In the past, silencing of granule-bound starch synthase (GBSSI) in potato was achieved by antisense technology, where it was observed that inclusion of the 3' end of the GBSSI coding region increased silencing efficiency. Since higher silencing efficiencies were desired, GBSSI inverted repeat constructs were designed and tested in potato. First, large inverted repeats comprising the 5' and the 3' half of the GBSSI cDNA were tested. The 5' IR construct gave a significantly higher silencing efficiency than the 3' IR construct. Since it was not known whether the observed difference was due to the sequence or the orientation of the inverted repeat, the GBSSI cDNA was divided into three regions, after which each region was tested in small inverted repeats in two orientations. To this end large numbers of independent transformants were produced for each construct. The results suggested that there was no effect of inverted repeat orientation on silencing efficiency. The percentage of transformants showing strong inhibition varied from 48% for a 3'-derived construct to 87% for a 5' as well as a middle region-derived construct. Similar to the large inverted repeats, the 3' sequences induced the least efficient silencing implying that the observed differences in silencing efficiency are caused by sequence differences. The small inverted repeat constructs with a repeat size of 500-600 bp and a spacer of about 150 bp were more efficient silencing inducers than the large inverted repeat constructs where the size of the repeat was 1.1 or 1.3 kb whilst the size of spacer was 1.3 or 1.1 kb. The results presented here show that size and sequence of the inverted repeat influenced silencing efficiency.

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The effect of introns on silencing efficiency was tested in inverted repeat constructs of Granule-Bound Starch Synthase (GBSSI) cDNA by comparing the silencing efficiencies induced by inverted repeat constructs with and without introns. No effect could be attributed to the presence of introns indicating that the introns neither enhance nor inhibit post-transcriptional gene silencing. The effect of a spliceable intron in the spacer was studied by comparing constructs harbouring a spliceable or a non-spliceable intron in the spacer. As opposed to the general belief that splicing of an intron increases silencing efficiency, the use of a spliceable intron in the spacer did not result in enhancement of silencing in our experimental system.