Dwarfing Rootstock 'Yunnan' Quince Promoted Fruit Sugar Accumulation by Influencing Assimilate Flow and PbSWEET6 in Pear Scion

'Yunnan' quince (Cydonia oblonga Mill.) is used as the dwarfing rootstock for pear (Pyrus spp.). Here, we reported that the sugar contents in mature 'Zaosu' pear fruit grafted on 'Yunan' quince (Z/Q) were higher than that in 'Zaosu' pear fruit grafted on 'Duli' (Pyrus betulifolia) (Z/D). To investigate the underlying mechanism, the leaf photosynthetic capacity and the leaf-to-fruit assimilate transport capacity were initially analyzed. The leaf photosynthetic capacity was similar between Z/Q and Z/D, but the assimilate transport capacity was greater for Z/Q than for Z/D. Sugar transporters mediate the distribution of assimilates; therefore, changes in PbSWEET transcriptional patterns were examined. PbSWEET6 was highly expressed in Z/Q fruit. Thus, the PbSWEET6 function related to assimilate transport was further verified. Sucrose and glucose contents increased in transgenic tomato fruit and pear fruit calli overexpressing PbSWEET6. Taken together, these results suggest that 'Yunnan' quince positively regulated fruit sugar contents by influencing the flow of PbSWEET6-involved assimilates in the scion.