The effect of NH4+ on phosphoenolpyruvate carboxykinase gene expression, metabolic flux and citrate content of citrus juice sacs.

Citrate is one of the most important metabolites determining the flavour of citrus fruit. It has been reported that nitrogen supply may have an impact on acid level of fruit. Here, the relationship between nitrogen metabolism and citrate catabolism was studied in pumelo juice sacs. Differences in metabolites, gene expression and flux distributions were analyzed in juice sacs incubated in medium with and without NH 4 +. Compared with those incubated with NH 4 +, juice sacs under nitrogen deficiency exhibited enhanced flux through phosphoenolpyruvate carboxykinase (PEPCK) and accelerated consumption of citrate, while the other two TCA cycle efflux points, through malic enzyme (ME) and glutamate dehydrogenase (GDH), were both repressed. Consistent with the estimated fluxes, the expression of PEPCK1 was upregulated under nitrogen deficiency, while that of GDH1 , GDH2 , NAD-ME1 and NADP-ME2 were all repressed. Thus, we propose that PEPCK1 contributes to citrate degradation under nitrogen limitation. • Metabolism in pomelo juice sacs was analyzed systematically and quantitatively with stoichiometric metabolic flux analysis. • Nitrogen deficiency accelerated decrease in citrate content of juice sacs. • Nitrogen deficiency enhanced reaction of PEPCK and suppressed other TCA cycle exporting reactions. • CitPEPCK1 gene expression was activated under different conditions where inorganic nitrogen is absent. [ABSTRACT FROM AUTHOR]