Tailored for swift recognition: A structural perspective on secondary active plant hormone transporters

Plant hormone transporters are essential for regulating phytohormone distribution at the cellular, tissue, and organ levels in plants. These proteins control hormone movement across membranes, shaping the spatial and temporal hormone gradients that underpin growth, development, and stress responses. This review surveys secondary active transporters implicated in the transport of auxin, cytokinin, abscisic acid, and gibberellins. We focus on five major transporter superfamilies: MFS, BART, APC, TOG, and DMT, highlighting their structural organization, transport mechanisms, and substrate specificity. Despite differences in substrate and regulation, members within each superfamily share conserved folds and mechanistic principles. Transport typically proceeds via alternating access models, including elevator, rocker-switch, and rocking-bundle mechanisms, which enable directional movement across chemically distinct environments. We show that structural classification offers an informative framework for predicting substrate interactions, energy coupling, and regulatory motifs, ultimately improving our understanding of hormone transport in plants. As more plant transporters are resolved structurally, this approach will be critical for uncovering how transport is coupled to energy sources and integrated into broader signaling networks.