High expression of GUS activities in sweet potato storage roots by sucrose-inducible minimal promoter.

KEY MESSAGE: We developed transgenic sweet potato with Spomin (sucrose-inducible minimal promoter)-GUS gene-fused constructs. Induced GUS activities by Spomin were higher than those by CaMV 35S promoter. We developed transgenic sweet potato (Ipomoea batatas L. Lam. cv. Kokei no. 14) plants with Spomin (sucrose-inducible minimal promoter)-GUS gene-fused constructs with signal peptides for sorting to cytosol, apoplast and ER, and we analyzed the GUS expression pattern of cut tissue after sucrose treatment. Induced GUS activities by Spomin were several hundred times higher than those by the CaMV 35S promoter. Also, GUS activities in storage roots induced with a Spomin-cytosol-GUS construct were higher than those with either Spomin-apoplast or -ER-GUS constructs. The induced GUS activities by Spomin were higher in storage roots without sucrose treatment than those with sucrose treatment. Chilling (4 °C) storage roots with Spomin constructs for 4 weeks produced higher GUS activities than in storage roots stored at 25 °C for 4 weeks. The calculated maximum GUS content in the storage roots was up to about 224.2 μg/g fresh weight. The chilling treatment increased the free sucrose content in the storage roots, and this increase in endogenous sugar levels induced increased GUS activities in the storage roots. Therefore, Spomin appears to be a useful promoter to develop protein production systems using sweet potato variety Kokei no. 14 storage roots by postharvest treatment.