Development of fire blight resistant apple cultivars by genetic engineering

To prove the concept that genetic engineering could be used to create fire blight (FB) resistant strains of apple cultivars, 'Royal Gala' ('RG') was transformed using Agrobacterium with genes for several heterologous lytic proteins (LP), driven by constitutive promoters. 'RG' lines transgenic for attacin LP showed elevated FB resistance and normal 'RG' fruit quality during up to 4 years of field trials. Attacin-transgenic lines of 'Galaxy' and 'M.26' rootstock have also shown increased FB resistance in the field. The fruit color of 'Galaxy' was maintained in the transgenic lines. 'RG' lines transgenic for LP's SB-37, and avian and T-4 lysozyme had increased FB resistance, but were generally less resistant than attacin-transgenic lines. Genetic engineering is now being applied with genes designed for ready acceptance by regulatory agencies, industry and the consumer. The FB bacterium, Erwinia amylovora, produces an effector, protein harpin, which induces resistance when applied topically to apple flowers. Some apple lines transgenic for the harpin gene (HrpN), driven by an inducible promoter were shown to have increased FB resistance in the growth chamber and in preliminary field tests. The NPR1 protein is thought to be pivotal in the plant disease resistance cascade, and when overexpressed, has increased resistance in Arabidopsis and rice. 'Galaxy' apple lines transgenic for the apple MpNPR1 gene are being evaluated for resistance in the growth chamber. E. amylovora secretes the DspE protein, which must interact with protein(s) in apple for fire blight disease to result. Four DspE-interacting kinases have been identified, and sequenced. Sense sequences have been transferred to 'Galaxy' with the goal of silencing the kinases and preventing disease development. Transgenic regenerants are now being propagated.