Tissue-specific accumulation of transcript encoding Delta(1)-pyrroline-5-carboxylate reductase in Arabidopsis thaliana

Delta(1)-pyrroline-5-carboxylate reductase (EC 1.5.1.2; PSCR) catalyses the final step in proline biosynthesis from glutamate and ornithine, where Delta(1)-pyrroline-5-carboxylate acts as an intermediate. In this study, differential accumulation of mRNA transcript encoding PSCR was investigated in tissues of the flowering stem of mature Arabidopsis thaliana (L.) Heynh. In situ hybridisation indicated that under normal growth conditions, the highest concentration of PSCR transcripts occurs in the cortical parenchyma, phloem, vascular cambium and pith parenchyma in the vicinity of the protoxylem. Levels of signal generated in these tissues increased in plants deprived of water for eight days. Genomic Southern analysis suggests the existence of only a single copy of the gene encoding P5CR in Arabidopsis. This conclusion eliminates the possibility that different genes encoding PSCR may exhibit different tissue-specific patterns of transcript accumulation. Our findings suggest that the upregulation of proline biosynthesis in response to water-deprivation stress may be regulated in a tissue-specific manner. The tissue-specific nature of proline biosynthesis suggests that this may be an important consideration in the production of transgenic plants capable of synthesising elevated levels of proline. It is possible that differential expression of P5CR may in part be due to differences in the water status of different tissues. The latter explanation for the patterns of expression observed would suggest that proline accumulation is strictly a cellular response to water deficit.