MiR858-Mediated Regulation of Flavonoid-Specific MYB Transcription Factor Genes Controls Resistance to Pathogen Infection in Arabidopsis

MicroRNAs (miRNAs) are a class of short endogenous non-coding small RNAs that direct post-transcriptional gene silencing in eukaryotes. In plants, the expression of a large number of miRNAs has been shown to be regulated during pathogen infection. However, the functional role of the majority of these pathogen-regulated miRNAs has not been elucidated. In this work, we investigated the role of Arabidopsis miR858 in the defense response of Arabidopsis plants to infection by fungal pathogens with necrotrophic (Plectosphaerella cucumerina) or hemibiotrophic (Fusarium oxysporum and Colletotrichum higginsianum) lifestyles. Whereas overexpression of MIR858 enhances susceptibility to pathogen infection, interference with miR858 activity by target mimics (MIM858 plants) results in disease resistance. Upon pathogen challenge, stronger activation of the defense genes PDF1.2 and PR4 occurs in MIM858 plants than in wild-type plants, whereas pathogen infection induced weaker activation of these genes in MIR858 overexpressor plants. Reduced miR858 activity, and concomitant up-regulation of miR858 target genes, in MIM858 plants, also leads to accumulation of flavonoids in Arabidopsis leaves. The antifungal activity of phenylpropanoid compounds, including flavonoids, is presented. Furthermore, pathogen infection or treatment with fungal elicitors is accompanied by a gradual decrease in MIR858 expression in wild-type plants, suggesting that miR858 plays a role in PAMP (pathogen-associated molecular pattern)-triggered immunity. These data support that miR858 is a negative regulator of Arabidopsis immunity and provide new insights into the relevant role of miR858-mediated regulation of the phenylpropanoid biosynthetic pathway in controlling Arabidopsis immunity.