening an expression library. Th e deduced mature CDSP 32 protein is 243 amino acids long and displays typical features of thioredoxins in the C-terminal region (122 residue s). In particular, CDSP 32 contains a CGPC motif corresponding to a th ioredoxin active site and a number of amino acids conferring thioredox in-type structure. The CDSP 32 C-terminal region was expressed as a fu sion protein in Escherichia coil and was shown to possess thioredoxin activity based on reduction assay of insulin disulfide bridges. RNA bl ot analysis showed that CDSP 32 transcript does not accumulate upon mi ld water deficit conditions corresponding to leaf relative water conte nts (RWC) around 85%, but high levels of CDSP 32 transcripts were obse rved for more severe stress conditions (RWC around 70%). In vivo label ling and immunoprecipitation revealed a substantial increase in CDSP 3 2 synthesis upon similar stress conditions. Rewatering of wilted plant s caused decreases in both transcript and protein abundances. In tomat o wild-type plants and ABA-deficient mutants, a similar accumulation o f a CDSP 32-related transcript was observed upon water deficit, most l ikely indicating no requirement for ABA in the regulation of CDSP 32 s ynthesis. Based on these results, it is proposed that CDSP 32 plays a role in preservation of the thiol:disulfide redox potential of chlorop lastic proteins during water deficit.