Iron (Fe) deficiency affects plant growth and development. The proton pump interactor (PPI) in plants responds to multiple abiotic stresses, although it has not been well characterized under Fe deficiency stress. In this study, we systematically identified and analyzed the PPI gene family in apple. Three PPI candidate genes were found, and they contained 318-1349 amino acids and 3-7 introns. Under Fe deficiency stress, we analyzed the expression of all the PPI genes in roots of apple rootstock Malus xiaojinensis. Expression of the gene MD11G1247800, designated PPI1, is obviously induced by Fe deficiency treatment in M. xiaojinensis. We first cloned MxPPI1 from M. xiaojinensis and determined its subcellular localization, which indicated that it is localized in the cell membrane and nucleus in tobacco. We found that the level of expression of the MxPPI1 protein increased significantly under Fe deficiency stress in apple calli. Moreover, overexpressing MxPPI1 in apple calli enhanced the activities of ferric chelate reductase and H
