Evidence for Apoplasmic Phloem Unloading in Pear Fruit

Pear is a tree fruit species with worldwide distribution and belongs to the Rosaceae family. In this study, the phloem unloading pathway in pear (Pyrus bretschneideri 'Yali') fruit was investigated using transmission electron microscopy coupled with a symplasmic tracer dye, carboxyfluorescein diacetate (CFDA). Structurally, the sieve element-companion cell (SE-CC) complex in the phloem cells of major and minor sepal bundles is symplasmically isolated to its surrounding parenchyma cells (PCs) throughout fruit development. This was demonstrated by movement of the fluorescent dye which was restricted to phloem cells only without apparent diffusion. Meanwhile, different sugar contents were examined in fruit phloem exudates, with the results indicating that sorbitol comprised about 70 % of total soluble sugars. Gene transcription of a putative plasma membrane sorbitol transporter (PbSOT1) in flesh tissues enclosing the phloem was examined by RT-qPCR, showing that the transcripts were present at all stages of fruit development, but highest in young fruits. These results potentially suggest that the phloem unloading pathway in the pear fruit is mainly via the apoplasm rather than the symplasm.