Sugar Transporter HmSWEET8 Cooperates with HmSTP1 to Enhance Powdery Mildew Susceptibility in Heracleum moellendorffii Hance

The powdery mildew caused by <i>Eeysiphe heraclei</i> is a serious concern in <i>Heracleum moellendorffii</i> Hance. Therefore, exploring the mechanisms underlying sugar efflux from host cells to the fungus during the plant-fungus interaction showed great significance. The study successfully cloned <i>HmSWEET8</i> and <i>HmSTP1</i> genes based on RNA-seq technology. The complementation assays in yeast EBY.VW4000 found HmSWEET8 and HmSTP1 transporting hexose. Over-expressing or silencing <i>HmSWEET8</i> in <i>H. moellendorffii</i> leaves increased or decreased powdery mildew susceptibility by changing glucose concentration in infective sites. Meanwhile, over-expressing <i>HmSTP1</i> in <i>H. moellendorffii</i> leaves also increased powdery mildew susceptibility by elevating the glucose content of infective areas. Additionally, <i>HmSTP1</i> expression was up-regulated obviously in <i>HmSWEET8</i> over-expressed plants and inhibited significantly in <i>HmSWEET8</i> silenced plants. Co-expressing <i>HmSWEET8</i> and <i>HmSTP1</i> genes significantly increased powdery mildew susceptibility compared with over-expressed <i>HmSWEET8</i> or <i>HmSTP1</i> plants alone. The results demonstrated that HmSTP1 may assist with HmSWEET8 to promote <i>E. heraclei</i> infection. Consequently, the infection caused by <i>E. heraclei</i> resulted in the activation of <i>HmSWEET8</i>, leading to an increased transfer of glucose to the apoplasmic spaces at the sites of infection, then, HmSTP1 facilitated the transport of glucose into host cells, promoting powdery mildew infection.