Transgenic apple lines over-expressing the apple gene MpNPR1 have increased resistance to fire blight

The NPR1 gene is thought to be pivotal in the defense cascade caused by systemic acquired resistance and by R gene resistance in plants. Its over-expression in Arabidopsis and rice has resulted in increased disease resistance and elevated PR-gene expression. An NPR1 homolog, MpNPR1, was cloned from Malus x domestica and over-expressed in two apple cultivars, 'Galaxy' and 'M.26'. Leaf pieces were transformed with MpNPR1 cDNA under the control of the Pin2 or CaMV35S promoter using Agrobacterium tumefaciens strain EHA105. PCR analysis confirmed the transgene integration in ca. 20 events for each construct in each cultivar. Overexpression of MpNPR1 mRNA was shown by RT-PCR and western blot. Activation of some PR proteins (PR2, PR5, PR8) was also demonstrated. Resistance to the serious bacterial disease, fire blight, was evaluated in the growth chamber by inoculation of the shoot tips of 30-cm high own-rooted potted plants with the virulent strain Ea273 of Erwinia amylovora. In the initial test, 'Galaxy' clones with an additional copy of MpNPR1 had shoot length infected of 10.6 to 53% compared with 79.5% in the control 'Galaxy' plants, 'M.26' clones had shoot length infected of 25.1% to 84.4%. MpNPR1-over-expressing 'Galaxy' clones are being propagated for field trials.