Arabidopsis AZG2, an auxin induced putative cytokinin transporter, regulates lateral root emergence

The phytohormones cytokinin (CK) and auxin are key regulators of plant growth and development. During the last decade specialised transport mechanisms turned out to be the key for the control of local and long distance hormone distributions. In contrast to auxin, CK transport is poorly understood. Here we show that AZG2, a member of the AZG purine transporter family, acts as CK transporter involved in the determination of the root system architecture. The expression of is primarily auxin dependent and restricted to a small group of cells surrounding the lateral root primordia. Compared to wild type, mutants carrying loss-of-function alleles of have higher density of lateral roots, suggesting AZG2 as being part of a regulatory pathway in lateral root emergence. Moreover, mutants are partially insensitive to exogenously applied CK, which is consistent with the observation that the CK reporter gene showed lower fluorescence signal in the roots of mutants compared to those of wild type. These results indicate a defective CK signalling pathway in the region of lateral root primordia. By the integration of AtAZG2 subcellular localization and CK transport capacity data, our results allowed us to propose a local Auxin/CK signalling model for the regulation of lateral root emergence.