Transcriptional differences in gene families of the ascorbate-glutathione cycle in wheat during mild water deficit

When comparing the responses of two wheat (Triticum aestivum L.) genotypes, the drought-tolerant Plainsman V and the drought-sensitive Cappelle Desprez, to reduced amounts of irrigation water, we found differences in ascorbate metabolism: both ascorbate oxidation and transcription levels of enzymes processing ascorbate were changed. Relative transcript levels of ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) isoenzymes, predicted to localize in distinct subcellular organelles, showed different transcriptional changes in the two genotypes. Among APX coding mRNAs, expression levels of two cytosolic (cAPX I, II) and a thylakoid-bound (tAPX) variants increased significantly in Plainsman V while a cytosolic (cAPX I) and a stromal (sAPX II) APX coding transcripts were found to be higher in Cappelle Desprez after a 4-week-long water-deficit stress. Examining the MDARs, two cytosolic isoforms (cMDAR I, II) displayed significant up-regulation of mRNA levels in the sensitive genotype, whereas only one of them (cMDAR II) did in the tolerant cultivar. We found an up-regulated chloroplastic DHAR (chlDHAR) mRNA only in the sensitive Cappelle Desprez. However, increased expression levels of a cytosolic GR (cGR) and a chloroplastic GR (chlGR) were detected only in the tolerant Plainsman V. After 4 weeks of reduced irrigation, a significantly lower ascorbate/dehydroascorbate ratio was detected in leaves of the sensitive Cappelle Desprez than in the tolerant Plainsman V. Our results indicate that more robust transcription of ascorbate-based detoxification machinery may prevent an adverse shift of the cellular redox balance.