Differential responses of root growth to nutrition with different ammonium/nitrate ratios involve auxin distribution in two tobacco cultivars

Nitrogen (N), the major forms of which are nitrate (NO3–) and ammonium (NH4+), plays an important role in plant growth and mediation of root development. However, the role of auxin in root growth in response to different NH4+/NO3– ratios remains unclear. Two tobacco cultivars (Nicotiana tabacum L.) were adopted in this study, which displayed variant growth features under the situations with sole NO3– nutrition ratio (NH4+/NO3– ratio: 0/100), low NO3– nutrition ratio (NH4+/NO3– ratio: 97/3), and optimal NH4+/NO3– ratio (50/50). We investigated the effects of the different NH4+/NO3– ratios on the formation and elongation of lateral roots (LRs), auxin concentration, DR5::GUS expression, 3H-labeled indole acetic acid ([3H]IAA) transport, and the expression of six PIN genes in tobacco roots. We also examined the effects of exogenous auxin and a transport inhibitor on LRs growth. The results are shown as follows, compared to optimal N nutrition conditions, the biomass and nitrogen (N) accumulation were largely reduced by sole and low NO3– nutrition treatment in NC89, but no difference was observed in Zhongyan 100. In most cases, sole and low NO3– nutrition impaired the elongation and formation of first-order lateral roots (1° LRs), only in NC89, thus reducing the root growth. IAA concentration and DR5::GUS expression levels decreased in roots when NC89 was subjected to sole and low NO3– nutrition media, suggesting that different NH4+/NO3– ratios affect the transport of auxin from leaves to roots. Results were similar following exogenous NAA application to low NO3– nutrition treated seedlings. Based on direct [3H]IAA transport measurement, the transport of polar auxin from shoots to roots decreased due to low NO3– nutrition. PIN4 expression levels were markedly decreased in roots of NC89 by sole and low NO3– nutrition, while they were unaffected in Zhongyan 100 roots. Overall, our findings suggest that LRs formation in tobacco seedlings is regulated by NH4+/NO3– ratios via modifying polar transport of auxin.