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.