Auxin distribution is differentially affected by nitrate in roots of two rice cultivars differing in responsiveness to nitrogen

Although ammonium (NH4) is the preferred form of nitrogen over nitrate (NO3) for rice (Oryza sativa), lateral root (LR) growth in roots is enhanced by partial NO3 nutrition (PNN). The roles of auxin distribution and polar transport in LR formation in response to localized NO3 availability are not known. Time-course studies in a split-root experimental system were used to investigate LR development patterns, auxin distribution, polar auxin transport and expression of auxin transporter genes in LR zones in response to localized PNN in Nanguang and Elio rice cultivars, which show high and low responsiveness to NO3, respectively. Patterns of auxin distribution and the effects of polar auxin transport inhibitors were also examined in DR5::GUS transgenic plants. Initiation of LRs was enhanced by PNN after 7 d cultivation in Nanguang but not in Elio. Auxin concentration in the roots of Nanguang increased by approx. 24 after 5 d cultivation with PNN compared with NH4 as the sole nitrogen source, but no difference was observed in Elio. More auxin flux into the LR zone in Nanguang roots was observed in response to NO3 compared with NH4 treatment. A greater number of auxin influx and efflux transporter genes showed increased expression in the LR zone in response to PNN in Nanguang than in Elio. The results indicate that higher NO3 responsiveness is associated with greater auxin accumulation in the LR zone and is strongly related to a higher rate of LR initiation in the cultivar Nanguang.