| Summary: | <p>Abstract</p> <p>Background</p> <p>Proline (Pro) accumulation is a widespread response of prokaryotic and eukaryotic cells subjected to osmotic stress or dehydration. When the cells are released from stress, Pro is degraded to glutamate by Pro-dehydrogenase (ProDH) and Pyrroline-5-carboxylate dehydrogenase (P5CDH), which are both mitochondrial enzymes in eukaryotes. While <it>P5CDH </it>is a single copy gene in Arabidopsis, two <it>ProDH </it>genes have been identified in the genome. Until now, only <it>ProDH1 </it>(At3g30775) had been functionally characterised.</p> <p>Results</p> <p>We demonstrate vasculature specific expression of the Arabidopsis <it>ProDH2 </it>gene (At5g38710) as well as enzymatic activity and mitochondrial localisation of the encoded protein. Expression levels of <it>ProDH2 </it>are generally low, but increased in senescent leaves and in the abscission zone of floral organs. While sucrose represses <it>ProDH2 </it>expression, Pro and NaCl were identified as inducers. Endogenous <it>ProDH2 </it>expression was not able to overcome Pro sensitivity of <it>ProDH1 </it>mutants, but overexpression of a GFP-tagged form of ProDH2 enabled the utilisation of Pro as single nitrogen source for growth. Amongst two intronic insertion mutants, one was identified as a null allele, whereas the other still produced native <it>ProDH2 </it>transcripts.</p> <p>Conclusions</p> <p>Arabidopsis possesses two functional ProDHs, which have non-redundant, although partially overlapping physiological functions. The two ProDH isoforms differ with respect to spatial, developmental and environmental regulation of expression. While <it>ProDH1 </it>appears to be the dominant isoform under most conditions and in most tissues, <it>ProDH2 </it>was specifically upregulated during salt stress, when <it>ProDH1 </it>was repressed. The characterisation of <it>ProDH2 </it>as a functional gene requires a careful re-analysis of mutants with a deletion of <it>ProDH1</it>, which were so far considered to be devoid of ProDH activity. We hypothesise that ProDH2 plays an important role in Pro homeostasis in the vasculature, especially under stress conditions that promote Pro accumulation.</p>
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