Increased biomass accumulation in maize grown in mixed nitrogen supply is mediated by auxin synthesis

The use of mixed nitrate and ammonium as a nitrogen source can improve plant growth. Here, we used metabolomics and transcriptomics to study the underlying mechanisms. Maize plants were grown hydroponically in the presence of three forms of nitrogen (nitrate alone, 75%/25% nitrate/ammonium, and ammonium alone). Plants grown with mixed nitrogen had a higher photosynthetic rate than those supplied only with nitrate, and had the highest leaf area and shoot and root biomass among the three nitrogen treatments. In shoot and root, the concentration of nitrogenous compounds (ammonium, glutamine, and asparagine) and carbohydrates (sucrose, glucose, and fructose) in plants with a mixed nitrogen supply was higher than that with nitrate supply, but lower than that with ammonium supply. The activity of the related enzymes (glutamate synthase, asparagine synthase, phosphoenolpyruvate carboxylase, invertase, and ADP-glucose pyrophosphorylase) changed accordingly. Specifically, the mixed nitrogen source enhanced auxin synthesis via the shikimic acid pathway, as indicated by the higher levels of phosphoenolpyruvate and tryptophan compared with the other two treatments. The expression of corresponding genes involving auxin synthesis and response was up-regulated. Supply of only ammonium resulted in high levels of glutamine and asparagine, starch, and trehalose hexaphosphate. We conclude that, in addition to increased photosynthesis, mixed nitrogen supply enhances leaf growth via increasing auxin synthesis to build a large sink for carbon and nitrogen utilization, which, in turn, facilitates further carbon assimilation and nitrogen uptake.