Transgenic sweet potato expressing thionin from barley gives resistance to black rot disease caused by Ceratocystis fimbriata in leaves and storage roots

Black rot of sweet potato caused by pathogenic fungus Ceratocystis fimbriata severely deteriorates both growth of plants and post-harvest storage. Antimicrobial peptides from various organisms have broad range activities of killing bacteria, mycobacteria, and fungi. Plant thionin peptide exhibited anti-fungal activity against C. fimbriata. A gene for barley alpha-hordothionin (alpha HT) was placed downstream of a strong constitutive promoter of E12 Omega or the promoter of a sweet potato gene for beta-amylase of storage roots, and introduced into sweet potato commercial cultivar Kokei No. 14. Transgenic E12X Omega:alpha HT plants showed high-level expression of alpha HT mRNA in both leaves and storage roots. Transgenic beta-Amy:alpha HT plants showed sucrose-inducible expression of alpha HT mRNA in leaves, in addition to expression in storage roots. Leaves of E12 Omega:alpha HT plants exhibited reduced yellowing upon infection by C. fimbriata compared to leaves of non-transgenic Kokei No. 14, although the level of resistance was weaker than resistance cultivar Tamayutaka. Storage roots of both E12 Omega:alpha HT and beta-Amy: alpha HT plants exhibited reduced lesion areas around the site inoculated with C. fimbriata spores compared to Kokei No. 14, and some of the transgenic lines showed resistance level similar to Tamayutaka. Growth of plants and production of storage roots of these transgenic plants were not significantly different from non-transgenic plants. These results highlight the usefulness of transgenic sweet potato expressing antimicrobial peptide to reduce damages of sweet potato from the black rot disease and to reduce the use of agricultural chemicals.