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Microbial nitrification in soils is a major contributor to nitrogen (N) loss in agricultural systems. Some plants can secrete organic substances that act as biological nitrification inhibitors (BNIs), and a small number of BNIs have been identified and characterized. However, virtually no research has focused on the important food crop, rice (Oryza sativa). Here, 19 rice varieties were explored for BNI potential on the key nitrifying bacterium Nitrosomonas europaea. Exudates from both indica and japonica genotypes were found to possess strong BNI potential. Older seedlings had higher BNI abilities than younger ones; Zhongjiu25 (ZJ25) and Wuyunjing7 (WYJ7) were the most effective genotypes among indica and japonica varieties, respectively. A new nitrification inhibitor, 1,9-decanediol, was identified, shown to block the ammonia monooxygenase (AMO) pathway of ammonia oxidation and to possess an 80% effective dose (ED80) of 90μl−1. Plant N-use efficiency (NUE) was determined using a 15N-labeling method. Correlation analyses indicated that both BNI abilities and 1,9-decanediol amounts of root exudates were positively correlated with plant ammonium-use efficiency and ammonium preference. These findings provide important new insights into the plant–bacterial interactions involved in the soil N cycle, and improve our understanding of the BNI capacity of rice in the context of NUE.

Increased use of nitrogen fertilizers has deleterious impact on the environment. Increase in yield potential at low nitrogen supply is regarded as a cereal breeding goal for future agricultural sustainability. Although natural variations of nitrogen transporters have been investigated, key genes associated with assimilation remain largely unexplored for nitrogen use efficiency (NUE) enhancement. Here, we identified a NIN‐like protein NLP4 associated with NUE through a GWAS in rice. We found that OsNLP4 transactivated OsNiR encoding nitrite reductase that was critical in nitrogen assimilation in rice. We further constructed quadrupling NREs (Nitrate‐responsive cis‐elements) in the promoter of OsNiR (p4xNRE:OsNiR) and enhanced nitrogen assimilation significantly. We demonstrated that OsNLP4‐OsNiR increased tiller number and yield through enhancing nitrogen assimilation and NUE. Our discovery highlights the genetic modulation of OsNLP4‐OsNiR signalling cascade as a strategy for high NUE and yield breeding in rice.

Nitrogen (N) is an extremely important macronutrient for plant growth and development. It is the main limiting factor in most agricultural production. However, it is well known that the nitrogen use efficiency (NUE) of rice gradually decreases with the increase of the nitrogen application rate. In order to clarify the underlying metabolic and molecular mechanisms of this phenomenon, we performed an integrated analysis of the rice transcriptome and metabolome. Both differentially expressed genes (DEGs) and metabolite Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis indicated that carbon and nitrogen metabolism is significantly affected by nitrogen availability. Further analysis of carbon and nitrogen metabolism changes in rice under different nitrogen availability showed that high N inhibits nitrogen assimilation and aromatic metabolism pathways by regulating carbon metabolism pathways such as the tricarboxylic acid (TCA) cycle and the pentose phosphate pathway (PPP). Under low nitrogen, the TCA cycle is promoted to produce more energy and α-ketoglutarate, thereby enhancing nitrogen transport and assimilation. PPP is also inhibited by low N, which may be consistent with the lower NADPH demand under low nitrogen. Additionally, we performed a co-expression network analysis of genes and metabolites related to carbon and nitrogen metabolism. In total, 15 genes were identified as hub genes. In summary, this study reveals the influence of nitrogen levels on the regulation mechanisms for carbon and nitrogen metabolism in rice and provides new insights into coordinating carbon and nitrogen metabolism and improving nitrogen use efficiency in rice.

Summary The importance of the nitrate (NO3−) transporter for yield and nitrogen‐use efficiency (NUE) in rice was previously demonstrated using map‐based cloning. In this study, we enhanced the expression of the OsNRT2.1 gene, which encodes a high‐affinity NO3− transporter, using a ubiquitin (Ubi) promoter and the NO3−‐inducible promoter of the OsNAR2.1 gene to drive OsNRT2.1 expression in transgenic rice plants. Transgenic lines expressing pUbi:OsNRT2.1 or pOsNAR2.1:OsNRT2.1 constructs exhibited the increased total biomass including yields of approximately 21% and 38% compared with wild‐type (WT) plants. The agricultural NUE (ANUE) of the pUbi:OsNRT2.1 lines decreased to 83% of that of the WT plants, while the ANUE of the pOsNAR2.1:OsNRT2.1 lines increased to 128% of that of the WT plants. The dry matter transfer into grain decreased by 68% in the pUbi:OsNRT2.1 lines and increased by 46% in the pOsNAR2.1:OsNRT2.1 lines relative to the WT. The expression of OsNRT2.1 in shoot and grain showed that Ubi enhanced OsNRT2.1 expression by 7.5‐fold averagely and OsNAR2.1 promoters increased by about 80% higher than the WT. Interestingly, we found that the OsNAR2.1 was expressed higher in all the organs of pUbi:OsNRT2.1 lines; however, for pOsNAR2.1:OsNRT2.1 lines, OsNAR2.1 expression was only increased in root, leaf sheaths and internodes. We show that increased expression of OsNRT2.1, especially driven by OsNAR2.1 promoter, can improve the yield and NUE in rice.

