Regulation of carbon and nitrogen metabolisms in rice roots by 2-oxoglutarate at the level of hexokinase

Feeding experiments were designed, to investigate the role of 2-oxoglutarate (2-OG) in regulation of carbon and nitrogen metabolisms in non-photosynthetic tissues of rice (Oryza sativa L.), and enzyme activities involved in the metabolisms as well as contents of several relating metabolites were determined in the roots. The enhancement of 2-OG level by feeding 2-OG or metabolizable sugars [sucrose (Suc) or glucose (Glc)], rather than by feeding non-metabolizable carbon sources (mannose or mannitol), led to increase in enzyme activities, including hexokinase (HXK, EC 2.7.1.1), nicotinamide adenine dinucleotide phosphate (NADP)(+)-dependent isocitrate dehydrogenase (NADP(+)-ICDH, EC 1.1.1.42), phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), glutamine synthetase (GS, EC 6.3.1.2) and the reduced form of nicotinamide adenine dinucleotide (NADH)-dependent glutamate synthase (NADH-GOGAT, EC 1.4.1.14). In addition, the increase in ammonium uptake, glutamine and glutamate (Glu) as well as the decrease in soluble carbohydrates were observed. The effects of feeding 2-OG or metabolizable sugars were reversed by feeding of N-acetyl-glucosamine (NAG; a HXK inhibitor). The decreased 2-OG level by the feeding of NAG alone led to increase in soluble carbohydrates and decrease in the enzyme activities, ammonium uptake as well as Glu content. The effects of NAG were reversed by supply of 2-OG, Suc and Glc. These results suggest that nitrogen uptake and assimilation as well as their related carbohydrate metabolism in rice roots were regulated in coordination by 2-OG level, and HXK activity was involved in the regulation of 2-OG.