Comprehensive Biotechnology - 4.17 - Improving Crop Nitrogen Use Efficiency

Nitrogen (N) is a critical nutrient for plant development. Crop N use efficiency (NUE) is necessary to prevent wastage of expensive N fertilizers as well as to prevent N pollution from unutilized fertilizers. Therefore, NUE seeks to maximize crop output with minimal input of N fertilizers. While agronomic improvements optimize the form, dose and timing of fertilizers in accordance with the soil and cropping conditions, plant breeders and biotechnologists focus on the improvement of the crop itself. This requires well-defined phenotype and genotype for NUE, which are beginning to be understood, aided by the recent progress in functional genomics and reverse genetics. Most biotechnological efforts to enhance NUE currently rely on plant nitrate uptake and assimilation or internal remobilization of secondary N metabolites and related pathways. This review summarizes a variety of ways to measure NUE, the limitations of agronomic and microbiological N-fixation and other approaches, while detailing the biotechnological interventions to improve it through transgenics so far. It concludes with newer opportunities emerging from the recent convergence of genomics, functional genomics, phenomics and genome-wide association mapping into systems biology.