Structure of Two Solanum tuberosum Steroidal Glycoalkaloid Glycosyltransferase Genes and Expression of their Promoters in Transgenic Potatoes

The Sgt2 gene in potato encodes a solanidine glucosyltransferase and is present as two distinct alleles expressed in cultivated potatoes. Promoter regions of both steroidal glycoalkaloid biosynthetic gene alleles, Sgt2.1 and Sgt2.2, were isolated from Solanum tuberosum cv. Russet Burbank genomic DNA. The genomic sequences of Sgt2.1 and Sgt2.2 were isolated by PCR amplification using a conserved region of Sgt2 and artificial upstream primers. The longest sequences for each allele were used to create beta-glucuronidase (GUS) reporter gene fusions. Fusion constructs were mobilized into stable transgenic lines for analysis of promoter expression in leaves and tubers under control and wounded conditions. S. tuberosum promoters from Sgt2.1 and from Sgt2.1 produced GUS activity in transgenic potato leaves and tubers comparable to GUS activity produced by the CaMV35S promoter. The CaMV35S promoter is a strong promoter frequently used in plant biotechnology. Both Sgt2 promoters exhibited activities similar to the CaMV35S promoter in tubers and lower relative activities in leaves. On average the Sgt2.2 promoter exhibited higher activity in both leaves and tubers relative to the Sgt2.1. There was no consistent effect of wounding on GUS activity from the Sgt2.2 promoter in leaves or tubers. The Sgt2.1 promoter supported higher transgene activity in tubers versus leaves and exhibited small but consistent increases in response to wounding in tubers only. This may be due to the presence of a MITE sequence in the Sgt2.1 promoter.