Silicon-Induced Changes in Antifungal Phenolic Acids, Flavonoids, and Key Phenylpropanoid Pathway Genes during the Interaction between Miniature Roses and the Biotrophic Pathogen Podosphaera pannosa

Application of 3.6 mM silicon (Si+) to the rose cultivar Smart increased the concentration antimicrobial phenolic acids and flavonoids in response to infection by rose powdery mildew (Podosphaera pannosa). Simultaneously, the expression of genes coding for key enzymes in the phenylpropanoid pathway (phenylalanine ammonia lyase, cinnamyl-alcohol dehydrogenase and chalcone synthase) were up-regulated. The increase in phenolic compounds correlated with a 46% reduction in disease severity compared to inoculated leaves without Si application (Si-). Furthermore, Si-application without pathogen inoculation induced gene expression and primed the accumulation of several phenolics compared to the uninoculated Si- control. Chlorogenic acid was the phenolic acid detected in the highest concentration, with an increase of more than 80% in ‘Si+ inoculated’ compared to the ‘Si- uninoculated’ plants. Among the quantified flavonoids, rutin and quercitrin were detected in the highest concentrations and the rutin concentration increased > 20 fold in ‘Si+ inoculated’ compared to ‘Si- uninoculated’ plants. Both rutin and chlorogenic acid had an antimicrobial effect on P. pannosa, evidenced by reduced conidial germination and appressorium formation of the pathogen, both after spray application and infiltration into leaves. The application of rutin and chlorogenic acid reduced powdery mildew severity with 40-50% and observation of an effect after leaf infiltration indicate that these two phenolics can be transported to the epidermal surface. In conclusion, we provide evidence that Si plays an active role in disease reduction in rose by inducing production of antifungal phenolic metabolites, as a response to powdery mildew infection.