Coordinated Transport of Nitrate, Potassium, and Sodium

Abstract

Potassium (K⁺) and nitrogen (N) are essential nutrients, and their absorption and distribution within the plant must be coordinated for optimal growth and development. Potassium is involved in charge balance of inorganic and organic anions and macromolecules, control of membrane electrical potential, pH homeostasis and the regulation of cell osmotic pressure, whereas nitrogen is an essential component of amino acids, proteins, and nucleic acids. Nitrate (NO₃ ^-) is often the primary nitrogen source, but it also serves as a signaling molecule to the plant. Nitrate regulates root architecture, stimulates shoot growth, delays flowering, regulates abscisic acid-independent stomata opening, and relieves seed dormancy. Plants can sense K⁺/NO₃ ^- levels in soils and adjust accordingly the uptake and root-to-shoot transport to balance the distribution of these ions between organs. On the other hand, in small amounts sodium (Na⁺) is categorized as a "beneficial element" for plants, mainly as a "cheap" osmolyte. However, at high concentrations in the soil, Na⁺ can inhibit various physiological processes impairing plant growth. Hence, plants have developed specific mechanisms to transport, sense, and respond to a variety of Na⁺ conditions. Sodium is taken up by many K⁺ transporters, and a large proportion of Na⁺ ions accumulated in shoots appear to be loaded into the xylem by systems that show nitrate dependence. Thus, an adequate supply of mineral nutrients is paramount to reduce the noxious effects of salts and to sustain crop productivity under salt stress. In this review, we will focus on recent research unraveling the mechanisms that coordinate the K⁺-NO₃ ^-; Na⁺-NO₃ ^-, and K⁺-Na⁺ transports, and the regulators controlling their uptake and allocation.

Keywords: long-distance transport; nitrate; plant nutrition; potassium; salinity; sodium.

FIGURE 1

Transporters involved in the main pathways for the uptake of nitrate and potassium. The diagram represents an idealized root cell disregarding developmental differentiation. Major signaling pathways regulating the expression and biochemical activity of these transporters are also represented. Solid lines represent signaling events and connections that have been confirmed experimentally. Dotted lines signify known or suspected connections for which the molecular events involved remain to be defined. PLC represents an unidentified phospholipase C. Further details are given in the main text.

FIGURE 2

Simplified representation of the source (root) and sink (shoot) organs in terms of mineral uptake and distribution, connected by xylem and phloem. Transporters involved in the root-shoot partition of nitrate and potassium are represented in their preferential placement based on gene expression and protein activity. AKT2 is represented as bidirectional owing to its facultative inward-rectification.

FIGURE 3

Depicted are the main transporters mediating anion and cation fluxes at the plasma membrane of guard cells and involved in stomatal movements. Left side, stomata opening; right side, stomata closure. PK represents protein kinases, including OST1 and CPK3.

FIGURE 4

Simplified representation of plant organs and vascular tissues, and the Na⁺ and K⁺ fluxes mediated by ion transporters involved in root uptake and root-shoot partition. Further details are given in the main text.