Inventory and Functional Characterization of the HAK Potassium Transporters of Rice

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Abstract
Plants take up large amounts of K+ from the soil solution and distribute it to the cells of all organs, where it fulfills important physiological functions. Transport of K+from the soil solution to its final destination is mediated by channels and transporters. To better understand K+ movements in plants, we intended to characterize the function of the large KT-HAK-KUP family of transporters in rice (Oryza sativacv Nipponbare). By searching in databases and cDNA cloning, we have identified 17 genes (OsHAK1–17) encoding transporters of this family and obtained evidence of the existence of other two genes. Phylogenetic analysis of the encoded transporters reveals a great diversity among them, and three distant transporters, OsHAK1, OsHAK7, and OsHAK10, were expressed in yeast (Saccharomyces cerevisiae) and bacterial mutants to determine their functions. The three transporters mediate K+ influxes or effluxes, depending on the conditions of the experiment. A comparative kinetic analysis of HAK-mediated K+ influx in yeast and in roots of K+-starved rice seedlings demonstrated the involvement of HAK transporters in root K+ uptake. We discuss that all HAK transporters may mediate K+ transport, but probably not only in the plasma membrane. Transient expression of the OsHAK10-green fluorescent protein fusion protein in living onion epidermal cells targeted this protein to the tonoplast.