Characterization of the expression of Phaseolus vulgaris OCT1, a dehydration-regulated gene that encodes a new type of phloem transporter.

A cDNA coding for a putative organic cation transporter (OCT) was isolated from Phaseolus vulgaris roots by differential display RT-PCR and the corresponding full-length cDNA (named PvOCT1) was subsequently obtained by RACE-PCR. Hydropathy profiles of the deduced amino acid sequence (547 residues) predicted the existence of twelve membrane-spanning domains, which are highly conserved in the major facilitator superfamily (MFS). Three specific domains, which characterize organic ion transporters in animals, can also be observed in the predicted protein. In the non-stressed plants, northern analysis showed that PvOCT1 is strongly expressed in roots and stems, while in situ hybridization revealed the presence of PvOCT1 transcripts in phloem cells. In roots PvOCT1 transcript levels transitorily increased after one hour of dehydration and then dramatically decreased. This decrease was associated with enhanced abundance of PvNeED1 mRNA encoding the enzyme thought to catalyze the limiting step of abscisic acid biosynthesis.