Mutation of the OsSAC1 Gene, which Encodes an Endoplasmic Reticulum Protein with an Unknown Function, Causes Sugar Accumulation in Rice Leaves

Sugars are the most abundant organic compounds produced by plants, and can be used to build carbon skeletons and generate energy. The sugar accumulation 1 (OsSAC1) gene encodes a protein with an unknown function that exhibits four N-terminal transmembrane regions and two conserved domains of unknown function, DUF4220 and DUF594. OsSAC1 was found to be poorly and specifically expressed at the bottoms of young leaves and in the developing leaf sheaths. Subcellular location results showed that OsSAC1 was co-localized with ER:mCherry and targeted the endoplasmic reticulum (ER). OsSAC1 has been found to affect sugar partitioning in rice (Oryza sativa). I2/KI starch staining, ultrastructure observations and starch content measurements indicated that more and larger starch granules accumulated in ossac1 source leaves than in wild-type (WT) source leaves. Additionally, higher sucrose and glucose concentrations accumulated in the ossac1 source leaves than in WT source leaves, whereas lower sucrose and glucose concentrations were observed in the ossac1 young leaves and developing leaf sheaths than in those of the WT. Much greater expression of OsAGPL1 and OsAGPS1 (responsible for starch synthesis) and significantly less expression of OscFBP1, OscFBP2, OsSPS1 and OsSPS11 (responsible for sucrose synthesis) and OsSWEET11, OsSWEET14 and OsSUT1 (responsible for sucrose loading) occurred in ossac1 source leaves than in WT source leaves. A greater amount of the rice plasmodesmatal negative regulator OsGSD1 was detected in ossac1 young leaves and developing leaf sheaths than in those of the WT. These results suggest that ER-targeted OsSAC1 may indirectly regulate sugar partitioning in carbon-demanding young leaves and developing leaf sheaths.