To investigate the expression of a gene encoding an auxin carrier protein in trees of different apple rootstocks, a 1433-bp full-length cDNA fragment of the putative auxin carrier protein gene RF-PIN1, encoding 476 amino acids, was cloned from Fuji apple. Using semi-quantitative RT-PCR analyses, RF-PIN1 transcripts were detected in various organs at different times of the year; the transcript abundance was higher in leaves and roots of Red Fuji/Malus micromalus Makino than in those of Red Fuji/M9 and Red Fuji/M9/M. micromalus Makino. An in situ hybridization analysis revealed significant hybridization signals for RF-PIN1 transcripts in leaves and roots in Red Fuji/M. micromalus Makino and Fuji/M9/M. micromalus Makino. RF-PIN1 transcripts were specifically distributed in the main vein of leaves, root vascular tissues (mainly in cambium and xylem parenchyma) and the lower epidermis of apical vascular pillar cells. The hybridization signals of RF-PIN1 were markedly stronger in leaves and roots of Red Fuji/M. micromalus Makino than in those of apple trees with M9 as interstock. Presumably, the introduction of the dwarfing stock M9 resulted in various changes in biological characteristics, eventually leading to differences in expression levels of RF-PIN1.
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