Expression of a synthetic antimicrobial peptide, D4E1, in Gladiolus plants for resistance to Fusarium oxysporum f. sp. gladioli

The main pathogen of Gladiolus plants is Fusarium oxysporum, a soilborne fungus that infects roots and corms resulting in death of the plant. Purified D4E1, a synthetic antimicrobial peptide, was previously reported (De Lucca et al. 1998) to inhibit F. oxysporum spores from forming mycelial colonies in vitro at a concentration of 3 μM making it a candidate gene for genetic engineering of Gladiolus for resistance to F. oxysporum. Gladiolus cv. Peter Pears plants were transformed by particle bombardment with plasmid DNA containing a 90 bp D4E1 gene that was under the control of the duplicated CaMV 35S promoter. Five of the 14 independently transformed plant lines were evaluated for resistance to F. oxysporum. Transgenic plants were tested in vitro for resistance to F. oxysporum, and several lines appeared to be more resistant than the control plants that lacked D4E1. Cell extracts of transgenic Gladiolus lines 6(1) and 7(1) inhibited germinated spores of F. oxysporum f. sp. gladioli from forming mycelial colonies by 34 and 38 %, respectively, in vitro. F. oxysporum f. sp. gladioli was transformed with the ECFP (cyan) gene allowing us to follow the growth of F. oxysporum during infection of D4E1-transformed and non-transformed roots. Fluorescence observations using confocal laser scanning microscopy showed that 3–10 days after infection, F. oxysporum covered the surface of the root and formed pseudo-appressoria, but hyphae were never observed to penetrate cells of the root. Ten days after infection with F. oxysporum, non-transformed roots had completely disintegrated whereas transgenic roots of line 7(1) were just beginning to lose their cellular integrity. Cell extracts from the five transgenic lines showed either an inhibition of F. oxysporum mycelial colony formation or less fungal hyphae were observed to infect their roots as compared to non-transformed Gladiolus plants.