Rice P-1B-type heavy-metal ATPase, OsHMA9, is a metal efflux protein

Abstract

P-1B-type heavy-metal ATPases (HMAs) are transmembrane metal- transporting proteins that play a key role in metal homeostasis. Despite their importance, very little is known about their functions in monocot species. We report the characterization of rice (Oryza sativa) OsHMA9, a member of the P (1B)- type ATPase family. Semiquantitative reverse transcription- polymerase chain reaction analyses of seedlings showed that OsHMA9 expression was induced by a high concentration of copper (Cu), zinc (Zn), and cadmium. We also determined, through promoter:: beta- glucuronidase analysis, that the main expression was in the vascular bundles and anthers. The OsHMA9: green fluorescence protein fusion was localized to the plasma membrane. Heterologous expression of OsHMA9 partially rescued the Cu sensitivity of the Escherichia coli copA mutant, which is defective in Cu- transporting ATPases. It did not rescue the Zn sensitivity of the zntA mutant, which is defective in Zn- transporting ATPase. To further elucidate the functional roles of OsHMA9, we isolated two independent null alleles, oshma9- 1 and oshma9-2, from the T- DNA insertion population. Mutant plants exhibited the phenotype of increased sensitivity to elevated levels of Cu, Zn, and lead. These results support a role for OsHMA9 in Cu, Zn, and lead efflux from the cells. This article is the first report on the functional characterization of a P-1B- type metal efflux transporter in monocots.