Plant-derived, stress-responsive microRNA mimics reduce powdery mildew infections of barley.

Barley powdery mildew, caused by Blumeria hordei, leads to economic losses worldwide. Here, in silico analyses and proof-of-concept experiments were conducted to determine if specific plant-derived synthetic microRNAs (miRNAs) may be considered as a new direction for the control of barley powdery mildew. Previous studies have demonstrated that barley miRNAs regulate plant immunity against B. hordei. Based on the predicted targets of stress-responsive miRNAs in barley, four miRNAs that are upregulated during biotic stresses were selected and applied through Spray-Induced Gene Silencing (SIGS). Double-stranded RNA (dsRNA) constructs were designed and synthesized for the four selected miRNAs and sprayed on detached barley leaf segments that were subsequently infected with B. hordei. Visual disease symptoms and the B. hordei micro-colony index were significantly reduced in the treated leaf segments compared to the control. Quantitative PCR analysis confirmed a significant reduction in the relative B. hordei DNA abundance six days post infection. These results confirmed that the selected plant-derived synthetic miRNAs suppress the development of barley powdery mildew under experimental conditions. This is the first study to show that plant-derived, stress-responsive synthetic miRNAs reduced infection caused by fungal pathogens when applied through SIGS. Our approach could be applied in further experiments focusing on functional analyses of miRNAs in a time-efficient manner, and without requiring miRNA overexpression via genetic transformation.