Glucose elevates NRT2.1 protein levels and nitrate transport activity independently of its HXK1-mediated stimulation of NRT2.1 expression.

Mineral nutrient uptake and assimilation is closely coordinated with the production of photosynthate to supply nutrients for growth. In Arabidopsis, nitrate uptake from the soil is mediated by genes encoding high- and low-affinity transporters that are transcriptionally regulated by both nitrate and photosynthate availability. In this study we have studied the interactions of nitrate and glucose on gene expression, nitrate transport and growth using gin2-1, which is defective in sugar responses. We confirm and extend previous work (Lejay et al., 2003; Lejay et al., 2008) by showing that HXK1-mediated oxidative pentose phosphate pathway (OPPP) metabolism is required for glucose-mediated NRT2.1 expression. Treatment with pyruvate and shikimate, two products derived from intermediates of the oxidative pentose phosphate pathway OPPP destined for amino acid production, restores wild-type levels of NRT2.1 expression, suggesting that metabolites derived from OPPP metabolism can, together with glucose, directly stimulate high levels of NRT2.1 expression. Nitrate-mediated NRT2.1 expression is not influenced by gin2-1, showing that glucose does not influence NRT2.1 expression through nitrate-mediated mechanisms. We also show that glucose stimulates NRT2.1 protein levels and transport activity independently of its HXK1-mediated stimulation of NRT2.1 expression, demonstrating another possible post-transcriptional mechanism influencing nitrate uptake. In gin2-1 plants nitrate responsive biomass growth was strongly reduced, showing that the supply of OPPP metabolites is essential for assimilating nitrate for growth.