A Major Latex Protein-Encoding Gene from Populus simonii × P. nigra (PsnMLP328) Contributes to Defense Responses to Salt and Cadmium Stress

Heavy metal pollution and soil salinization harm human health and the environment. Phytoremediation is a widely accepted soil decontamination method, with woody plants being particularly effective due to their large biomass and extensive root systems. In this study, we identified and cloned <i>PsnMLP328</i> from <i>Populus simonii × P. nigra</i> and demonstrated its role in mitigating salt and cadmium stress. <i>PsnMLP328</i> expression was up-regulated under both stress conditions, and its overexpression in tobacco enhanced resistance to these stresses, albeit through distinct mechanisms. Transgenic plants exhibited increased Cd²⁺ uptake and a higher biomass, alleviating Cd²⁺-induced growth inhibition. Additionally, <i>PsnMLP328</i> boosted proline content, chlorophyll levels, and antioxidative enzyme activities (POD, SOD) under Cd²⁺ stress, likely by protecting cells from oxidative damage. Expression analysis revealed that <i>PsnMLP328</i> down-regulated the cadmium transporter <i>Nramp2</i> while up-regulating <i>YSL2</i> (another cadmium transporter) and potassium channels (<i>AKT1</i> and <i>AKT2/3</i>), suggesting its role in modulating K⁺ and Cd²⁺ homeostasis. These findings indicate that <i>PsnMLP328</i> enhances tobacco resistance to salt and cadmium stress, particularly the latter. This study is the first to elucidate the function of poplar <i>MLP</i> family genes under salt and cadmium stress, advancing our understanding of <i>MLP</i> gene roles in heavy metal stress and offering new insights for remediating salinized and heavy metal-contaminated soils.