1. Supplementary Fig. S1 Determination of T-DNA copy number by Southern blot analysis. Fifteen µg genomic DNA of Bintje and mutant nikku were digested with restriction enzymes (PciI or PsiI) that cut within the T-DNA, and probed with an AlkPhos direct labeled 600 bp probe, and the signal detected with the CDP-Star detection kit

2. Supplementary Fig. S2 Revertant plant phenotypes appeared during in vitro tuberization experiment. From left to right, plants with increasing degree of phenotype reversion, as denoted by increased rooting and leaf/stem development. Stem sections of 3-5 cm in length were grown in 40 ml liquid propagation medium at 24±1°C, 16 h photoperiod and 70-100 μmol photons m -2 s -1 light intensity for 4 weeks followed by 30 days of plant culture in microtuberization medium at 18-20°C in the dark. (Scale bar = 1 cm)

3. Supplementary Fig. S3 Bintje explants cultured with different amounts of abscisic acid (ABA) in the culture medium. Data were recorded after 1 month of growth on MS medium with vitamins or MS medium with vitamins supplemented with different levels of ABA (Scale bar = 1cm)

4. Supplementary Fig. S4 Growth data for Bintje explants cultured with different amounts of abscisic acid (ABA) in the culture medium. Data were recorded after 1 month of growth on MS medium with vitamins or MS medium with vitamins supplemented with different levels of ABA. The values are means ± SE (sample size, n=12) of n observations per treatment. The asterisks indicate statistical significance of means in the same parameter estimated using Tukey’s HSD test (* = P < 0.05-0.005, ** = P < 0.005- 0.0005, *** = P < 0.0005)

5. Supplementary Fig. S5 Bintje explants cultured with different amounts of NaCl in the culture medium. Data were recorded after 1 month of growth on MS medium with vitamins or MS medium with vitamins supplemented with different levels of NaCl (Scale bar = 1cm)

6. Supplementary Fig. S6 Growth data for Bintje explants cultured with different amounts of NaCl in the culture medium. Data were recorded after 1 month of growth on MS medium with vitamins or MS medium with vitamins supplemented with different levels of NaCl. The values are means ± SE (sample size, n=12). The asterisks indicate statistical significance of means in the same parameter estimated using Tukey’s HSD test (* = P < 0.05-0.005, ** = P < 0.005- 0.0005, *** = P < 0.0005)

7. Supplementary Fig. S7 Potato explants cultured with different amounts of indole-3-butyric acid (IBA) in the culture medium. Bintje (upper panel) and nikku (lower panel) plants after 1 month of culture on MS medium with vitamins (a, f) and MS medium with vitamins supplemented with 20 µM (b, g), 50 µM (c, h), 100 µM (d, i) and 200 µM (e, j) IBA. Arrowheads represent callus-like structures. (Scale bar= 1 cm)

8. Supplementary Fig. S8 GenomeWalker secondary PCR products. Gene specific primers (GSP1) and nested gene specific primers (GSP2) were designed against the mannopine synthase gene sequences and used with arbitrary primer 1 (AP1) and arbitrary primer 2 (AP2) to amplify potato genomic regions flanking the left border of the T-DNA, from different mutant nikku genomic DNA GenomeWalker libraries. Secondary PCR products from GenomeWalker are shown here. Invitrogen 1 Kb Plus DNA mass ladder (M), SspI library (1), ScaI library (2), NaeI library (3), and MscI library (4). Arrows indicate fragments which were cloned and sequenced