Transcriptome and physiological studies reveal key players in wheat nitrogen use efficiency under both high and low N.

The effective use of available nitrogen (N) to improve crop grain yields provides an important strategy to reduce environmental N pollution and promote sustainable agriculture. However, little is known about the common genetic basis of N use efficiency (NUE) at varying N availability. Two wheat (Triticum aestivum L.) cultivars were grown in the field with high, moderate and low N supply. Zhoumai 27 outperformed Aikang 58 independent of the N supply and showed improved growth, canopy leaf area index, flag leaf surface area, grain number and yield, and enhanced NUE due to both higher N uptake and utilization efficiency. Further, transcriptome and proteome analyses were performed using flag leaves that provide assimilates for grain growth. Results showed that many genes or proteins that are up- or down-regulated under all N regimes are associated with N and carbon metabolism and transport. This was reinforced by cultivar differences in photosynthesis, assimilate phloem transport, and grain protein/starch yield. Overall, our study supports that improving NUE at both high and low N requires distinct adjustments in leaf metabolism and assimilate partitioning. Identified key genes/proteins may individually or concurrently regulate NUE and are promising targets for maximizing crop NUE irrespective of the N supply.