Unraveling differential characteristics and mechanisms of nitrogen uptake in wheat cultivars with varied nitrogen use efficiency.

Nitrogen uptake is crucial to wheat nitrogen use efficiency (NUE). The study's findings indicate that both high- and low-NUE cultivars exhibited highest nitrogen uptake efficiency (NupE) under 0.2 mM nitrogen. Under 2 mM nitrogen, their NupEs decrease significantly, while uptakes to NO were notably higher than that of NH. Strikingly, high-NUE cultivars exhibited a significantly higher NH uptake rate than low NUE cultivars, resulting in a marked improvement in their ability to take up nitrogen. The NUEs of the cultivars with 5 mM nitrogen were almost half that of 2 mM nitrogen. NO uptake primarily occurred in the mature zone of roots, while NH uptake took place in the root tip meristem and elongation zones. Notably, the NH uptake in root tip meristematic zone of high-NUE cultivar was significantly higher than that of low NUE cultivar. Furthermore, the NO uptake of high-NUE cultivar in the root mature zone was significantly higher than that of low-NUE cultivar under 2 mM nitrogen. These findings were consistent with the significantly higher expression levels of TaAMT in root tip and of TaNRT in root mature area of high-NUE cultivar compared to low-NUE cultivar, respectively, enabling efficient absorption of NO and NH and transport of NO to shoot. The high-NUE cultivars showed elevated expression of amino acid transporters further promoting nitrogen uptake, and conversion of nitrogen into ureides and amino acids further facilitated inorganic nitrogen uptake by roots. The differential findings offer valuable insights into novel variety breeding of high NUE in the future.