Root extension and nitrate transporter up-regulation induced by nitrogen deficiency improves nitrogen status and plant growth at the seedling stage of winter wheat (Triticum aestivum L.)

- Low-nitrate conditions reduced plant growth and N content, but increased root extension to increase N uptake area at 120 h after LN treatment.
- A low NO3− concentration induced upregulation of TaNRT1.1, TaNRT2.1 and TaNRT2.2, followed by upregulation of TaNAR2.1 after short treatment time.
- Both NR/GS activity increased initially and H+-ATPase activity increased due to increase of TaNR and TaHA1 transcript abundance, which could supply energy and increase N assimilation in low-nitrate conditions.
- The low-nitrate tolerant phenotype showed greater acclimation to low-nitrate conditions than the low-nitrate sensitive phenotype by maintaining higher N uptake area and higher TaNRTs transcript levels.

Characterizing the responses of plant growth and nitrogen (N) uptake to low-nitrogen conditions will provide information towards elucidating the acclimation mechanism of N-efficient wheat seedlings. Two winter wheat cultivars, Yangmai158 (low-nitrate tolerant) and Zaoyangmai (low-nitrate sensitive), were supplied either 0.25 mmol L−1 nitrate (low nitrate nutrition; LN) or 5 mmol L−1 nitrate (control; CK), to investigate the responses of root morphology and nitrate transporter expression in a hydroponic experiment. Decreases in plant dry weight and N content were greater in Zaoyangmai than in Yangmai158, suggesting that Yangmai158 has a better growth status and N-storage ability in low-nitrate treatment. Additionally, root dry weight and uptake area increased to a greater extent in Yangmai158, especially in nodal roots. The decrement in nitrate concentration stimulated simultaneous upregulation of TaNRT1.1, TaNRT2.1 and TaNRT2.2 in Yangmai158, followed by upregulation of TaNAR2.1 in low-nitrate conditions. Plasma membrane H+-ATPase activity (PM H+-ATPase), which is involved in N-uptake energy supply, increased significantly in nodal roots of Yangmai158, consistent with increasing TaHA1 transcript abundance. Furthermore, the activities of nitrate reductase (NR) and glutamine synthetase (GS) were much higher in Yangmai158 initially, which could be a feedback signal for N-uptake. However, no difference of NR and GS activity between LN and CK was found in Zaoyangmai. These results suggest that larger root extension and nitrate transporter upregulation are the major reasons for the superior acclimation of Yangmai158 to nitrogen deficient growth conditions.