The poplar <scp>SWEET1c</scp> glucose transporter plays a key role in the ectomycorrhizal symbiosis

The mutualistic interaction between ectomycorrhizal fungi and trees is characterized by the coordinated exchange of soil nutrients with soluble sugars. Despite the importance of this process, the precise mechanism by which sugars are transported from host roots to colonizing hyphae remains unclear. This study aimed to identify the specific membrane transporters responsible for the unloading of sugars at the symbiotic interface, with a focus on the role of the root Sugars Will Eventually Be Exported Transporter (SWEET) uniporters. Our study used RNA sequencing and quantitative PCR to identify PtaSWEET gene expression in Populus tremula × alba-Laccaria bicolor ectomycorrhizal root tips. Our results suggest that symbiosis-induced PtaSWEET1c is primarily responsible for transporting glucose and sucrose, as demonstrated by the yeast assays. Moreover, we used a promoter-YFP reporter to confirm the localization of the PtaSWEET1c expression in cortical cells of ectomycorrhizal rootlets, supporting its major role in supplying glucose at the symbiotic interface. Furthermore, our observations confirmed the localization of PtaSWEET1c-GFP in the plasma membrane. The inactivation of PtaSWEET1c reduced ectomycorrhizal root formation and ¹³C translocation to ectomycorrhizal roots. Our findings highlight the crucial role of PtaSWEET1c in facilitating glucose and sucrose transport at the symbiotic interface of Populus-L. bicolor symbiosis.