Genes for Use in Improving Nitrate Use Efficiency in Crops

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Nitrogen (N) is a major factor in plant growth and crop yield (Marschner, 1995 ). The growth and development of plants are often profoundly affected by the form and abundance of the nitrogen supply because the form of nitrogen signifi cantly alters intracellular metabolisms. Restricted or inappropriate nitrogen supply or form alters development including shoot to root ratio, root development, seed development, and the rate of senescence. Activities of enzymes of primary metabolism respond to N supply, but so do the enzymes of photosynthesis, secondary metabolism, and metabolic control (Figure 7.1 ). Since N fertilizers are rapidly depleted from most soil types and symbiotic nitrogen fi xation in many legumes ceases in mid season, all fi eld crops have some degree of dependence on organic or inorganic nitrogenous fertilizer (Marschner, 1995 ). Nitrogen must often be supplied to the soils surrounding growing crops in excess, before or during the growing season. Nitrogen that is not assimilated can contaminate the environment (Cherfas, 1990 ; Burkholder et al., 1992 ; David et al., 1997 ) and have negative effects on human health in food and water (Tannenbaum et al., 1978 ; Mirvish, 1985 ; Moller, 1990 ; Duncan et al., 1998 ). Therefore, nitrogen use by crops has to be optimized. Nitrogen use effi ciency (NUE) is the metric commonly targeted for improvement by breeders and biotechnologists in this fi eld (Lightfoot et al., 2007 , 2008 ). Enhanced NUE by plants should also enable crops to be cultivated under low nitrogen availability, slow release fertilizers, stress conditions, or poor soil quality (Hirel et al., 2007 ). NUE is defi ned as the percentage of fertilizer N recovered by a crop. NUE is the product of many components (Moll et al., 1982 ). On a coarse scale NUE is determined by nitrogen productivity (NP) and the mean residence time of nitrogen (MRT). In turn they are expressions of (1) N uptake effi ciency; (2) the fraction of N translocated to the seed; (3) the translocation index; (4) soil supply effi ciency; and (5) developmental effi ciency. Consequently, on a fi ne scale, thousands of genes and hundreds of regulatory networks contribute to NUE from seed germination to fi nal harvest in the plant and hundreds more to the microbial activity in the soil. Improvement of cereal NUE could enable practices directed toward reducing groundwater contamination by nitrates in Illinois and the USA (Lightfoot et al., 1999 ; Ameziane et al., 2000 ). The decreased undesirable environmental effects and reduced dietary nitrates could decrease several human and animal health problems. For brevity this chapter focuses on genes and regulatory networks with major effects on NUE with an emphasis on patented technologies near commercialization and pending patents.