Medicago truncatula ABCG40 is a cytokinin importer negatively regulating lateral root density and nodule number

ABSTRACT

Numerous studies suggest a relevant role of cytokinin (CK) distribution in shaping of plant morphology upon changing environment. Nonetheless, our knowledge about an involvement of short-distance CK translocation in root mineral nutrition is still scarce and specific role of CK transporters in root morphology has yet to be established. Revealing the molecular identity of CK transporters is thereby crucial for better understanding of root plasticity during soil fertility, as well as more frequently encountered plant nutrients deficiencies. In this work, we identified and characterized the Medicago truncatula full-size ATP-binding cassette (ABC) transporter belonging to the G subfamily, namely MtABCG40 as a CK importer. Its expression is root-specific and is induced by nitrogen deprivation and CKs. Our analyses indicate that MtABCG40 has a negative impact on lateral root density through decreased lateral root initiation and enhancement of primary root elongation. Moreover, in line with postulated resemblance to lateral roots, we also observed an inhibitory influence of this transporter on nodule number. Our results suggest that MtABCG40 action affects CK signaling which impacts on the cellular response to auxin. We present data that demonstrate a full-size ABCG transporter with a novel function in legumes and CK transport.