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Overexpression of the Apple MpNPR1 Gene Confers Increased Disease Resistance in Malus × domestica

The NPR1 gene plays a pivotal role in systemic acquired resistance in plants. Its overexpression in Arabidopsis and rice results in increased disease resistance and elevated expression of pathogenesis-related (PR) genes. An NPR1 homolog, MpNPR1-1, was cloned from apple (Malus × domestica) and overexpressed in two important apple cultivars, Galaxy and M26. Apple leaf pieces were transformed with the MpNPR1 cDNA under the control of the inducible Pin2 or constitutive Cauliflower mosaic virus (CaMV)35S promoter using Agrobacterium tumefaciens. Overexpression of MpNPR1 mRNA was shown by reverse transcriptase-polymerase chain reaction. Activation of some PR genes (PR2, PR5, and PR8) was observed. Resistance to fire blight was evaluated in a growth chamber by inoculation of the shoot tips of our own rooted 30-cm-tall plants with virulent strain Ea273 of Erwinia amylovora. Transformed Galaxy lines overexpressing MpNPR1 had 32 to 40% of shoot length infected, compared with 80% in control Galaxy plants. Transformed M26 lines overexpressing MpNPR1 under the control of the CaMV35S promoter also showed a significant reduction of disease compared with control M26 plants. Some MpNPR-overexpressing Galaxy lines also exhibited increased resistance to two important fungal pathogens of apple, Venturia inaequalis and Gymnosporangium juniperi-virginianae. Selected transformed lines have been propagated for field trials for disease resistance and fruit quality.