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Main conclusion  Certain apple cultivars accumulate to high levels in their nectar and stigma exudate an acidic chitinase III protein that can protect against pathogens including fire blight disease causing  Erwinia amylovora.      To prevent microbial infections, flower nectars and stigma exudates contain various antimicrobial compounds.  Erwinia amylovora , the causing bacterium of the devastating fire blight apple disease, is the model pathogen that multiplies in flower secretions and infects through the nectaries. Although  Erwinia -resistant apples are not available, certain cultivars are tolerant. It was reported that in flower infection assay, the ‘Freedom’ cultivar was  Erwinia  tolerant, while the ‘Jonagold’ cultivar was susceptible. We hypothesized that differences in the nectar protein compositions lead to different susceptibility. Indeed, we found that an acidic chitinase III protein (Machi3-1) selectively accumulates to very high levels in the nectar and the stigma exudate of the ‘Freedom’ cultivar. We show that three different  Machi3 - 1  alleles exist in apple cultivars and that only the  5B - Machi3 - 1  allele expresses the Machi3-1 protein in the nectar and the stigma exudate. We demonstrate that the  5B - Machi3 - 1  allele was introgressed from the  Malus floribunda  821 clone into different apple cultivars including the ‘Freedom’. Our data suggest that MYB-binding site containing repeats of the 5B- Machi3 - 1  promoter is responsible for the strong nectar- and stigma exudate-specific expression. As we found that in vitro, the Machi3-1 protein impairs growth and biofilm formation of  Erwinia  at physiological concentration, we propose that the Machi3-1 protein could partially protect  5B - Machi3 - 1  allele containing cultivars against  Erwinia  by inhibiting the multiplication and biofilm formation of the pathogen in the stigma exudate and in the nectar.

Nectar- and stigma exudate-specific expression of an acidic chitinase could partially protect certain apple cultivars against fire blight disease