Nitrogen is the main limiting nutrient after carbon, hydrogen and oxygen for photosynthetic process, phyto-hormonal, proteomic changes and growth-development of plants to complete its lifecycle. Excessive and inefficient use of N fertilizer results in enhanced crop production costs and atmospheric pollution. Atmospheric nitrogen (71%) in the molecular form is not available for the plants. For world’s sustainable food production and atmospheric benefits, there is an urgent need to up-grade nitrogen use efficiency in agricultural farming system. The nitrogen use efficiency is the product of nitrogen uptake efficiency and nitrogen utilization efficiency, it varies from 30.2 to 53.2%. Nitrogen losses are too high, due to excess amount, low plant population, poor application methods etc., which can go up to 70% of total available nitrogen. These losses can be minimized up to 15–30% by adopting improved agronomic approaches such as optimal dosage of nitrogen, application of N by using canopy sensors, maintaining plant population, drip fertigation and legume based intercropping. A few transgenic studies have shown improvement in nitrogen uptake and even increase in biomass. Nitrate reductase, nitrite reductase, glutamine synthetase, glutamine oxoglutarate aminotransferase and asparagine synthetase enzyme have a great role in nitrogen metabolism. However, further studies on carbon–nitrogen metabolism and molecular changes at omic levels are required by using “whole genome sequencing technology” to improve nitrogen use efficiency. This review focus on nitrogen use efficiency that is the major concern of modern days to save economic resources without sacrificing farm yield as well as safety of global environment, i.e. greenhouse gas emissions, ammonium volatilization and nitrate leaching.

Nitrogen (N) plays an important role in sugarcane (Saccharum spp. hybrids) growth and development; however, long-term effects of N application levels on cane and sugar production in different sugarcane cultivars under field conditions remain unclear. In this study, we investigate the agronomic, yield, and quality traits in three sugarcane cultivars (GT11, B9, and ROC22) under different N levels (0, 150, and 300 kg/ha urea) from 2015 to 2019. Continuous four-year field experiments of plant and ratoon crops were carried out by using two-factor split-plot design. The results showed that N fertilizer application improved the tillering rate, stalk diameter, plant height, stalk weight, millable stalks/ha, cane yield, sugar yield and juice rate of cane, and the difference between N application and non-N application was significant. The cane yield, millable stalks/ha, juice rate, and juice gravity purity increased with the increase of N application, but the milled juice brix and sucrose % cane decreased with the increase of N application. The sugar yield was the highest at 150 kg/ha urea application, while the cane yield was the highest at 300 kg/ha urea application. Different N fertilizer application levels significantly regulated the activities of glutamic pyruvic transaminase (GPT) and glutamic oxaloacetic transaminase (GOT) and the contents of chlorophyll and nitrate N in plant leaves, which reflected the regulation in nitrogen metabolism and alteration in dry matter production and distribution, cane yield and sugar accumulation in different sugarcane cultivars. During the four-year experiment duration, the cane yield and sugar yield generally showed ROC22 > B9 > GT11. These data suggested that 300 kg/ha urea application was suitable for the plant and first ratoon crops, and 150 kg/ha urea application was suitable for the second and third ratoon crops. Both cane and sugar yields could be the highest in a four-year production cycle under this circumstance.

Sugarcane (Saccharum spp. hybrid) is a noteworthy crop in the world for sugar and bioenergy production. In the present study, transcriptomic analysis was steered for a high sucrose mutant Guixuan B9 (GXB9) and the low sucrose mother genotype B9 of sugarcane during sugar accumulating stage. A total of 112,170 unigenes were obtained, and 106,026 (94.52%) were annotated by using the available public databases nonredundant, Gene ontology, COG, Kyoto Encyclopedia of Genes and Genomes, Swiss‐Prot, and Pfam. Functional assignment analysis of unigenes exhibited that they were active in a diversity of metabolic pathways. The vital unigenes for sucrose metabolism and accumulation were relatively characterized in the immature internodes (int. 5, 6) and maturing internodes (int. 13, 14) of GXB9 in comparison with B9. The differentially expressed unigenes (DEGs) encoding sucrose phosphate synthase (SPS, http://www.chem.qmul.ac.uk/iubmb/enzyme/EC2/4/1/14.html), sucrose synthase (SuSy, http://www.chem.qmul.ac.uk/iubmb/enzyme/EC2/4/1/13.html), and invertase (http://www.chem.qmul.ac.uk/iubmb/enzyme/EC3/2/1/26.html), which are the vital enzymes functioning in the biosynthesis and splitting of sucrose, in the internodal tissues were revealed by using Illumina Hiseq 2000 platform, and verified by quantitative reverse transcription‐polymerase chain reaction analysis. The DEGs encoding SPS proteins SPS1, SPS2, SPS4, and SPS5 were upregulated in the maturing internodes of GXB9 compared with B9. The DEGs encoding SuSy proteins were majorly upregulated in the immature internodes of GXB9, and those encoding the enzymes involved in trehalose synthesis were downregulated in GXB9 compared with B9. The results in this study have revealed the opportunities for future sugarcane improvement by concentrating on the DEGs linked to sucrose metabolic pathways by applying omics technologies.

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Background The significance of sugarcane as a major crop is due to the production of sucrose and a significant source of ethanol. Sucrose phosphate synthase (SPS), Sucrose synthase (SuSy), and cell wall invertase (CWIN) genes regulate sucrose metabolism and accumulation in high sucrose sugarcane; such genes are significant for manipulating sucrose content. The current study explored the genes associated with sucrose metabolism and accumulation in a high sucrose sugarcane mutant clone GXB9 compared to its low sucrose parental clone B9. Results A total of 100, 262 differentially expressed genes were obtained, and 69, 637 (69.46%) got annotation in public databases COG, GO, KEGG, KOG, Pfam, eggNOG, NR, and Swiss-Prot. The SPS5 gene was significantly up-regulated in the maturing internodes of GXB9 and downregulated in B9. The SuSy and CWIN genes were up-regulated in the immature internodes; however, no expression was found in the maturing internodes of GXB9. The genes involved in trehalose synthesis were downregulated in GXB9 while up-regulated in B9. The cellulose synthase (CeS) genes were significantly up-regulated in the immature internodes than in maturing internodes of GXB9. Conclusions The key finding of this research suggests that the SPS5 gene has a predominant role in enhancing the sucrose accumulation ability of GXB9. No expression of SuSy and CWIN genes in maturing internodes and downregulation of trehalose genes in GXB9 may also have contributed to the higher sucrose accumulation in the stalk of high sucrose mutant. The current finding is a source for sugarcane breeding, particularly improving sucrose content. Graphical Abstract