In plants, potassium serves an essential role as an osmoticum and charge carrier. Its uptake by roots occurs by poorly defined mechanisms. To determine the role of potassium channels in planta, we performed a reverse genetic screen and identified an Arabidopsis thaliana mutant in which the AKT1 channel gene was disrupted. Roots of this mutant lacked inward-rectifying potassium channels and displayed reduced potassium (rubidium-86) uptake. Compared with wild type, mutant plants grew poorly on media with a potassium concentration of 100 micromolar or less. These results and membrane potential measurements suggest that the AKT1 channel mediates potassium uptake from solutions that contain as little as 10 micromolar potassium.

A transferred-DNA insertion mutant of Arabidopsis that lacks AKT1 inward-rectifying K+ channel activity in root cells was obtained previously by a reverse-genetic strategy, enabling a dissection of the K+-uptake apparatus of the root into AKT1 and non-AKT1 components. Membrane potential measurements in root cells demonstrated that the AKT1 component of the wild-type K+ permeability was between 55 and 63% when external [K+] was between 10 and 1,000 μM, and NH4 + was absent. NH4 + specifically inhibited the non-AKT1 component, apparently by competing for K+ binding sites on the transporter(s). This inhibition by NH4 + had significant consequences for akt1 plants: K+ permeability, 86Rb+ fluxes into roots, seed germination, and seedling growth rate of the mutant were each similarly inhibited by NH4 +. Wild-type plants were much more resistant to NH4 +. Thus, AKT1 channels conduct the K+ influx necessary for the growth of Arabidopsis embryos and seedlings in conditions that block the non-AKT1 mechanism. In contrast to the effects of NH4 +, Na+ and H+ significantly stimulated the non-AKT1 portion of the K+ permeability. Stimulation of akt1 growth rate by Na+, a predicted consequence of the previous result, was observed when external [K+] was 10 μM. Collectively, these results indicate that the AKT1 channel is an important component of the K+ uptake apparatus supporting growth, even in the “high-affinity” range of K+ concentrations. In the absence of AKT1 channel activity, an NH4 +-sensitive, Na+/H+-stimulated mechanism can suffice.

The Arabidopsis thaliana KAT1 cDNA encodes a voltage-gated inward-rectifying K+ channel. A KAT1 genomic DNA clone was isolated and sequenced, and a 5' promoter and coding sequences containing eight introns were identified. Reporter gene analysis of transgenic plants containing the KAT1 promoter fused to bacterial beta-glucuronidase showed robust beta-glucuronidase activity primarily in guard cells.

A reverse genetic strategy was used to isolate Arabidopsis plants containing "knockout" mutations in AKT1 and AKT2, two members of a K ϩ channel gene family. Comparative studies of growth and membrane properties in wild-type and mutant seedlings were performed to investigate the physiological functions of these two related channels. The growth rates of plants supplied with rate-limiting concentrations of K ϩ depended on the presence of AKT1 but not AKT2 channels. This result indicates that AKT1 but not AKT2 mediates growth-sustaining uptake of K ϩ into roots, consistent with the expression patterns of these two genes. K ϩ -induced membrane depolarizations were measured with microelectrodes to assess the contribution each channel makes to the K ϩ permeability of the plasma membrane in three different organs. In apical root cells, AKT1 but not AKT2 contributed to the K ϩ permeability of the plasma membrane. In cotyledons, AKT1 was also the principal contributor to the K ϩ permeability. However, in the mesophyll cells of leaves, AKT2 accounted for approximately 50% of the K ϩ permeability, whereas AKT1 unexpectedly accounted for the remainder. The approximately equal contributions of AKT1 and AKT2 in leaves detected by the in vivo functional assay employed here are not in agreement with previous RNA blots and promoter activity studies, which showed AKT2 expression to be much higher than AKT1 expression in leaves. This work demonstrates that comparative functional studies of specific mutants can quantify the relative contributions of particular members of a gene family, and that expression studies alone may not reliably map out distribution of gene functions.The most abundant inorganic solute in plant cells is K ϩ . The transport in and out of cells of this essential element is a highly regulated process mediated by specific transporters within the plasma membrane (Maathuis et al., 1997; Chrispeels et al., 1999). Present at concentrations on the order of 100 mm, K ϩ serves as an osmoticum important to turgor pressure, and may act as an essential cofactor for certain enzymes. Its abundance contributes to the electrolyte character of cytoplasm and affects electrostatic interactions between charged entities such as proteins and other biopolymers. The transport of K ϩ helps set the electric potential difference across the plasma membrane, which powers the transport of other substances. Because K ϩ serves such fundamental functions throughout the plant, understanding the molecular mechanisms of its uptake and redistribution is an important goal. Progress in this regard may also spawn novel strategies for improving plant mineral nutrition and fertilizer application in the field.The first isolation of Arabidopsis genes encoding plasma membrane K ϩ channels by complementation of yeast K ϩ uptake mutants marked a major step toward this goal (Anderson et al., 1992;Sentenac et al., 1992). The transport properties displayed by AKT1 and KAT1 channels expressed in heterologous systems (Schachtman et al., 1992; Bertl et al., 1995 ...

No abstract