Strategies for Identification of Genes Toward Enhancing Nitrogen Utilization Efficiency in Cereals

Global cereal demand will increase up to 38% by 2025, and to achieve it in a sustainable way, 60% increase in global nitrogen (N) use will be necessary. In cereals ~30 to 50% of the applied N is taken up by the crop, and the rest is lost in the environment causing pollution. Hence, improvement of N use efficiency (NUE) in cereals is really important. The NUE is the total biomass or grain yield produced per unit of applied N fertilizer. Soil and plant management practices play a key role toward enhancing N recovery, but again it greatly depends on environmental conditions. Another option for improvement of NUE is the genetic strategy. Broadly, NUE has two components, N uptake efficiency (NUpE), which is N acquisition by the plant per unit of available N in the soil, and N utilization efficiency (NUtE), which is yield per unit of acquired N by the plant. As NUtE is directly related to the crop yield, it depends on subcomponent N assimilation, remobilization, and finally efficient utilization of assimilated N for starch biosynthesis in the grain. Understanding the mechanisms and gene regulating of these processes, exploiting genotypic variant in each subcomponent (N uptake, assimilation, and remobilization) to find genes and superior alleles is crucial for the improvement of NUE in crop plants. In addition, the studies on starch metabolism during grain filling are an important factor for N utilization. To study this, genotypes with similar background of uptake and assimilation but differing in grain filling should be taken into consideration. Global metabolomic profiling of these genotypes, transcriptome profiling, identification, and mapping of quantitative trait loci (QTLs) in combination with marker-assisted selection (MAS), analyzing mutants defective in their normal response to N limitation, and studying plants that show better growth under N-limiting conditions are different options to study the N-utilization efficiency and gene identification. In the first topic, we have highlighted the N application and its effect on yield in cereals. Introduction of N-responsive genotype during green revolution has enhanced yield, but indiscriminate use of fertilizer mainly N fertilizer has caused severe damage to environment. In the subsequent topic, we have defined NUE as a whole; later the main focus was on biological NUE and their different components. Thereafter we described strategies for genetic improvement to reduce N use without much compromising yield. Primarily we tried to highlight candidate genes and their role in NUE reported in cereals as well as model plant system. We have also described the advance molecular techniques to identify the gene in strategic manner. As a part of molecular breeding, QTL identification and its introgression are described in one of the topics at the last part.