Potassium and Phosphate Uptake. Uptake of Other Nutrient Elements

While K⁺ uptake is driven by the negative membrane potential, external concentrations can be so low—that is, the concentration gradient between inside and outside can be so high—that energisation is required. Thus, in contrast to the low-affinity systems, which are channels enabling passive diffusion, the high-affinity systems are usually symporters co-transporting K⁺ and protons. In A. thaliana the K⁺ channel AKT1 accounts for much of the K⁺ root uptake. AKT1 is a representative of the Shaker-type of K⁺ channels (named after a Drosophila mutant), which are present in a wide range of species across kingdoms and are involved in processes as diverse as action potentials in neurons, potassium nutrition and stomatal regulation (Cherel et al. 2014).

Low external K⁺ causes an even more negative membrane potential than normal because K⁺ influx into cells is a major force attenuating the surplus of positive charges on the outside of cells. This more negative membrane potential further enhances the electrical potential gradient, which can drive K⁺ into cells. This is the reason why AKT1, as a channel mediating only facilitated diffusion, is still able to support growth even at rather low external K⁺. Still, below such concentrations, proton-coupled import is required. The responsible proteins are designated KUP/HAK (K⁺ uptake permease/High affinity K⁺). They account for the high-affinity uptake (mechanism I in Fig. 7.12).

Phosphate Uptake. Phosphate availability is one of the major constraints of plant growth. In the soil, P is present in the inorganic oxidised form (P_i; orthophosphate) or as part of organic molecules. Only inorganic phosphate can be utilised. Phosphate in organic molecules has to be liberated by enzymes such as plant or microbial phosphatases (Fig. 7.5). Concentrations of inorganic phosphate in the soil solution are often very low (around 1 μM) because of the poor solubility of phosphate and its tendency to adsorb to soil particles. Its low mobility relative to other macronutrients is explained by these characteristics too.

The negative membrane potential and a steep concentration gradient between the soil solution and the cytoplasm (with a concentration of around 5 mM) under most conditions demand energised high-affinity uptake systems for the uptake of phosphate anions such as the major form H₂PO₄^-. The systems mainly responsible in both mycorrhized and non-mycorrhized plants are the Pht1 family transporters, of which several are encoded in the genomes of higher plants (Lopez-Arredondo et al. 2014).

Uptake of Other Nutrient Elements. Sulphate acquisition shows many similarities to nitrate acquisition. High-affinity uptake by proton-coupled symporters (in the case of sulphate these are referred to as sulphate permeases) mediate entry into the cytosol. All steps of sulphate assimilation occur in plastids. The metabolite that is analogous to glutamate in N assimilation is cysteine for S assimilation.

Uptake of Fe(II) by strategy I plants such as A. thaliana is mainly dependent on IRT1. The absence of this uniporter causes severe growth inhibition when Fe availability in the soil is low (Fig. 7.14).

Fig. 7.14. The transporter IRT1 is essential for Fe(II) uptake in Arabidopsis thaliana. Loss of the functional transporter causes severe growth inhibition and chlorosis. Left: Wild-type plants. Right: irt1 mutant plants. (Vert et al. 2002)

Boron and the beneficial element Si are taken up as boric acid and silicic acid, respectively, by aquaglyceroporins of the nodulin 26-like intrinsic protein (NIP) family. These proteins form pores in the membrane for facilitated diffusion. The protein responsible for boron uptake in A. thaliana is NIP5;1. The very pronounced uptake of Si in rice is mediated by Lsi 1. This transporter represents an example of the polar localisation of at least some nutrient transporters in root cells. Lsi 1 is exclusively localised on the distal side facing the soil (Ma et al. 2006).