The expression of genes coding for auxin carriers in different tissues and along the organ can explain variations in auxin transport and the growth pattern in etiolated lupin hypocotyls.

Novel cDNA clones encoding putative auxin influx and efflux carriers have been isolated and characterized from etiolated lupin (Lupinus albus L) hypocotyls. The full length of LaAUX1 and LaPIN1 and the partial length of LaPIN3 were obtained and the deduced amino acid sequence revealed a high degree of identity with the corresponding auxin carrier proteins from several species. The expression of these genes depended on the tissue, the hypocotyl zone and seedling age. LaAUX1 and LaPIN3 were expressed in stele and outer tissues, while LaPIN1 was restricted to the stele. From the above-mentioned results and taking into account the role proposed for the efflux carrier PIN1, it is suggested that LaPIN1 could mediate the basipetal auxin transport already described in this organ. LaAUX1 might facilitate auxin influx in the transport cells. The expression of the three genes decreased down the hypocotyl. The basipetally decreasing gradient in the expression of LaPIN1 coincides with previous results showing a similar gradient in the intensity and polarity of auxin transport. The decisive role ascribed to PIN1 in polar auxin transport due to its localization in the basal end of transporting cells and the existence of such a gradient in the expression of LaPIN1 support the hypothesis of a barrier effect (generated by decreasing auxin transport) previously proposed by our research group as being responsible for the auxin gradient, which controls the growth pattern in etiolated lupin hypocotyls.