Post-transcriptional gene silencing of chalcone synthase genes is involved in white spotted pattern formation in petunia flowers.

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conclusionPost-transcriptional gene silencing of CHS genes contributes to spotted pattern formation of petunia flowers, and the duplicated tandem CHS-A repeat is one of the key factors inducing the spotted pattern. Regardless of the importance of flower color pattern, the mechanism and the gene(s) controlling pattern formation have not been fully elucidated yet. Previous studies suggested that, in picotee and star petunias, post-transcriptional gene silencing (PTGS) of chalcone synthase (CHS) is involved in pattern formation, and these patterns were recessive traits. In this study, we analyzed a petunia (Petunia hybrida) 'Night Sky' that has irregular white spotted flowers. We demonstrated that PTGS of CHS is also involved in spotted pattern formation, however, spotted pattern was a dominant trait. In picotee and star petunias, possession of homozygous CHS-A repeats, consisting of two CHS-A genes arranged in tandem, is required for pattern formation. Spotted cultivars also had CHS-A repeats, but surprisingly, CHS-A copy number was higher in spotted cultivars than in picotee and star cultivars. We sequenced 'Night Sky' genome and found that approximately 71.7 kb genomic region including the CHS-A repeat was duplicated. Segregation analysis showed tight correlation between the duplicated region and the spotted phenotype. All spotted progenies harbored the duplicated region, while progenies without the duplicated region did not produce spotted flowers. Fine mapping using the BC