Lateral root elongation in maize is related to auxin synthesis and transportation mediated by N metabolism under a mixed NO3− and NH4+ supply

A mixed nitrate (NO3−) and ammonium (NH4+) supply can promote root growth in maize (Zea mays), however, the changes in root morphology and the related physiological mechanism under different N forms are still unclear. Here, maize seedlings were grown hydroponically with three N supplied in three different forms (NO3− only, 75/25 NO3−/NH4+ and NH4+ only). Compared with sole NO3− or NH4+, the mixed N supply increased the total root length of maize but did not affect the number of axial roots. The main reason was the increased total lateral root length, while the average lateral root (LR) length in each axle was only slightly increased. In addition, the average LR density of 2nd whorl crown root under mixed N was also increased. Compared with sole nitrate, mixed N could improve the N metabolism of roots (such as the N influx rate, nitrate reductase (NR) and glutamine synthase (GS)enzyme activities and total amino content of the roots). Experiments with exogenously added NR and GS inhibitors suggested that the increase in the average LR length under mixed N was related to the process of N assimilation, and whether the NR mediated NO synthesis participates in this process needs further exploration. Meanwhile, an investigation of the changes in root-shoot ratio and carbon (C) concentration showed that C transportation from shoots to roots may not be the key factor in mediating lateral root elongation, and the changes in the sugar concentration in roots further proved this conclusion. Furthermore, the synthesis and transportation of auxin in axial roots may play a key role in lateral root elongation, in which the expression of ZmPIN1B and ZmPIN9 may be involved in this pathway. This study preliminarily clarified the changes in root morphology and explored the possible physiological mechanism under a mixed N supply in maize, which may provide some theoretical basis for the cultivation of crop varieties with high N efficiency.