Ammonium transport and CitAMT1 expression are regulated by N in Citrus plants

Citrus seedlings (Citrus sinensis L. Osbeck x Poncirus trifoliata Blanco) were used to describe the effects of different N treatments on the NH4+ influx mediated by high- and low-affinity transport systems (HATS and LATS, respectively) and CitAMT1 gene expression. Results show that Citrus plants favor NH4+ over NO3- influx mediated by HATS and LATS when both N sources are present in the nutrient solution and Citrus plants display a much higher capacity to take up NH4+ than NO3-. Furthermore, NH4+ exerts a regulatory effect on NH4+ HATS activity and CitAMT1 expression, both are down-regulated by high N status of the plant, but specifically stimulated by NH4+ and the balance between these two opposite effects depends on the prior nutrition regime of the plant. On the other hand, supply of NO3- inhibits CitAMT1 expression but doesn't affect NH4+ HATS activity on the roots. To explain this discrepancy, it is possible that other CitAMT1 transporters, up-regulated by N limitation, but not repressed by NO3- could be involved in the stimulation of NH4+ HATS activity under pure NO3- nutrition or CitAMT1 transporter could be regulated at the post-transcriptional level.