Plackova, L. | bioRxiv Science

bioRxiv2022

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

Numerous studies suggest that cytokinin (CK) distribution plays a relevant role in shaping plant morphology in changing environments. Nonetheless, our knowledge about the involvement of short-distance CK translocation in root mineral nutrition remains scarce, and the specific role of CK transporters in root morphology has yet to be established. Therefore, the molecular identity of CK transporters should be determined to increase knowledge on root plasticity during soil fertility, as well as more frequently encountered plant nutrient deficiencies. In this work, we identified and characterized the Medicago truncatula full-size ATP-binding cassette (ABC) transporter of the G subfamily MtABCG40 as a plasma membrane CK importer. Its expression is root-specific and is induced by nitrogen deprivation and CKs. Our analyses indicate that MtABCG40 exerts a negative impact on lateral root density by decreasing lateral root initiation and enhancing primary root elongation. Moreover, we also observed that this transporter negatively influenced the nodule number. Our results suggest that MtABCG40 action affects CK signalling, which impacts the cellular response to auxin. In summary, we identified a novel ABCG-type CK transporter that regulates lateral root density and nodule number.

plant biology↗

bioRxiv2019

Cell-surface receptors enable perception of extracellular cytokinins

Cytokinins are mobile multifunctional plant hormones with roles in development and stress resilience 1,2. Although cytokinin receptors are substantially localised to the endoplasmic reticulum 3-5, the cellular sites of cytokinin perception continue to be debated 1,6,7. Several cytokinin types display bioactivity 8,9 and recently a cell-specific cytokinin gradient was reported in roots 10. Yet, the importance of spatially heterogeneous cytokinin distribution and the specific cytokinin(s) that account for the different responses remain unclear. Here we show that cytokinin perception by plasma membrane receptors is an effective path for cytokinin response in root cells. Readout from a Two Component Signalling cytokinin-specific reporter (TCSn::GFP;11) is closely matched to intracellular cytokinin content, yet a proportion of bioactive cytokinins are detected in the extracellular fluid. Using cytokinins covalently linked to beads that could not pass the plasma membrane, we demonstrate that strong TCSn activation still occurs and that this response is greatly diminished in cytokinin receptor mutants. Although intracellular receptors play significant roles, we argue for a revision of concepts of cytokinin perception to include the spatial dimensions. In particular, selective ligand-receptor affinities, cellular localisation and tissue distribution of bioactive cytokinins, their receptors, transporters and inactivation enzymes appear all to be components of the signalling regulatory mechanisms.

plant biology↗