Genome-wide analyses of metal tolerance protein genes in apple (Malus domestica): Identification, characterization, expression and response to various metal ion stresses

Abstract

The growth and production of fruit trees are adversely affected by heavy metal pollution in orchard lands. Plant metal homeostasis and resistance are regulated by metal tolerance proteins (MTP), which function as divalent cation transporters. To understand the possible role of MTPs in fruit trees, we identified 20 MdMTPs in the apple genome. Phylogenetic and collinear relationship analyses revealed that they belonged to Zn-MTP, Fe/Zn-MTP and Mn-MTP subfamilies, and have gone through two tandem and six segmental duplication events under purifying selection. All MdMTPs were predicted to form homodimer, with each monocase containing a conserved cation efflux domain (CED), the typical structural feature of MTP family, and one or two HXXXD or DXXXD residues. Further spatiotemporal expression analyses indicated that all MdMTP genes were ubiquitously expressed, and most of them were induced by at least three heavy metal ions, in roots, stems or leaves. Yeast and Arabidopsis mutant complementation experiments verified the function of MdMTP11.1 in response to heavy metal stress. Our findings provide an important genetic foundation for the functional elucidation of MdMTP genes, which can be used as the potential gene candidates for the breeding of new fruit tree cultivars with improved tolerance to heavy metal stress. • A total number of 20 metal tolerance protein (MTP) genes were identified in apple. • All MdMTP proteins contained a cation efflux domain to work for metal ion efflux. • Tandem and segmental duplication events occurred in MdMTPs under purifying selection. • Most of MdMTPs were induced by at least three heavy metal ions in various organs.