<i>Symplocarpus renifolius</i> SWEET4 and SWEET6 may contribute to stamen development via hexose transport during floral thermogenesis

Sugar transporter SWEETs play diverse roles in plants. To assess the role of SWEETs in floral thermogenesis, we analyzed Symplocarpus renifolius SWEET2, SWEET4, and SWEET6, whose mRNA expression increased from the prethermogenic to thermogenic stages in RNA-seq data. Yeast complementation using hexose transport-deficient strains showed that SrSWEET4 and SrSWEET6 transport both glucose and fructose; SrSWEET4 also transports galactose. SrSWEET2 did not complement hexose transport deficiency in the yeast. Intracellular localization of GFP-tagged proteins in S. renifolius leaf protoplasts revealed that SrSWEET4 and SrSWEET6 localize to the plasma membrane, whereas SrSWEET2 may localize to the tonoplast. In qPCR, SrSWEET4 and SrSWEET6 exhibited prominent organ-specific expression in the spadix, particularly in the stamen, whereas SrSWEET2 was not differentially expressed at any stage or organ. In situ localization of SrSWEET6 mRNA was detected in developing pollen. These results suggest that SrSWEET4 and SrSWEET6 are involved in stamen development via hexose transport during thermogenesis.