Relative contribution of AtHAK5 and AtAKT1 to K⁺ uptake in the high‐affinity range of concentrations

Abstract

The relative contribution of the high-affinity K(+) transporter AtHAK5 and the inward rectifier K(+) channel AtAKT1 to K(+) uptake in the high-affinity range of concentrations was studied in Arabidopsis thaliana ecotype Columbia (Col-0). The results obtained with wild-type lines, with T-DNA insertion in both genes and specific uptake inhibitors, show that AtHAK5 and AtAKT1 mediate the NH4+-sensitive and the Ba(2+)-sensitive components of uptake, respectively, and that they are the two major contributors to uptake in the high-affinity range of Rb(+) concentrations. Using Rb(+) as a K(+) analogue, it was shown that AtHAK5 mediates absorption at lower Rb(+) concentrations than AtAKT1 and depletes external Rb(+) to values around 1 muM. Factors such as the presence of K(+) or NH4+ during plant growth determine the relative contribution of each system. The presence of NH4+ in the growth solution inhibits the induction of AtHAK5 by K(+) starvation. In K(+)-starved plants grown without NH4+, both systems are operative, but when NH4+ is present in the growth solution, AtAKT1 is probably the only system mediating Rb(+) absorption, and the capacity of the roots to deplete Rb(+) is reduced.

Analysis of the Root System Architecture of Arabidopsis Provides a Quantitative Readout of Crosstalk between Nutritional Signals

“…By contrast, induction of second-order LRs by low-K was significantly weaker in mutant plants defective for the K-channel AKT1 (akt1). This finding is surprising since AKT1 is not required for K-nutrition in low-K as long as HAK5 is functional (Rubio et al, 2008). Nevertheless, a role of AKT1 was further supported by the observation that induction of second-order LRs by low-K was completely abolished in mutant plants defective for the The effects of individual and double nutrient starvation on RSA, visualized as radar charts.…”

“…The physiological relevance of this pathway, however, has remained a mystery because AKT1 does not make a major contribution to K-uptake in low-K conditions as long as HAK5 is active (Rubio et al, 2008). Our study not only identified LR branching as a developmental, target process of this signaling module ( Figures 7A and 1B), but also linked it with the nitrate-transceptor NRT1.1, another target of CIPK23 (Ho et al, 2009).…”

Sphingolipids in the Root Play an Important Role in Regulating the Leaf Ionome inArabidopsis thaliana

“…The elevated suberin in tsc10a may also be responsible for the enhanced Na, K, and Rb observed in the mutant, since similar changes were also observed in the esb1 mutant (Baxter et al, 2009). One possible mechanism to explain the elevated leaf Na and K may be that increased resistance to Na + and K + transport via the apoplastic pathway increases the movement of these ions via a more efficient symplastic transport pathway mediated by high affinity transporters, such as the Na transporter HKT1 (Uozumi et al, 2000) and the K transporters HAK5 and AKT1 (Rubio et al, 2008). We are currently unsure if reduced Mg in tsc10a is related to the changes in suberin observed in this mutant, since leaf Mg is unchanged in esb1 (Baxter et al, 2009).…”

Ion Exchangers NHX1 and NHX2 Mediate Active Potassium Uptake into Vacuoles to Regulate Cell Turgor and Stomatal Function in Arabidopsis

“…Avoidance of the high concentration of NH 4 + in the Murashige and Skoog medium routinely used for Arabidopsis growth was important to prevent inhibition of K + uptake at low external K + concentrations (Spalding et al, 1999;Rubio et al, 2008). As recently described by Bassil et al (2011b), plants of the nhx1-1 nhx2-1 genotype were smaller than the wild-type and single mutant plants when grown in soil under standard growth conditions and in hydroponic culture in LAK medium (see Supplemental Figure 2 online).…”

“…Decreases in cytosolic K + content are thought to be required for induction and development of high-affinity K + uptake in Arabidopsis (Rubio et al, 2008). Three-week-old plants of Col-0 and mutant lines L2 and L14 growing in LAK hydroponic medium with 1 mM K + where starved in K + -free medium for 3 d and then assayed for ion uptake rates by roots using rubidium (Rb + ) at low (100 mM) concentration as a tracer for K + .…”