Phytohormone-ROS Crosstalk Regulates Metal Transporter Expression in Sedum alfredii

<i>Sedum alfredii</i> is a cadmium (Cd) hyperaccumulator, but the regulatory mechanisms linking phytohormones and redox balance to Cd transporter expression remain unclear. In this study, we omitted external cadmium (Cd) stress to isolate and examine the interplay between phytohormone and reactive oxygen species (ROS) signaling. Exogenous treatments with abscisic acid (ABA), indole-3-acetic acid (IAA), gibberellic acid (GA₃), trans-zeatin (t-Z), and H₂O₂ were combined with analyses of hormone levels, antioxidant enzyme activities, and transporter gene expression. Correlation and PLS-SEM analyses identified the CAT-H₂O₂ module as a key node: ABA and IAA enhanced CAT activity and alleviated ROS-mediated repression of transporters, while GA₃ and t-Z exerted opposite effects. Functional validation using an H₂O₂ scavenger revealed that the regulation of <i>HMA3</i> and <i>Nramp5</i> by ABA and t-Z is H₂O₂-dependent. In contrast, IAA modulates <i>Nramp5</i> through a ROS-independent pathway, while the regulatory effects of GA₃ were negligible. Functional validation under Cd exposure suggests a model wherein <i>HMA3</i> and <i>Nramp5</i> act in a complementary manner to sequester and redistribute Cd in leaves, thereby supporting hyperaccumulation. These findings highlight hormone-specific ROS pathways as central to transporter regulation and provide mechanistic insights to improve phytoremediation efficiency.