Molecular cloning and characterization of the genes encoding an auxin efflux carrier and the auxin influx carriers associated with the adventitious root formation in mango (Mangifera indica L.) cotyledon segments.

Polar auxin transport (PAT) plays an important role in the adventitious root formation of mango cotyledon segments, but the molecular mechanism remains unclear. In this study, we cloned a gene encoding an auxin efflux carrier (designated as MiPIN1), and we cloned four genes encoding auxin influx carriers (designated as MiAUX1, MiAUX2, MiAUX3 and MiAUX4). The results of a phylogenetic tree analysis indicated that MiPIN1 and the MiAUXs belong to plant PIN and AUXs/LAXs groups. Quantitative real-time PCR indicated that the expression of MiPIN1 and the MiAUXs was lowest at 0 days but sharply increased on and after day 4. During the root formation in the mango cotyledon segments, the MiPIN1 expression in the distal cut surface (DCS) was always higher than the expression in the proximal cut surface (PCS) whereas the expression of the MiAUXs in the PCS was usually higher than in the DCS. This expression pattern might be result in the PAT from the DCS to the PCS, which is essential for the adventitious root formation in the PCS. Our previous study indicated that a pre-treatment of embryos with indole-3-butyric acid (IBA) significantly promoted adventitious rooting in PCS whereas a pre-treatment with 2,3,5-triiodobenzoic acid (TIBA) completely inhibited this rooting. In this study, however, IBA and TIBA pre-treatments slightly changed the expression of MiPIN1. In contrast, while the MiAUX3 and MiAUX4 expression levels were significantly up-regulated by the IBA pre-treatment, the expression levels were down-regulated by the TIBA pre-treatment. These findings imply that MiAUX3 and MiAUX4 are more sensitive to the IBA and TIBA treatments and that they might play important roles during adventitious root formation in mango cotyledon segments.