N Demand and the Regulation of Nitrate Uptake

Abstract

Uptake of nitrate by root cells followed by reduction and assimilation in plant tissues is the main route by which mineral N is converted into organic N by living organisms. Like photosynthesis, these are life-dependent processes that members of the animal kingdom are unable to perform for themselves. Nitrate and other mineral nutrients required for optimal plant growth and development frequently exist at relatively low concentrations in soil. To thrive on these dilute nutrients, plants have developed high-performance uptake systems in their root cells. To cope with wide variations in mineral concentrations in soil, plants have evolved mechanisms to regulate the activity of uptake systems so that net intake of a nutrient depends on the plant's need for this element rather than its concentration in the rooting medium. Indeed, uptake rates of most ions are seemingly controlled by specific demand-driven regulatory mechanisms. Such processes set the uptake rate of a given element to match the plant's current growth rate and developmental stage. Nitrate uptake is of special interest because nitrate is absorbed at a relatively high rate and because compounds that function as uptake sensors may have been identified. This paper focuses on whole-plant signaling processes involved in the regulation of nitrate uptake by N demand. NITRATE UPTAKE LlMlTS NITRATE ASSlMlLATlONNitrate-fed plants seldom accumulate an excess of either nitrite or ammonia, indicating that reduction of nitrate to nitrite usually is the rate-limiting step in nitrate assimilation. The rate of nitrate reduction in situ, however, is controlled primarily by the rate of nitrate uptake rather than by alterations in NR activity (Wilkinson and Crawford, 1993) or limitations in reducing power (Warner and Huffaker, 1989). Thus, nitrate uptake appears to control N assimilation in nitrate-fed plants. WHAT LlMlTS NITRATE UPTAKE?Experiments in which both nitrate availability and plant growth rate are manipulated independently show that nitrate uptake rates are determined mainly by regulatory processes that coordinate nitrate uptake and biomass production. This generalization is seemingly independent of plant species. Legumes, however, are a particularly interesting family because they can obtain N via nitrate uptake, symbiotic NZ fixation, or both. Hence, even when N concentrations of legumes are rather constant, their rates of nitrate uptake vary appreciably, depending on nitrate availability, the extent of nodulation, and plant developmental stage.In higher plants, two types of variation in nitrate uptake rate have been identified: (a) temporal responses to modifications of environmental factors such as light intensity, temperature, or stress conditions; and (b) variations that occur during ontogeny. In the first case, when a discrepancy exists between interna1 N supply and growth rate, nitrate uptake varies so that the amount of N in the different interna1 pools remains relatively constant. During ontogeny, the rate of nitrate uptake varies dramatically. Nitrat...

The nutritional control of root development

“…For example, the evidence that the uptake of inorganic ions by roots is regulated in accordance with the shoot's demand for the ions (Marschner, 1995), has led to a search for the means by which the shoot can communicate its requirements to the transport systems of the root. For N, it has been proposed that the rapid cycling of amino acids which occurs between shoot and root would serve to provide the root with the necessary information about changes in the shoot's N status (Cooper and Clarkson, 1989;Imsande and Touraine, 1994). However, contrary to the requirements of this model it is sometimes found that N deficiency can lead to an increase rather than a decline in amino acid cycling (e.g., Peuke et al, 1994).…”

Populus deltoides females are more selective in nitrogen assimilation than males under different nitrogen forms supply

“…Inhibition of nitrate uptake by ammonium is well described in woody species (Serna et al 1992;Kreuzwieser et al 1997;Gessler et al 1998). Some research considered that the products of nitrate and ammonium assimilation resulted in the inhibition, because the products, such as amino acids, were responsible for the down-regulation of nitrate uptake when ammonium is supplied simultaneously (Lee et al 1992;Imsande and Touraine 1994). After uptake, ammonium is assimilated mainly in the roots, while the NO 3 -is assimilated in both roots and shoots of plants (Andrews 1986;Munzarova et al 2006), and the ammonium can be converted into amino acids directly, whereas NO 3 -needs to be reduced to NH 4…”

Field and laboratory experiments on net uptake of nitrate and ammonium by the roots of spruce (Picea abies) and beech (Fagus sylvatica) trees

“…Nitrate uptake is thought to be strongly regulated by the N demand of the plant (Imsande & Touraine, 1994). A possible mechanism of control is the transport of products of nitrate reduction, namely carboxylates, from the site of nitrate reduction to that of nitrate uptake.…”