AKT3, a phloem-localized K ⁺ channel, is blocked by protons

Abstract

The potassium-channel gene, AKT3, has recently been isolated from an Arabidopsis thaliana cDNA library. By using the whole-mount and in situ hybridization techniques, we found AKT3 predominantly expressed in the phloem. To study the physiological role of this channel type, AKT3 was heterologously expressed in Xenopus oocytes, and the electrical properties were examined with voltage-clamp techniques. Unlike the plant inward-rectifying guard cell K ؉ channels KAT1 and KST1, the AKT3 channels were only weakly regulated by the membrane potential. Furthermore, AKT3 was blocked by physiological concentrations of external Ca 2؉ and showed an inverted pH regulation. Extracellular acidification decreased the macroscopic AKT3 currents by reducing the single-channel conductance. Because assimilate transport in the vascular tissue coincides with both H ؉ and K ؉ f luxes, AKT3 K ؉ channels may be involved in K ؉ transport accompanying phloem loading and unloading processes.The plant vascular system, which consists of xylem and phloem, is specialized for long-distance solute and water transport. In both tissues, potassium represents one of the major mineral nutrients and is likely to assist in osmotic homeostasis. After uptake from the soil, K ϩ ions circulate between roots and shoots through the xylem and phloem to adopt the specific demands for this cation in the various tissues (1, 2). Recently, in vivo and in vitro analysis demonstrated the presence of K ϩ uptake and release channels in xylem parenchyma cells (3-6). In comparison, information about K ϩ transport across the plasma membrane of phloem cells, the underlying mechanisms, and the physiological role in long-distance solute transport is still limited. Phloem loading with assimilates is accompanied by ionic movements (7). Protons are pumped into the apoplast by a H ϩ -ATPase, generating transmembrane gradients in electropotential and pH that in turn enable the uphill transport of assimilates into the phloem through assimilate/ H ϩ -cotransporters (8-10). The phloem loading coincides with an increase in the symplastic K ϩ concentration likely to maintain electrical neutrality that is required for creating the pH gradient (2, 7). In addition, the K ϩ concentration in the sieve tube may affect the volume flow rate in the phloem (11). Furthermore, the membrane potential that transiently changes during phloem-propagating action potentials is possibly reestablished on K ϩ release from the sieve tube (12). Evidence for the expression of K ϩ channels in the phloem was recently provided (P.A., K. Philippar, and R.H., unpublished data) to support the idea that K ϩ channels may also mediate the transmembrane K ϩ fluxes in the phloem.In the present paper, we localized the cloned Arabidopsis K ϩ channel AKT3 to the phloem and revealed its unique dependence on voltage, Ca 2ϩ , and pH properties, which are well suited for meeting its supposed role in processes associated with the phloem. MATERIALS AND METHODSRNA Extraction and Northern Blot Analysis. Total RNA was isolate...

Increased Functional Diversity of Plant K+ Channels by Preferential Heteromerization of the Shaker-like Subunits AKT2 and KAT2

“…In our experimental conditions, 10 mM Ca 2ϩ produced a voltage-dependent block of the instantaneous inward current recorded on AKT2-expressing oocytes (Fig. 5A, white bars), in line with previously published data (44,46). In AKT2/KAT2 co-expressing oocytes, the instantaneous inward current was poorly affected (less than 10% inhibition) in an essentially voltage-independent way (Fig.…”

“…The AKT2 instantaneous current has been shown to decrease when the external concentration of Ca 2ϩ or H ϩ is increased (44,46,63), whereas the KAT2 current has been reported to be essentially insensitive to external Ca 2ϩ (59) and stimulated by lowering the external pH (36). We have therefore compared the effect of extracellular Ca 2ϩ and pH on the instantaneous fraction of the currents recorded on AKT2-expressing or AKT2/KAT2 co-expressing oocytes (Fig.…”

“…The two genes are co-expressed in the phloem vasculature. AKT2 is expressed in phloem throughout the whole plant (44,46,47), and KAT2 is expressed in the leaf minor vein phloem (36). AKT2 and KAT2 transcripts are also co-localized in guard cells (41).…”

Arabidopsis NPCC6/NaKR1 Is a Phloem Mobile Metal Binding Protein Necessary for Phloem Function and Root Meristem Maintenance

“…Differences in K + concentration between source and sink tissues have been detected and are thought to contribute to the turgor gradient (Fischer, 1987;Fromm and Eschrich, 1989;Hayashi and Chino, 1990) that drives phloem sap movement (Mengel and Haeder, 1977;Lang, 1983). The K + channel AKT2/3 is specifically expressed in the phloem and functions in regulating the membrane potential (Marten et al, 1999;Deeken et al, 2000;Lacombe et al, 2000). The akt2/3-1 mutant exhibited reduced sucrose concentration in the phloem sap, but K + concentration was not significantly affected (Deeken et al, 2002).…”

Identification of regions responsible for the function of the plant K+ channels KAT1 and AKT2 in Saccharomyces cerevisiae and Xenopus laevis oocytes

“…AKT3 13 is shorter than AKT2 14 and lacks the first 15 amino acids encoded by the AKT2 gene 15 ; but both show the same electrophysiological properties when expressed in Xenopus oocytes. 15,16 The weak-rectification of AKT2 is due to a bi-modal gating behavior 17 regulated by phosphorylation. [18][19][20][21] This peculiar feature allows to tap a 'potassium battery' to energize membrane transport processes in plant vascular tissues.…”