Growth of maize on mixed nitrate and ammonium promotes auxin synthesis and biomass accumulation

Mixed nitrate (NO3-) and ammonium (NH4+) as nitrogen source can improve plant growth. The underlying mechanism was explored by using metabolomics and transcriptome. Maize plants were grown hydroponically with three nitrogen forms (nitrate, 75/25 nitrate/ammonium and ammonium). Mixed N increased photosynthetic rate compared to nitrate supply, and had the highest leaf area, shoot and root biomass among the three nitrogen treatments. In shoot and root, the concentration of nitrogenous compounds (NH4+, Gln, Asn) and carbohydrates (sucrose, glucose, fructose) in plant with mixed N supply was higher than that with nitrate supply, but lower than that with ammonium supply. The activity of the related enzymes (GS, ASNS, PEPCase, invertase, AGPase) changed accordingly. Specifically, mixed N enhanced IAA synthesis via shikimic acid pathway, as indicated by the higher PEP and tryptophan compared to the other two treatments. The expression of corresponding genes involving IAA synthesis and response ...