Environmental and developmental signals modulate proline homeostasis: evidence for a negative transcriptional regulator

In many plants, osmotic stress induces a rapid accumulation of proline through de novo synthesis from glutamate, This response is thought to play a pivotal role in osmotic stress tolerance [Kishor, P. B. K., Hong, Z., Miao, G.-H., Hu, C.-A. A. and Verma, D. P. S. (1995) Plant Physiol. 108, 1387-1394], During recovery from osmotic stress, accumulated proline is rapidly oxidized to glutamate and the first step of this process is catalyzed by proline oxidase, We have isolated a full-length cDNA from Arabidopsis thaliana, At-POX. which maps to a single locus on chromosome 3 and that encodes a predicted polypeptide of 499 amino acids showing significant similarity with proline oxidase sequences from Drosophila and Saccharomyces cerevisiae (55.5% and 45.1%, respectively), The predicted location of the encoded polypeptide is the inner mitochondrial membrane, RNA gel blot analysis revealed that At-POX mRNA levels declined rapidly upon osmotic stress and this decline preceded proline accumulation, On the other hand, At-POX mRNA levels rapidly increased during recovery, Free proline, exogenously added to plants, was found to be an effective inducer of At-POX expression; indeed, At-POX was highly expressed in flowers and mature seeds where the proline level is higher relative to other organs of Arabidopsis, Our results indicate that stress- and developmentally derived signals interact to determine proline homeostasis in Arabidopsis