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.

中文翻译：

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酸性几丁质酶的花蜜和柱头分泌物特异性表达可以部分保护某些苹果品种免受火疫病的侵害


某些苹果品种的花蜜和柱头会分泌出一种酸性几丁质酶 III 蛋白，这种蛋白可以预防病原体，包括引起火疫病的梨火疫病菌。为了防止微生物感染，花蜜和柱头分泌物含有各种抗菌化合物。梨火疫病菌是毁灭性苹果火疫病的致病细菌，是在花朵分泌物中繁殖并通过蜜腺感染的模式病原体。尽管还没有抗欧文氏菌的苹果，但某些品种具有抗欧文氏菌的能力。据报道，在花卉感染试验中，“Freedom”品种对欧文氏菌具有耐受性，而“Jonagold”品种则对欧文氏菌敏感。我们假设花蜜蛋白质组成的差异导致不同的易感性。事实上，我们发现酸性几丁质酶 III 蛋白 (Machi3-1) 在“自由”品种的花蜜和柱头渗出液中选择性地积累到非常高的水平。我们发现苹果品种中存在三种不同的 Machi3-1 等位基因，并且只有 5B-Machi3-1 等位基因在花蜜和柱头分泌物中表达 Machi3-1 蛋白。我们证明 5B-Machi3-1 等位基因从 Malus floribunda 821 克隆基因渗入到不同的苹果品种中，包括“Freedom”。我们的数据表明，含有 5B-Machi3-1 启动子重复序列的 MYB 结合位点负责强花蜜和柱头分泌物特异性表达。 由于我们在体外发现，Machi3-1 蛋白在生理浓度下会损害欧文氏菌的生长和生物膜形成，因此我们提出 Machi3-1 蛋白可以通过抑制欧文氏菌的增殖和生物膜来部分保护含有 5B-Machi3-1 等位基因的品种免受欧文氏菌的侵害。病原体在柱头分泌物和花蜜中形成。