Salt stress and glycolytic regulation in suspension-cultured cells of the mangrove tree, Bruguiera sexangula

Suspension-cultured cells derived from seedlings of Bruguiera sexangula are tolerant to NaCl. To examine the influence of long-term salt stress on glycolysis, we determined the effect of 100 mM NaCl on the activities of two key enzymes, phosphofructokinase (PFK, EC 2.7.1.11) and pyruvate kinase ( PK, EC 2.7.1.40), and on the bypass enzymes, pyrophosphate: fructose-6-phosphate phosphotransferase (PFP, EC 2.7.1.90), phosphoenolpyruvate carboxylase ( PEPC, EC 4.1.1.49) and phosphoenolpyruvate phosphatase (PEPase, EC 3.1.3.60). From 10 days after NaCl treatment, increases were found in the activities of PFK, PK and PEPC. In contrast, there was little or no difference in the activities of PFP or PEPase. The short-term effect of salt stress was also investigated. NaCl (150 mM) caused a 1.4-fold increase in respiratory O-2 uptake at 24 h after treatment. Alongside this respiratory rise, drastic changes in the levels of glycolytic metabolites were found: a decrease in the levels of glucose, glucose- 6-phosphate and fructose-6-phosphate, and an increase in the levels of fructose-1, 6-bisphosphate and metabolites of the later steps of the glycolytic pathway. The crossover diagram of metabolites suggests that NaCl stimulates those steps catalysed by PFK and/or PFP. The in vitro activities of partially purified PFK and PFP were increased by the addition of 150 mM NaCl. The effect of salt on the kinetic properties of PFK and PFP was studied, and possible control mechanisms of glycolysis on salt stress are discussed.