Regulation of replenishment of carbon skeletons for nitrogen assimilation in roots.

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Both increased activity of nitrogen assimilating enzymes and replenishment of carbon skeletons as acceptors of nitrogen are required to continue the assimilation of nitrogen that enters into roots and export of assimilates to shoot. The objective of this study is to make clear regulation of dark carbon fixation in roots necessary for replenishment of carbon skeletons. The activity of phosphoenolpyruvate carboxylase (PEPC) in wheat, barley and tomato roots increased in response to NH_4^+ , and the NH_4^+-fed plants exhibited a 2-2.5-fold higher PEPC activity than NO_3 -fed plants at 7 days after the onset of nitrogen supply. Western blot analysis revealed that the amounts of PEPC subunit proteins increased as reflected in the extractable PEPO activity. Malate, citrate and acidic amino acids inhibited the activity of PEPC purified from tomato roots. These results suggest that the PEPC activity is regulated by de novo protein synthesis and by changes in related-metabolite levels. The dark carbon fixation rate increased remarkably in roots of wheat plants that had been grown with NH_4^+ for 1 day. In response to, PEPC activity increased and PEPC proteins NH_4^+ accumulated in wheat roots. Neither activity nor amounts of PEPC in roots did not increase in the presence of MSX.These findings suggest that the primary assimilation of NH_4^+ in roots is a crucial event for stimulation of dark carbon fixation, which coincides with the increased root PEPC activity when NH_4^+ is supplied.