Zinc Enhances Cadmium Accumulation in Shoots of Hyperaccumulator Solanum nigrum by Improving ATP-Dependent Transport and Alleviating Toxicity

<i>Solanum nigrum</i> is a cadmium (Cd) and zinc (Zn) accumulator with potential for phytoextraction of soil contaminated with heavy metals. However, how Zn affects Cd accumulation in <i>S. nigrum</i> remains unclear. In this study, <i>S. nigrum</i> seedlings were treated with 100 μmol·L^-1 Zn (Zn100), 100 μmol·L^-1 Cd (Cd100), and the Zn and Cd combination (Zn100+Cd100) for 10 days under hydroponic culture. Compared with Cd100, the Cd content in stems, leaves, and xylem saps was 1.8, 1.6, and 1.3 times more than that in Zn100+Cd100, respectively. In addition, the production of reactive oxygen species in leaves was significantly upregulated in Cd100 compared with the control, and it was downregulated in Zn100. Comparative analyses of transcriptomes and proteomes were conducted with <i>S. nigrum</i> leaves. Differentially expressed genes (DEGs) were involved in Cd uptake, transport, and sequestration, and the upregulation of some transporter genes of Zn transporters (<i>ZIPs</i>), a natural resistance associated macrophage protein (<i>Nramp1</i>), a metal-nicotianamine transporter (<i>YSL2</i>), ATP-binding cassette transporters (<i>ABCs</i>), oligopeptide transporters (<i>OPTs</i>), and metallothionein (<i>MTs</i>) and glutathione S-transferase (<i>GSTs</i>) genes was higher in Zn100+Cd100 than in Cd100. In addition, differentially expressed proteins (DEPs) involved in electron transport chain, ATP, and chlorophyll biosynthesis, such as malate dehydrogenases (MDHs), ATPases, and chlorophyll <i>a</i>/<i>b</i> binding proteins, were mostly upregulated in Zn100. The results indicate that Zn supplement increases Cd accumulation and tolerance in <i>S. nigrum</i> by upregulating ATP-dependent Cd transport and sequestration pathways.