MeCIPK10 regulates the transition of the K+ transport activity of MeAKT2 between low- and high-affinity molds in cassava

Abstract

AKT1 is an inward-rectifying K+ channel that was originally thought to function only within a low-affinity K+ concentration range. However, the growth of an akt1 mutant of Arabidopsis was shown to be severely inhibited within a high-affinity range. This suggested that AKT1 may also be a high-affinity K+ transporter, but it remains unclear how the two modes of AKT1 coordinate to uptake K+. One gene (MeAKT2) encodes for a putatively inward-rectifying K+ channel and was isolated from cassava. Relative to other tissues, the MeAKT2 gene was expressed mainly in roots, and its transcriptional level was observed to be significantly increased under low-K+ conditions. Functional analyses were performed using a yeast expression system. When MeAKT2 was expressed alone in yeast cells, transgenic yeast could grow only in nutrient media supplied with >0.5 mM potassium. A yeast two-hybrid assay showed that both MeCIPK10 and MeCIPK12 clearly interacted with MeAKT2. Additionally, 0.05 mM K+ was sufficient for the growth of yeast cells co-expressing MeAKT2 with MeCIPK10, but also their co-expression significantly enhanced the growth capacity of yeast cells in the low range of K+ concentrations. Change in K+ uptake rate in co-transgenic yeast cells grown across a wide range of K+ concentrations showed that MeAKT2-mediated K+ uptake displayed a biphasic pattern, but also the switching from low-to high-affinity K+ uptake was regulated by CIPK10. This indicated that MeAKT2 functioned as a dual-affinity transporter to uptake K+ under both low- and high-affinity K+ conditions.