In plant-pathogen interaction, both host and pathogen have evolved very sophisticated strategies to survive. Plants need to resist infection and pathogens require colonizing their hosts to attain nutrients for reproduction. Plants have evolved several mechanisms to resist pathogen invasion that consists of several defence layers. Meanwhile, nonexpressor of pathogenesis-related genes 1 (NPR1) plays in the model plant Arabidopsis an essential role in systemic resistance against pathogen infection. Previous reports and recent findings have indicated a similar function for it s homologous in rice. The significant of NPR1 in disease resistance have been proven by transient transformation of the Arabidopsis gen AtNPR1 in barley (Hordeum vulgare)-powdery mildew (Blumeria graminis f.sp. hordei, Bgh) interaction. In this respect, silencing of the NPR1 homologous gen HvNH1 using RNA interference method led to suppress NH1 transcript up to 3.7-fold at 12 hours after inoculation (hai) in challenging to Bgh. As well, in transgenic lines, the expression levels of pathogenesis-related proteins (PR-1b, PR-2 and PR-5) attenuated between 50-100% in compare to WT barley at 24 hai. The rate of susceptibility to Bgh in NH1-silenced lines increased about 29-33%. In contrary, wheat expressing AtNPR1 showed 31% increased resistance to Blumeria graminis f.sp. tritici. Histochemical observation showed suppression of hypersensitive response (HR) and cell wall apposition (CWA) by 28 and 22% in epidermal cells, respectively in NH1-silenced lines, resulted in increased fungal penetration rate. Application of systemic acquired resistance inducer, benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH) induced up to 48% resistance against Bgh in WT plants, whereas this rate in transgenic plants was only 5.5%. Therefore, BTH failed in breaking down the susceptibility to Bgh in NH1-silenced line. Consistently, MLO (Mildew resistance Locus O) as a negative cell death regulator was induced up to 63-64% more in NH1-silenced lines than in WT plants at 12 and 36 hai, respectively. However, the expression level of Bax inhibitor 1 as a cell death suppressor was not affected in NH1-silenced plant. On the other hand, NH1-silenced barley had shown no difference in susceptibility to necrotrophic pathogen Fusarium graminearum, the causal agent of barley root rot. Similarly in response to leaf spot fungus, Bipolaris sorokiniana, the rate of spore penetration associated with necrotic lesions was not significantly changed in transgenic lines in compared with WT plants at 60 hai. The described phenotypes in NH1-silenced barley are inheritable and stable in different generations. In conclusion, silencing of NH1 conferred susceptibility to Bgh but not to necrotrophic and hemibiotrophic fungi. The probable mechanisms behind Bgh susceptibility in NH1-silenced plants are MLO induction as well as suppression of PR proteins.
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