ZMK1 Is Involved in K(+) Uptake and Regulated by Protein Kinase ZmCIPK23 in Zea mays

Potassium (K(+)) is one of essential mineral elements for plant growth and development. K(+) channels, especially AKT1-like channels, play crucial roles in K(+) uptake in plant roots. Maize is one of important crops; however, the K(+) uptake mechanism in maize is little known. Here, we report the physiological functions of K(+) channel ZMK1 in K(+) uptake and homeostasis in maize. ZMK1 is a homolog of Arabidopsis AKT1 channel in maize, and mainly expressed in maize root. Yeast complementation experiments and electrophysiological characterization in Xenopus oocytes indicated that ZMK1 could mediate K(+) uptake. ZMK1 rescued the low-K(+)-sensitive phenotype of akt1 mutant and enhanced K(+) uptake in Arabidopsis. Overexpression of ZMK1 also significantly increased K(+) uptake activity in maize, but led to an oversensitive phenotype. Similar to AKT1 regulation, the protein kinase ZmCIPK23 interacted with ZMK1 and phosphorylated the cytosolic region of ZMK1, activating ZMK1-mediated K(+) uptake. ZmCIPK23 could also complement the low-K(+)-sensitive phenotype of Arabidopsis cipk23/lks1 mutant. These findings demonstrate that ZMK1 together with ZmCIPK23 plays important roles in K(+) uptake and homeostasis in maize.