In sub-Saharan Africa (SSA), rice production from smallholder farms is challenged because of a lack of fertilizer inputs and nutrient-poor soils. Therefore, improving nutrient efficiency is particularly important for increasing both fertilizer use and rice yield. This review discusses how to improve the return from fertilizer input in terms of agronomic N use efficiency (AE N ), that is, the increase in grain yield per kg of applied N, for rice production in SSA. The AE N values we summarized here revealed large spatial variations even within small areas and a certain gap between researcher-led trials and smallholder-managed farms. Experimental results suggest AE N can be improved by addressing spatial variations in soil-related factors such as P, S, Zn, and Si deficiencies and Fe toxicity in both irrigated and rainfed production systems. In rainfed production systems, differences in small-scale topography are also important which affects AE N through dynamic changes in hydrology and variations in the contents of soil organic carbon and clay. Although empirical evidence is further needed regarding the relationship between soil properties and responses to fertilizer inputs, recent agricultural advances have generated opportunities for integrating these micro-topographical and soil-related variables into field-specific fertilizer management. These opportunities include UAV (unmanned aerial vehicle) technology to capture microtopography at low cost, database on soil nutrient characteristics at high resolution and more numbers of fertilizer blending facilities across SSA, and interactive decision support tools by use of smartphones on site. Small-dose nursery fertilization can be also alternative approach for improving AE N in adverse field conditions in SSA. ABBREVIATIONS: AE N : agronomic nitrogen use efficiency; FISP: farm input subsidy program; VCR: value cost ratio; SOC: soil organic carbon; SSA: sub-Saharan Africa; UAV: unmanned aerial vehicle

Drought stress which often occurs during early growth stage is one constraint in sugarcane production. In this study, the response of sugarcane to drought and nitrogen application for physiological and agronomical characteristics was investigated. Two water regimes (well-watered and drought stress from 60 to 120 day after transplanting) and four nitrogen levels (0, 4.4, 8.8 and 13.2 g pot −1 equivalent to 0, 90, 180 and 270 kg ha −1 , respectively) were assigned in a Split-plot design with three replications. The results showed that photosynthetic responses to light intensity and intercellular CO 2 concentrations of sugarcane were different between fertilized and non-fertilized treatments. Photosynthetic rates of 180 and 270 N treatments, normally, were significantly higher than that of 90 N, but not significant at drought conditions. Photosynthetic rates of 0 N treatment were the lowest under both conditions. Higher nitrogen application supported higher photosynthetic rate, stomatal conductance, and chlorophyll content because of higher nitrogen concentration accumulated into the leaf. Drought significantly reduced the potential photosynthetic rate, stomatal conductance, SPAD, leaf area, and biomass production. Higher nitrogen applications with larger root system could support higher photosynthetic activities to accumulate more dry mass. Strong positive coefficient between photosynthetic and biomass nitrogen use efficiency and drought tolerance index may suggest that higher nitrogen use efficiency could help plants have higher ability to tolerate drought stress.

The limited availability of nitrogen (N) is a fundamental challenge for many crop plants. We have hypothesized that the relative crop photosynthetic rate ( ) is exponentially constrained by certain plant-specific enzyme activities, such as ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), NADP-glyceraldehyde-3-phosphate dehydrogenase (NADP-G3PDH), 3-phosphoglyceric acid (PGA) kinase, and chloroplast fructose-1,6-bisphosphatase (cpFBPase), in and . We conducted a literature search to compile information from previous studies on C and C crop plants, to examine the photosynthetic rate responses to limited leaf [N] levels. We found that in s, NADP-malic enzyme (NADP-ME), PEP carboxykinase (PCK), and Rubisco activities were positively correlated with . A positive correlation was also observed between both phosphoenolpyruvate carboxylase (PEPC) and Rubisco activity with leaf [N] in . Key enzyme activities responded differently to in C and C plants, suggesting that other factors, such as leaf [N] and the stage of leaf growth, also limited specific enzyme activities. The relationships followed the best fitting exponential relationships between key enzymes and the rate in both C and C plants. It was found that C species absorbed less leaf [N] but had higher [N] assimilation rates ( ) and higher maximum photosynthesis rates ( ), i.e., they were able to utilize and invest more [N] to sustain higher carbon gains. All C species studied herein had higher [N] storage (N ) and higher absorption of [N], when compared with the C species. N was the main [N] source used for maintaining photosynthetic capacity and leaf expansion. Of the nine C species assessed, rice had the greatest , thereby absorbing more leaf [N]. Elevated CO (eCO ) was also found to reduce the leaf [N] and in rice but enhanced the leaf [N] and N use efficiency of photosynthesis in maize. We concluded that eCO affects [N] allocation, which directly or indirectly affects . These results highlight the need to further study these physiological and biochemical processes, to better predict how crops will respond to eCO concentrations and limited [N].

