Transgenic tobacco plants overexpressing pyruvate phosphate dikinase increase exudation of organic acids and decrease accumulation of aluminum in the roots

摘要

Pyruvate phosphate dikinase (PPDK) catalyses the phosphorylation of pyruvate (+ ATP + Pi) yielding phosphoenolpyruvate (+ AMP + PPi). PPDK levels are induced upon drought and salt stress and both cytosolic and plastidic isoforms are found in higher plants. Herein we investigate the role of this enzyme upon aluminum exposure. We generated transgenic tobacco plants expressing either a plastidic (PPDK) or a putative cytosolic (PPDK) Mesembryanthemum crystallinum PPDK under the transcriptional control of the patatin B33 promoter and found that they can ameliorate Al stress. Enhanced root growth in the presence of Al was observed in transgenic lines, while that of wild-type (WT) seedlings was strongly inhibited. Al content in root tips of stressed plants was reduced in the transgenic lines compared with WT, as judged by Eriochrome cyanine R (ECR) staining and atomic absorption spectrometry. Moreover, all transformants excreted higher levels of citric acid cycle intermediates (citrate and malate) than the WT seedlings did. Interestingly, we found different behavior between the two sets of transformants, based on results of ECR root staining, organic acid exudation and Al accumulation. In general, lines harboring the PPDK construct were more tolerant to Al stress than those expressing the PPDK construct. We conclude that the overexpression of PPDK can serve to protect roots against Al toxicity. Furthermore, we provide the first physiological evidence for a link between Al stress and PPDK in plants and postulate that such approach could be successful under certain edaphic conditions, namely acid soils.