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

摘要

Citrus seedlings (Citrus sinensis L. Osbeck × 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. Keywords Ammonium - Ammonium transporter - Citrus - High-affinity transport system - Low-affinity transport system