Genome-Wide Characterization of Sedum plumbizincicola HMA Gene Family Provides Functional Implications in Cadmium Response

Heavy-metal ATPase (HMA), an ancient family of transition metal pumps, plays important roles in the transmembrane transport of transition metals such as Cu, Zn, Cd, and Co. Although characterization of HMAs has been conducted in several plants, scarcely knowledge was revealed in Sedum plumbizincicola, a type of cadmium (Cd) hyperaccumulator found in Zhejiang, China. In this study, we first carried out research on genome-wide analysis of the HMA gene family in S. plumbizincicola and finally identified 8 SpHMA genes and divided them into two subfamilies according to sequence alignment and phylogenetic analysis. In addition, a structural analysis showed that SpHMAs were relatively conserved during evolution. All of the SpHMAs contained the HMA domain and the highly conserved motifs, such as DKTGT, GDGxNDxP, PxxK S/TGE, HP, and CPx/SPC. A promoter analysis showed that the majority of the SpHMA genes had cis-acting elements related to the abiotic stress response. The expression profiles showed that most SpHMAs exhibited tissue expression specificity and their expression can be regulated by different heavy metal stress. The members of Zn/Co/Cd/Pb subgroup (SpHMA1-3) were verified to be upregulated in various tissues when exposed to CdCl2. Here we also found that the expression of SpHMA7, which belonged to the Cu/Ag subgroup, had an upregulated trend in Cd stress. Overexpression of SpHMA7 in transgenic yeast indicated an improved sensitivity to Cd. These results provide insights into the evolutionary processes and potential functions of the HMA gene family in S. plumbizincicola, laying a theoretical basis for further studies on figuring out their roles in regulating plant responses to biotic/abiotic stresses.

SpARF4 reduces cadmium accumulation by negatively regulating SpABCG14 and SpACO4 in the cadmium/zinc hyperaccumulator Sedum plumbizincicola

Root development and apoplastic transport are respectively important for cadmium (Cd) absorption and transportation, which profoundly influence Cd bioremediation. However, molecular mechanisms underlying the two processes are not completely understood. In this study, we demonstrated that auxin response factor 4 (SpARF4) from a Cd hyperaccumulator Sedum plumbizincicola was a negative regulator for these processes. SpARF4 positively regulated by auxin was highly expressed in xylem. Overexpression of SpARF4 significantly decreased vessel area and declined lignin content of S. plumbizincicola. Meanwhile, less adventitious roots were found, and lateral root development was delayed in transgenic plants. Furthermore, ethylene production and auxin transportation were impaired. More importantly, SpARF4 negatively regulated Cd content of xylem saps and aerial tissues. Combining dual-LUC reporter, Y1H and qRT-PCR assays, SpARF4 was a repressor for two downstream genes (SpABCG14 and SpACO4) which influenced vascular bundle development and ethylene production, respectively. PIN1, 2, 3, 7 were downregulated and slowed down local auxin accumulation rate, which suspended root development. These results indicate that SpARF4 can decelerate Cd transportation rate from roots to aerial parts and reduce Cd content of aboveground tissues by delaying the root development and decreasing vessel area.