Previous studies have demonstrated that poplar is sexually dimorphic in its responses to environmental stresses and provided evidence of sex‐related differences in protein composition, gene transcription and morphological and physiological processes. However, sexual competition of poplar has not yet been studied. We employed Populus cathayana males and females as a model to investigate intra‐ and intersexual competition under well‐watered condition and drought stress. Our results indicated that competition significantly affected sexual dimorphism of P. cathayana under different watering regimes. Sexual competition was environment‐dependent and modified by drought. In females, drought intensified the negative effect of intrasexual competition. Similar resource utilization patterns of females may aggravate pressure for resources under drought stress. Under intersexual competition, females showed a greater competitive ability than males under well‐watered condition, while males showed a slight competitive advantage under drought stress. Intersexual competition seems to alleviate the sexual dimorphism of P. cathayana in response to drought stress, as females increase fitness and males decrease fitness compared with intrasexual competition. Sexual dimorphism in resource utilization patterns and niche segregation may contribute to sexual competition in a given environment. Sexual competition was found to affect competitive ability and fitness of both sexes, which may cause spatial segregation of sexes in P. cathyana populations.

We used light and confocal microscopy to visualize bacteria in leaf and bract cells of more than 30 species in 18 families of seed plants. Through histochemical analysis, we detected hormones (including ethylene and nitric oxide), superoxide, and nitrogenous chemicals (including nitric oxide and nitrate) around bacteria within plant cells. Bacteria were observed in epidermal cells, various filamentous and glandular trichomes, and other non-photosynthetic cells. Most notably, bacteria showing nitrate formation based on histochemical staining were present in glandular trichomes of some dicots (e.g., Humulus lupulus and Cannabis sativa). Glandular trichome chemistry is hypothesized to function to scavenge oxygen around bacteria and reduce oxidative damage to intracellular bacterial cells. Experiments to assess the differential absorption of isotopic nitrogen into plants suggest the assimilation of nitrogen into actively growing tissues of plants, where bacteria are most active and carbohydrates are more available. The leaf and bract cell endosymbiosis types outlined in this paper have not been previously reported and may be important in facilitating plant growth, development, oxidative stress resistance, and nutrient absorption into plants. It is unknown whether leaf and bract cell endosymbioses are significant in increasing the nitrogen content of plants. From the experiments that we conducted, it is impossible to know whether plant trichomes evolved specifically as organs for nitrogen fixation or if, instead, trichomes are structures in which bacteria easily colonize and where some casual nitrogen transfer may occur between bacteria and plant cells. It is likely that the endosymbioses seen in leaves and bracts are less efficient than those of root nodules of legumes in similar plants. However, the presence of endosymbioses that yield nitrate in plants could confer a reduced need for soil nitrogen and constitute increased nitrogen-use efficiency, even if the actual amount of nitrogen transferred to plant cells is small. More research is needed to evaluate the importance of nitrogen transfer within leaf and bract cells of plants.

Breeding high-yielding and nitrogen-efficient maize ( Zea mays L.) hybrid varieties is a strategy that could simultaneously solve the problems of resource shortages and environmental pollution. We conducted a 2-year field study using four nitrogen application rates (0, 150, 225, and 300 kg N hm −2 ) and two maize hybrid varieties (ZD958 and QS101) to understand the plant traits related to high grain yields and high nitrogen use efficiency (NUE). We found that ZD958 had a higher grain yield and nitrogen accumulation in the shoots at harvest as well as a higher NUE at lower nitrogen application rates (0 and 150 kg hm –2 ) than QS101. The grain yields and NUE were almost identical for the two hybrid varieties at nitrogen application rates of 225 and 300 kg N hm –2 . Compared with QS101, ZD958 had higher above-ground and below-ground biomass amounts, a deeper root distribution, longer root length, root active absorption area, greater grain filling rate, and higher photosynthetic NUE than QS101 at lower nitrogen application rates. Our results showed that ZD958 can maintain a higher grain yield at lower nitrogen rates in a similar manner to N-efficient maize hybrid varieties. The selection of hybrids such as ZD958 with a deeper root distribution and higher photosynthetic NUE can increase the grain yield and NUE under low nitrogen conditions.