Barley HvHMA1 Is a Heavy Metal Pump Involved in Mobilizing Organellar Zn and Cu and Plays a Role in Metal Loading into Grains

Abstract

Heavy metal transporters belonging to the P1B-ATPase subfamily of P-type ATPases are key players in cellular heavy metal homeostasis. Heavy metal transporters belonging to the P1B-ATPase subfamily of P-type ATPases are key players in cellular heavy metal homeostasis. In this study we investigated the properties of HvHMA1, which is a barley orthologue of Arabidopsis thaliana AtHMA1 localized to the chloroplast envelope. HvHMA1 was localized to the periphery of chloroplast of leaves and in intracellular compartments of grain aleurone cells. HvHMA1 expression was significantly higher in grains compared to leaves. In leaves, HvHMA1 expression was moderately induced by Zn deficiency, but reduced by toxic levels of Zn, Cu and Cd. Isolated barley chloroplasts exported Zn and Cu when supplied with Mg-ATP and this transport was inhibited by the AtHMA1 inhibitor thapsigargin. Down-regulation of HvHMA1 by RNA interference did not have an effect on foliar Zn and Cu contents but resulted in a significant increase in grain Zn and Cu content. Heterologous expression of HvHMA1 in heavy metal-sensitive yeast strains increased their sensitivity to Zn, but also to Cu, Co, Cd, Ca, Mn, and Fe. Based on these results, we suggest that HvHMA1 is a broad-specificity exporter of metals from chloroplasts and serve as a scavenging mechanism for mobilizing plastid Zn and Cu when cells become deficient in these elements. In grains, HvHMA1 might be involved in mobilizing Zn and Cu from the aleurone cells during grain filling and germination.

Functional and Biochemical Characterization of Cucumber Genes Encoding Two Copper ATPases CsHMA5.1 and CsHMA5.2

“…A. thaliana HMA1 was shown to be a Ca 2ϩ /heavy metal pump localized in the chloroplast envelope, which delivers copper from the cytosol into the chloroplast stroma and detoxifies chloroplasts from Zn 2ϩ under zinc excess (8,9). A similar protein from barley (Hordeum vulgaris), HvHMA1, was shown to be involved in mobilization of zinc and copper from plastids under zinc or copper deficiency or from the intracellular compartments of the mineral storage site of grains (aleurone cells) during grain filling and germination (10). In comparison, the P 1B1 subgroup of plant P 1B -ATPases includes proteins that are highly specific for monovalent Cu ϩ (11,12).…”

Influence of Soil Chemistry and Plant Physiology in the Phytoremediation of Cu, Mn, and Zn

“…AtHMA8 (PAA2), closely related to AtHMA6 (PAA1), is expressed in the thylakoid membrane and supplies Cu to plastocyanin (Tapken et al, 2012). AtHMA1 and HvHMA1, present in the chloroplast envelope, are broad-specificity exporters of metals from chloroplasts and may play a specialized role in Cu mobilization (Mikkelsen et al, 2012). AtHMA7 (RAN1), the first functionally characterized heavy metal ATPase, is responsible for the biogenesis of ethylene receptors by supplying Cu at the endoplasmic reticulum and also for Cu homeostasis in seedling development (Binder et al, 2010).…”

“…AtZIP1 may contribute to remobilizing Zn from the vacuole to the cytoplasm (Milner et al, 2013). In the chloroplasts of both Arabidopsis and barley, HMA1 contributes to Zn detoxification by exporting Zn from the plastids to the cytoplasm Mikkelsen et al, 2012). It has been proposed that the Golgi-localized transporter AtECA3 may have a role in the mobilization of Zn from the cytoplasm to the Golgi complex (Barabasz et al, 2011;Mills et al, 2008).…”

A Member of the Heavy Metal P-Type ATPase OsHMA5 Is Involved in Xylem Loading of Copper in Rice

“…HvHMA1 from barley might be involved in mobilizing Zn and Cu during the stage of grain filling (Mikkelsen et al, 2012). HvHMA2 from barley and TaHMA2 from wheat showed similar functions as OsHMA2 in rice Tan et al, 2013).…”