Superior glucose metabolism supports NH4+ assimilation in wheat to improve ammonium tolerance.

The use of slow-release fertilizers and seed-fertilizers cause localized high-ammonium (NH4 +) environments in agricultural fields, adversely affecting wheat growth and development and delaying its yield. Thus, it is important to investigate the physiological responses of wheat and its tolerance to NH4 + stress to improve the adaptation of wheat to high NH4 + environments. In this study, the physiological mechanisms of ammonium tolerance in wheat (Triticum aestivum) were investigated in depth by comparative analysis of two cultivars: NH4 +-tolerant Xumai25 and NH4 +-sensitive Yangmai20. Cultivation under hydroponic conditions with high NH4 + (5 mM NH4 +, AN) and nitrate (5 mM NO3 -, NN), as control, provided insights into the nuanced responses of both cultivars. Compared to Yangmai20, Xumai25 displayed a comparatively lesser sensitivity to NH4 + stress, as evident by a less pronounced reduction in dry plant biomass and a milder adverse impact on root morphology. Despite similarities in NH4 + efflux and the expression levels of TaAMT1.1 and TaAMT1.2 between the two cultivars, Xumai25 exhibited higher NH4 + influx, while maintaining a lower free NH4 + concentration in the roots. Furthermore, Xumai25 showed a more pronounced increase in the levels of free amino acids, including asparagine, glutamine, and aspartate, suggesting a superior NH4 + assimilation capacity under NH4 + stress compared to Yangmai20. Additionally, the enhanced transcriptional regulation of vacuolar glucose transporter and glucose metabolism under NH4 + stress in Xumai25 contributed to an enhanced carbon skeleton supply, particularly of 2-oxoglutarate and pyruvate. Taken together, our results demonstrate that the NH4 + tolerance of Xumai25 is intricately linked to enhanced glucose metabolism and optimized glucose transport, which contributes to the robust NH4 + assimilation capacity.

A low red/far-red ratio restricts nitrogen assimilation by inhibiting nitrate reductase associated with downregulated TaNR1.2 and upregulated TaPIL5 in wheat (Triticum aestivum L.).

Understanding the physiological mechanism underlying nitrogen levels response to a low red/far-red ratio (R/FR) can provide new insights for optimizing wheat yield potential but has been not well documented. This study focused on the changes in nitrogen levels, nitrogen assimilation and nitrate uptake in wheat plants grown with and without additional far-red light. A low R/FR reduced wheat nitrogen accumulation and grain yield compared with the control. The levels of total nitrogen, free amino acid and ammonium were decreased in leaves but nitrate content was temporarily increased under a low R/FR. The nitrate reductase (NR) activity in leaves was more sensitive to a low R/FR than glutamine synthetase, glutamate synthase, glutamic oxalacetic transaminase and glutamic-pyruvic transaminase. Further analysis showed that a low R/FR had little effect on the NR activation state but reduced the level of NR protein and the expression of encoding gene TaNR1.2. Interestingly, a low R/FR rapidly induced TaPIL5 expression rather than TaHY5 and other members of TaPILs in wheat, suggesting that TaPIL5 was the key transcription factor response to a low R/FR in wheat and might be involved in the downregulation of TaNR1.2 expression. Besides, a low R/FR downregulated the expression of TaNR1.2 in leaves earlier than that of TaNRT1.1/1.2/1.5/1.8 in roots, which highlights the importance of NR and nitrogen assimilation in response to a low R/FR. Our results provide revelatory evidence that restricted nitrate reductase associated with downregulated TaNR1.2 and upregulated TaPIL5 mediate the suppression of nitrogen assimilation under a low R/FR in wheat.

Enhanced Stomatal Conductance Supports Photosynthesis in Wheat to Improved NH4+ Tolerance.

The impact of ammonium (NH4+) stress on plant growth varies across species and cultivars, necessitating an in-depth exploration of the underlying response mechanisms. This study delves into elucidating the photosynthetic responses and differences in tolerance to NH4+ stress by investigating the effects on two wheat (Triticum aestivum L.) cultivars, Xumai25 (NH4+-less sensitive) and Yangmai20 (NH4+-sensitive). The cultivars were grown under hydroponic conditions with either sole ammonium nitrogen (NH4+, AN) or nitrate nitrogen (NO3-, NN) as the nitrogen source. NH4+ stress exerted a profound inhibitory effect on seedling growth and photosynthesis in wheat. However, these effects were less pronounced in Xumai25 than in Yangmai20. Dynamic photosynthetic analysis revealed that the suppression in photosynthesis was primarily attributed to stomatal limitation associated with a decrease in leaf water status and osmotic potential. Compared to Yangmai20, Xumai25 exhibited a significantly higher leaf K+ concentration and TaAKT1 upregulation, leading to a stronger stomatal opening and, consequently, a better photosynthetic performance under NH4+ stress. In conclusion, our study suggested stomatal limitation as the primary factor restricting photosynthesis under NH4+ stress. Furthermore, we demonstrated that improved regulation of osmotic substances contributed to higher stomatal conductance and enhanced photosynthetic performance in Xumai25.