Sugar will eventually be exported transporter (SWEET) is a family of sugar transporters that plays a critical role during host and pathogen interaction (Bezrutczyk et al., 2018). In rice, SWEET11/Xa13, SWEET13/Xa25, and SWEET14 were identified as targets of Xanthomonas oryzae pv. oryzae effectors (Antony et al., 2010; Hutin et al., 2015; Yang et al., 2006). However, the role of SWEET genes in rice and Rhizoctonia solani, the causative agent of sheath blight disease (ShB), is largely unknown. Our previous transcriptome data showed that SWEET14 expression is sensitive to R. solani infection (De Peng Yuan, 2020). qPCR results verified that R. solani infection dramatically induces SWEET14 expression along with a pathogen‐related protein PBZ1 (Figure 1a).

Sugars Will Eventually be Exported Transporter (SWEET) and SemiSWEET are recently characterized families of sugar transporters in eukaryotes and prokaryotes, respectively. SemiSWEETs contain 3 transmembrane helices (TMHs), while SWEETs contain 7. Here, we performed sequence-based comprehensive analyses for SWEETs and SemiSWEETs across the biosphere. In total, 3,249 proteins were identified and ≈60% proteins were found in green plants and Oomycota, which include a number of important plant pathogens. Protein sequence similarity networks indicate that proteins from different organisms are significantly clustered. Of note, SemiSWEETs with 3 or 4 TMHs that may fuse to SWEET were identified in plant genomes. 7-TMH SWEETs were found in bacteria, implying that SemiSWEET can be fused directly in prokaryote. 15-TMH extraSWEET and 25-TMH superSWEET were also observed in wild rice and oomycetes, respectively. The transporters can be classified into 4, 2, 2, and 2 clades in plants, Metazoa, unicellular eukaryotes, and prokaryotes, respectively. The consensus and coevolution of amino acids in SWEETs were identified by multiple sequence alignments. The functions of the highly conserved residues were analyzed by molecular dynamics analysis. The 19 most highly conserved residues in the SWEETs were further confirmed by point mutagenesis using SWEET1 from Arabidopsis thaliana. The results proved that the conserved residues located in the extrafacial gate (Y57, G58, G131, and P191), the substrate binding pocket (N73, N192, and W176), and the intrafacial gate (P43, Y83, F87, P145, M161, P162, and Q202) play important roles for substrate recognition and transport processes. Taken together, our analyses provide a foundation for understanding the diversity, classification, and evolution of SWEETs and SemiSWEETs using large-scale sequence analysis and further show that gene duplication and gene fusion are important factors driving the evolution of SWEETs.

Rhizoctonia solani, the causative agent of the sheath blight disease (ShB), invades rice to obtain nutrients, especially sugars; however, the molecular mechanism via which R. solani hijacks sugars from rice remains unclear. In this study, rice-R. solani interaction model was used to explore whether pathogen effector proteins affect plant sugar absorption during infection. Yeast one-hybrid assay was used to identify Activator of SWEET2a (AOS2) from R. solani. Localization and invertase secretion assays showed that nuclear localization and secreted function of AOS2. Hexose transport assays verified the hexose transporter activity of SWEET2a and SWEET3a. Yeast two-hybrid assays, Bimolecular fluorescence complementation (BiFC) and transactivation assay were conducted to verify the AOS2-WRKY53-Grassy tiller 1 (GT1) transcriptional complex and its activation of SWEET2a and SWEET3a. Genetic analysis is used to detect the response of GT1, WRKY53, SWEET2a, and SWEET3a to ShB infestation. Also, the soluble sugar contents were measured in the mutants and overexpression plants before and after the inoculation of R. solani. The present study found that R. solani protein AOS2 activates rice SWEET2a and localized in the nucleus of tobacco cells and secreted in yeast. AOS2 interacts with rice transcription factor WRKY53 and GT1 to form a complex that activates the hexose transporter gene SWEET2a and SWEET3a and negatively regulate rice resistance to ShB. These data collectively suggest that AOS2 secreted by R. solani interacts with rice WRKY53 and GT1 to form a transcriptional complex that activates SWEETs to efflux sugars to apoplast; R. solani acquires more sugars and subsequently accelerates host invasion.