Arabidopsis Azg2 Transports Cytokininsin Vivoand Regulates Lateral Root Emergence

Cytokinin and auxin are key regulators of plant growth and development. During the last decade transport mechanisms have turned out to be the key for the control of local and long-distance hormone distributions. In contrast with auxin, cytokinin transport is poorly understood. Here, we show thatArabidopsis thalianaAZG2, a member of the AZG purine transporter family, acts as cytokinin transporter involved in root system architecture determination. Even though purines are substrates for both AZG1 and AZG2, we found distinct transport mechanisms. The expression ofAZG2is restricted to a small group of cells surrounding the lateral root (LR) primordia and induced by auxins. Compared to the wild-type (WT), mutants carrying loss-of-function alleles ofAZG2have higher LR density, suggesting that AZG2 is part of a regulatory pathway in LR emergence. Moreover,azg2is partially insensitive to exogenous cytokinin, which is consistent with the observation that the cytokinin reporterTCSn(pro):GFPshowed lower fluorescence signal in the roots ofazg2compared to the WT. These results indicate a defective cytokinin signalling pathway in the region of LR primordia. The integration of AZG2 subcellular localization and cytokinin transport capacity data allowed us to propose a local cytokinin : auxin signalling model for the regulation of LR emergence.