Comparative expression analysis of heavy metal ATPase subfamily genes between Cd-tolerant and Cd-sensitive turnip landraces

The heavy metal ATPase (HMA) subfamily is mainly involved in heavy metal (HM) tolerance and transport in plants, but an understanding of the definite roles and mechanisms of most HMA members are still limited. In the present study, we identified 14 candidate HMA genes named BrrHMA1–BrrHMA8 from the turnip genome and analyzed the phylogeny, gene structure, chromosome distribution, and conserved domains and motifs of HMAs in turnip (Brassica rapa var. rapa). According to our phylogenetic tree, the BrrHMAs are divided into a Zn/Cd/Co/Pb subclass and Cu/Ag subclass. The BrrHMA members show similar structural characteristics within subclasses. To explore the roles of BrrHMAs in turnip, we compared the gene sequences and expression patterns of the BrrHMA genes between a Cd-tolerant landrace and a Cd-sensitive landrace. Most BrrHMA genes showed similar spatial expression patterns in both Cd-tolerant and Cd-sensitive turnip landraces; some BrrHMA genes, however, were differentially expressed in specific tissue in Cd-tolerant and Cd-sensitive turnip. Specifically, BrrHMA genes in the Zn/Cd/Co/Pb subclass shared the same coding sequence but were differentially expressed in Cd-tolerant and Cd-sensitive turnip landraces under Cd stress. Our findings suggest that the stable expression and up-regulated expression of BrrHMA Zn/Cd/Co/Pb subclass genes under Cd stress may contribute to the higher Cd tolerance of turnip landraces.