Methionine sulfoximine suppressed the stimulation of dark carbon fixation by ammonium nutrition in wheat roots

Abstract

To evaluate the role of NH4 + assimilates in dark carbon fixation in roots in providing carbon skeletons expended for NH4 + assimilation, the rate of dark carbon fixation in roots was measured using NaH14CO3. The 14C-metabolites were analyzed in wheat (Triticum aestivum L.) plants grown in NH4 + media for various periods of time with or without methionine sulfoximine (MSX) treatment. The dark carbon fixation rate in the roots of wheat plants that had been grown with NH4 + for 1 d was approximately 6-fold higher than the rate in control roots. The stimulation of dark carbon fixation in NH4 +-grown plants, however, was not observed in MSX-treated roots. In the roots of NH4 +-grown plants, the concentration and 14C-Iabeling of acidic metabolites such as citrate and malate considerably decreased whereas those of basic metabolites, especially asparagine, increased noticeably. With MSX treatment, the incorporation of 14C into basic metabolites was negligible. In response to NH4 +, phosphoenolpyruvate carboxylase (PEPC) activity increased, and PEPC proteins accumulated in wheat roots. Neither activity nor amounts of PEPC in roots increased in the presence of MSX. These findings suggest that primary assimilation of NH4 + in roots is essential for the stimulation of dark carbon fixation, which coincides with the increased activity of root PEPC, to sufficiently replenish carbon skeletons necessary for NH4 + assimilation.