Some aspects of the function of abscisic acid (ABA) in the control of proline (PRO) metabolism in canola leaf discs (CLD) subjected in vitro to consecutive hyper-osmotic (stress) and hypo-osmotic (recovery) treatments have been investigated. PRO accumulation in response to stress conditions relies on both stimulation of its synthesis via enhancement of transcription of the gene and activity of Delta(1) -pyrroline-5-carboxylate synthetase (P5CS) and inhibition of its degradation via inactivation of PRO dehydrogenase (PDH). These changes were partly reversed under recovery conditions. Thus both PDH mRNA and PDH activity increased while P5CS mRNA decreased. Surprisingly P5CS activity remained high even after a 20-h period of rehydration. Exogenously supplied ABA at recovery inhibited net PRO consumption and this could be associated with downregulation of PDH gene expression and PDH activity. Under these conditions ABA hardly upregulated P5CS gene expression while P5CS activity first transiently decreased and then reached a value close to that found under stressing conditions. Experiments with CLD supplied with either methionine sulphoximine or gabaculine, brought preliminary evidence for a significant synthesis of PRO from glutamate during recovery that replenished the proline pool(s) and provoked a negative effect on the net rate of PRO consumption. Consequently, it is suggested that the availability of both PRO precursors and ABA could be determinant in the control of the amount of residual PRO present in CLD after the period of recovery. This level also seemed to depend on the amount of P5CS transcripts induced under stress conditions. However, the results obtained with turgid leaf discs treated with ABA indicate that the ABA status of the tissues, necessary for inducing the proline response, is not sufficient to determine their PRO content because it remained relatively low despite the stimulation of P5CS expression and P5CS activity.
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