Expression analysis of sorbitol transporters in pear tissues reveals that PbSOT6/20 is associated with sorbitol accumulation in pear fruits

Sorbitol is a primary substrate that is translocated from its source to its sink in pear and apple by sorbitol transporters (SOTs). However, little is known about the expression profiles of SOTs in tree tissues, and their association with sorbitol accumulation. In this study, 64 SOT genes were isolated from the sequenced genomes of six rosaceae trees, which clustered phylogenetically into four groups. Groups I and III contained genes from pome trees, group II those from drupe trees, and group IV those from pome, drupe, and berry trees. A gene duplication analyses showed that the 24 SOT genes of pear arose from tandem, proximal, and dispersed duplications, which differs from their evolution in the other rosaceae fruit trees. A qRT-PCR analysis showed that the PbSOT6/20 gene in group IV is most strongly expressed in the leaf, branch, flower, fruit, and seed of pear. The expression pattern of PbSOT6/20 correlates more strongly with the pattern of sorbitol accumulation in the pear fruits than do those of other SOT genes. A low concentration (100 mg/g) of exogenous sorbitol induced the expression of PbSOT6/20 in the pear fruits, but not in the leaves. These results suggest that PbSOT6/20 is associated with sorbitol accumulation in pear fruits and plays important role in sorbitol translocation from its source to its sink. This study provides new insights into sorbitol translocation mediated by the SOT genes in pear.