Plant Nitrogen Acquisition Under Low Availability: Regulation of Uptake and Root Architecture

Abstract

Nitrogen availability is a major factor determining plant growth and productivity. Plants acquire nitrogen nutrients from the soil through their roots mostly in the form of ammonium and nitrate. Since these nutrients are scarce in natural soils, plants have evolved adaptive responses to cope with the environment. One of the most important responses is the regulation of nitrogen acquisition efficiency. This review provides an update on the molecular determinants of two major drivers of the nitrogen acquisition efficiency: (i) uptake activity (e.g. high-affinity nitrogen transporters) and (ii) root architecture (e.g. low-nitrogen-availability-specific regulators of primary and lateral root growth). Major emphasis is laid on the regulation of these determinants by nitrogen supply at the transcriptional and post-transcriptional levels, which enables plants to optimize nitrogen acquisition efficiency under low nitrogen availability.

Keywords: Acquisition efficiency; Limitation; Nitrogen nutrient; Root architecture; Uptake.

Fig. 1

Schematic illustration summarizing the function of NRT2 transporters in Arabidopsis roots under low N availability. Spatial and temporal localization of AtNRT2.1 (NRT2.1, purple), AtNRT2.4 (NRT2.4, blue) and AtNRT2.5 (NRT2.5, red) in (A) root tissues and (B) whole root systems under low N availability. (A) The NRT2.4/NAR2 complex is localized to the outer (soil) side of the epidermal cells of the roots of young seedlings. The NRT2.5/NAR2 complex is expressed in the epidermal cells of the roots of adult plants. NRT2.4 and NRT2.5 are responsible for nitrate uptake directly from the soil. Nitrate can apoplastically penetrate toward cortex cells to be absorbed by the NRT2.1/NAR2 complex. NAR2 (AtNAR2.1) is shown as green circles. Orange circles indicate a putative high-affinity exporter involved in xylem loading of nitrate. (B) NRT2.1 is strongly expressed in the older part of the root system, while NRT2.4 and NRT2.5 are preferentially expressed in the younger part of the roots of young seedlings and adult plants, respectively. ep, epidermis; co, cortex; en, endodermis; pe, pericycle; xy, xylem; cs, casparian strip

Fig. 2

A model of low N availability signaling pathways involved in the regulation of high-affinity N transporter gene expression and root architecture in Arabidopsis. Signaling pathways regulating (A) the expression of high-affinity N transporter genes (AtNRT2 and AtAMT genes) and (B) primary root (PR) growth, and/or lateral root (LR) growth and development under low N availability are depicted. Only pathways described in this review are shown. Red arrows and black blunted lines indicate positive and negative interactions, respectively. CK, cytokinin