Genome-wide identification of ZmHMAs and association of natural variation in ZmHMA2 and ZmHMA3 with leaf cadmium accumulation in maize

P-1B-type ATPases, known as heavy metal ATPases (HMAs), play an important role in the control of cadmium (Cd) accumulation in plants. In this study, a total of 12 ZmHMA genes were identified in the maize genome and particularly classified into six clusters based on their phylogenetic relationship and motif compositions. Furthermore, the expression patterns of different ZmHMA genes varied with developmental stages, and were tissue specific under normal conditions. ZmHMA2 and ZmHMA3 genes exhibited significant up-regulation under Cd treatment. Eventually, the association analysis between 103 inbred lines and alleles in ZmHMA2 and ZmHMA3 revealed that one insertion-deletion (InDel) in the intron from ZmHMA2 was associated with leaf Cd concentration under low Cd condition at the seedling stage. Twenty polymorphisms in ZmHMA3 were significantly associated with leaf Cd concentration under various Cd levels at seedling and maturing stages. Five single nucleotide polymorphisms (SNPs) and two InDels of these significantly associated polymorphic loci from ZmHMA3 caused the amino acid substitutions and insertion or deletion events. Importantly, the proteins encoded by ZmHMA2 and ZmHMA3 genes were located in the plasma membrane. This comprehensive analysis will provide an important theoretical basis for future functional verification of ZmHMA genes to unravel the mechanisms of Cd accumulation in leaves of maize. Additionally, the favorable alleles in ZmHMA3 will lay a foundation for the marker-assisted selection of low Cd accumulation in maize.