Agrobacterium-mediated transformation of Ipomoea trifida, a diploid relative of sweet potato

Ipomoea trifida (Convolvulaceae), a diploid relative of sweet potato, displays a sporophytic self-incompatibility (SSI) controlled by a single multi-allelic S locus. To prove an involvement of S gene candidates in the SSI system through analysis of transgenic plants, we developed an efficient method for Agrobacterium-mediated transformation in I. trifida by modifying the protocol used for the transformation of sweet potato. We used A. tumefaciens strain EHA101, which carries a binary vector, pIG121-Hm, containing GUS and HPT genes driven by the CaMV 35S promoter. Embryogenic calluses (ECs) were infected with it and then selected on medium supplemented with 10mg/L hygromycin. The frequency of callus clusters producing hygromycin-resistant (Hyg^R) ECs reached a maximum of 84.5%, from which several transgenic plants could be regenerated. All the regenerated Hyg^R plants examined were confirmed to carry both transgenes. The ubiquitous expression of GUS and the stable inheritance of transgenes were also demonstrated. This transformation method is applicable to different genotypes in I. trifida and would thus be useful for the identification of S genes as well as functional analyses of genes involved in self-incompatibility in this species.