Effects of exogenous putrescine on glycolysis and Krebs cycle metabolism in cucumber leaves subjected to salt stress

The effects of exogenous putrescine (8 mmol L-1) on the carbohydrate content, glycolytic metabolism, citric acid cycle intermediates, key enzyme activities and their corresponding gene expression in cucumber (cucumis sativus L. cv. Jingyou NO.4) subjected to salt stress (75 mmol L-1 NaCl) in solution culture was studied. Phosphofructokinase (PFK), pyruvate kinase (PK) and phosphoenolpyruvate pyruvate kinase (PEPC) activities were significantly decreased in leaves when exposed to salt stress. NaCl stress caused the PFK and PK gene expression and the content of pyruvate to significantly decrease, while PEPC gene expression increased. Salt stress also significantly reduced isocitrate dehydrogenase, malate dehydrogenase and succinate dehydrogenase and their levels of transcription, causing decreases in the citric acid, succinic acid and malic acid content in leaves. Exogenous putrescine reversed the salt stress and increased gene expression and, in turn, the key enzymes involved in the glycolysis pathway and Krebs cycle, which significantly increased the amounts of organic acids and pyruvate produced, promoting the release of more energy currency (ATP and ADP). These results suggest that Put effectively participate glycolysis pathway and Krebs cycle, reducing the excessive accumulation of carbohydrates in the leaves, and providing more material and energy for the defense salt-induced injury, thus enhances cucumber seedling to salt stress tolerance.