Arabidopsis ABCC4 encodes a cytokinin efflux transporter and is involved in root system development

The directional and sequential flow of cytokinin in plants is organized by a complex network of transporters. Genes involved in several aspects of cytokinin transport have been characterized, but a large part of the elaborate system remains elusive. In this study, we have identified ABCC4 as a cytokinin efflux transporter gene. Using a transient expression system in tobacco leaves, we screened Arabidopsis transporter genes and isolated ATP-BINDING CASSETTE TRANSPORTER C4 (ABCC4). Further validation through drug-induced expression in Arabidopsis and heterologous expression in budding yeast revealed that ABCC4 effluxes the active form of cytokinins. During the seedling stage, ABCC4 was highly expressed in roots, and its expression was up-regulated in response to cytokinin application. Loss-of-function mutants of ABCC4 displayed enhanced primary root elongation, similar to mutants impaired in cytokinin biosynthesis or signaling, which was suppressed by exogenous trans-zeatin treatment. In contrast, overexpression of the gene led to suppression of root elongation. These results suggest that ABCC4 plays a role in the efflux of active cytokinin, thereby contributing to root growth regulation. Our findings contribute to unraveling the many complexities of cytokinin flow and enhance our understanding of the regulatory mechanisms underlying root system development in plants.