MtABCG40 is a cytokinin transporter that limits lateral root density and nodule formation in Medicago truncatula

Numerous studies have already demonstrated that cytokinin (CK) distribution plays a pivotal role in shaping plant morphology in response to environmental changes. However, the contribution of short-distance CK translocation to root mineral nutrition remains poorly understood, and the specific roles of CK transporters in root morphology are still unclear. Identifying the molecular identity of CK transporters is, therefore, essential for advancing our understanding of root plasticity, under varying soil fertility conditions and common nutritional deficiencies. In this study, we identified and characterised MtABCG40, a full-size ATP-binding cassette (ABC) transporter of the G subfamily in Medicago truncatula, as a CK transporter. MtABCG40 expression is root-specific, and it is induced both by nitrogen deficiency and CK treatment. We propose that MtABCG40 mediates the lateral translocation of CKs from the xylem to surrounding cells, giving rise to lateral organs, and thereby influencing root morphology, by suppressing the lateral root and nodule formation under nitrogen-limited conditions. Furthermore, MtABCG40 reduces apoplastic CK concentrations in the root meristematic zone, lowering the responsiveness of the root apical meristem to CKs. In summary, the activity of MtABCG40 indirectly influences CK perception and, consequently, modulates auxin-mediated cellular responses.