TOWARDS AN UNDERSTANDING OF THE MOLECULAR-BASIS OF PLANTS K+ TRANSPORT - CHARACTERIZATION OF CLONED K+ TRANSPORT CDNAS

Recently, two K+-transport cDNAs, KAT1 and AKT1, were cloned in Arabidopsis thaliana. These cDNAs had structural similarities to K+ channel genes in animals, and also conferred the ability for growth on micromolar levels of K+ when expressed in K+ transport-defective yeast mutants. In this study, we examined the possibility that KAT1 encodes the high-affinity K+ transport system that has been previously characterized in plant roots, by studying the concentration-dependent kinetics of K+ transport for KAT1 expressed in Xenopus oocytes and Saccharomyces cerevisiae. In both organisms, the K+ transport system encoded by KAT1 yielded Michaelis-Menten kinetics with a high K-m for K+ (35 mM in oocytes, 0.6 mM in yeast cells). Furthermore, Northern analysis indicated that KAT1 is expressed primarily in the Arabidopsis shoot. These results strongly suggest that the system encoded by KAT1 is not a root high-affinity K+ transporter.