Cucumber STACHYOSE SYNTHASE is regulated by its cis-antisense RNA asCsSTS to balance source-sink carbon partitioning

Photosynthate partitioning between source and sink is a key determinant of crop yield. In contrast to sucrose-transporting plants, cucumber (Cucumis sativus) plants mainly transport stachyose and stachyose synthase (CsSTS) synthesizes stachyose in the vasculature for loading. Therefore, CsSTS is considered a key regulator of carbon partitioning. We found that CsSTS expression and CsSTS enzyme activity were upregulated in the vasculature and downregulated in mesophyll tissues at fruiting. In situ hybridization and tissue enrichment experiments revealed that a cis-natural antisense noncoding transcript of CsSTS, named asCsSTS, is mainly expressed in mesophyll tissues. In vitro overexpression (OE), RNA interference (RNAi), and dual luciferase reporter experiments indicated that CsSTSs are negatively regulated by asCsSTS. Fluorescence in situ hybridization revealed that asCsSTS transcript localized in leaf cytoplasm, indicating that the regulation of CsSTS by asCsSTS is a posttranscriptional process. Further investigation revealed that this regulation occurred by reducing CsSTS transcript stability through a DICER-like protein-mediated pathway. Chemically induced OE and RNAi of asCsSTS led to promotion or inhibition, respectively, of assimilate export from leaves and altered fruit growth rates. Our results suggest that the regulation of CsSTSs between the mesophyll and vasculature reduces sugar storage in mesophyll tissue and promotes assimilate export from the leaf when the plant carries fruit.