A Stowaway transposon disrupts the InWDR1 gene controlling flower and seed coloration in a medicinal cultivar of the Japanese morning glory

Abstract

Floricultural cultivars of the Japanese morning glory (Ipomoea nil) carry transposons of the Tpn1 family as active spontaneous mutagens. Half of the characterized mutations related to floricultural traits were caused by insertion of Tpn1 family elements. In addition, mutations comprising insertions of several bp, presumed to be footprints generated by transposon excisions, were also found. Among these, ca-1 and ca-2 are 7-bp insertions at the same position in the InWDR1 gene, which encodes a multifunctional transcription regulator. InWDR1 enhances anthocyanin pigmentation in blue flowers and red stems, and promotes dark brown seed pigmentation as well as seed-trichome formation. The recessive ca mutants show white flowers and whitish seeds. We characterized here a white flower and whitish seed line that is used as a medicinal herb. The mutant line carries a novel ca allele named ca-3, which is the InWDR1 gene carrying an insertion of a Stowaway-like transposon, InSto1. The ca-3 allele is the first example of a mutation induced by transposons other than those in the Tpn1 family in I. nil. Because InSto1 and the 7-bp putative footprints are inserted at identical positions in InWDR1, ca-3 is likely to be the ancestor of ca-1 and ca-2. According to Japanese historical records on whitish seeds of I. nil, putative ca mutants appeared at the end of the 17th century, at the latest. This is around one hundred years before the appearance of many floricultural mutants. This suggests that ca-3 is one of the oldest mutations, and that its origin is different from that of most floricultural mutations in I. nil.