Control of Nitrate Uptake/Assimilation and Plant Productivity

Abstract

Nitrogen is one of the major elements that limit plant growth and productivity in both natural environments and plowed fields. To increase the harvest yield, nitrogen fertilizers are widely used in agriculture. The form of nitrogen commonly used as fertilizer is ammonium (NH 4 ), but in temperate aerobic soils, ammonium is rapidly oxidized to nitrate (NO 3 ) by nitrification. The resulting nitrate hence serves as the major source of nitrogen for most cultivated plants. Nitrate is, however, known to cause environmental and health problems. This paper summarizes current knowledge about the critical steps that determine the nitrate flow in plants and discusses possible strategies for improving the nitrate use efficiency. Corresponding author: omata@agr.nagoya-u.ac.jp Control of relative contribution of roots and shoots in nitrate assimilation Another possible means to lower the nitrate levels, specifically in leaves, is to modify the relative contribution of the roots and shoot in nitrate assimilation. The relative contribution of roots and shoots is variable among different species of plants; in some plants, the contribution of roots in nitrate assimilation is small and most of the nitrate is transported to the shoot. In certain plant species, on the other hand, nitrate is assimilated mostly in the root, and the fixed nitrogen is transported to the shoot as amino acids. It is therefore, theoretically possible to genetically engineer a plant that assimilates nitrate entirely in the root. To achieve this type of metabolic engineering, we need to control the expression levels of the enzymes involved in nitrate assimilation and also of the enzymes involved in supply of carbon skeletons and metabolic energy for nitrogen assimilation. Since the root cells are dependent on the supply of carbon and energy from the leaf cells, understanding of the metabolic interactions between the shoot and root is also necessary. Importance of basic research for genetic modification of nitrate assimilation in plants As has been d i scussed above , cur ren t ly available information about nitrate assimilation in plants allows us to design some strategies for improvement of the nitrate use efficiency in plants. However, to actually construct a genetically modified plant, more detailed information about the regulation of the relevant genes is necessary; the molecular mechanism(s) regulating the nitrate transporter genes needs to be fully understood. The molecular basis for the different contribution of roots in nitrate assimilation in different plant species also needs to be elucidated. Thus, the importance of basic research on the nitrate assimilation pathway cannot be overemphasized. Chapter 2: Biotechnology for Sustainable Bioproduction