Molecular and developmental biology of inorganic nitrogen nutrition.

Unique among the major mineral nutrients, inorganic N is available to plants in both anionic and cationic forms (NO3− and NH4+, respectively). The relative abundance of these two ions in natural soils is highly variable and to a large degree depends on the relative rates of two microbial processes: mineralisation (the release of NH4+ from organic N) and nitrification (the conversion of NH4+ to NO3−) (Marschner, 1995). In well-aerated soils nitrification is rapid, so that NH4+ concentrations are low and NO3− is the main N source, but in waterlogged or acidic soils nitrification is inhibited and NH4+ accumulates. Most plants (including Arabidopsis) seem to be able to use either form of N, although exceptions to this rule are known (e.g. Kronzucker et al., 1997). Nitrogen's importance in plant biology extends far beyond its role as a nutrient. It is now clear that several different N compounds, including NO3−, NH4+ and some of the products of their assimilation, exert strong regulatory effects on both metabolic and developmental pathways (Redinbaugh and Campbell, 1991; Crawford, 1995; Forde and Clarkson, 1999; Stitt, 1999; Zhang and Forde, 2000; Coruzzi and Bush, 2001; Coruzzi and Zhou, 2001). Both the biochemical and the regulatory aspects of inorganic N nutrition with emphasis on Arabidopsis will be considered in this chapter.