Abstract
NRT1.1 is the first nitrate transport protein cloned in plants and has both high- and low-affinity functions. It imports and senses nitrate, which is modulated by the phosphorylation on Thr101 (T101). Structural studies have revealed that the phosphorylation of T101 either induces dimer decoupling or increases structural flexibility within the membrane, thereby switching the NRT1.1 protein from a low- to high-affinity state. Further studies on the adaptive regulation of NRT1.1 in fluctuating nitrate conditions have shown that, at low nitrate concentrations, nitrate binding only at the high-affinity monomer initiates NRT1.1 dimer decoupling and priming of the T101 site for phosphorylation activated by CIPK23, which functions as a high-affinity nitrate transceptor. However, nitrate binding in both monomers retains the unmodified NRT1.1, maintaining the low-affinity mode. This NRT1.1-mediated nitrate signalling and transport may provide a key to improving the efficiency of plant nitrogen use. However, recent studies have revealed that NRT1.1 is extensively involved in plant tolerance of several adverse environmental conditions. In this context, we summarise the recent progress in the molecular mechanisms of NRT1.1 dual-affinity nitrate transport/signalling and focus on its expected and unexpected roles in plant abiotic stress resistance and their regulation processes.
Highlights
Nitrogen (N) is a primary constituent of proteins and nucleotides that are essential for life
We briefly summarise the important milestones in the discovery process, dual-affinity features, and structural basis of the dual transport/sensing function of nitrate transporter 1 (NRT1).1 in Arabidopsis
NRT1.1 was first characterised as a low-affinity nitrate transporter (LAT), as disruption of NRT1.1 function in nrt1.1 mutants led to a >80% decrease in nitrate uptake in sufficient nitrate (25 mm KNO3) growth medium compared with that of the wild-type plants (Huang et al, 1996)
Summary
Nitrogen (N) is a primary constituent of proteins and nucleotides that are essential for life. NRT1.1 was first characterised as a low-affinity nitrate transporter (LAT), as disruption of NRT1.1 function in nrt1.1 mutants led to a >80% decrease in nitrate uptake in sufficient nitrate (25 mm KNO3) growth medium compared with that of the wild-type plants (Huang et al, 1996) Consistent with this result, a recent study by Ye et al (2019) reported that the nrt1.1 mutants showed approximately 50% less nitrate uptake than the wild type under 4 mm nitrate conditions, indicating that the contribution of LATS of NRT1.1 at high nitrate supply was at least 50%. NRT1.1 is indispensable for maintaining plant growth under both high- and low nitrate growth conditions
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