Physcion, a natural anthraquinone derivative, enhances the gene expression of leaf-specific thionin of barley against Blumeria graminis.

Background: Physcion is a key active ingredient of the ethanol extract from roots of Chinese rhubarb (Rheum officinale Baill.) that has been commercialised in China for controlling powdery mildews. The biological mechanism of action of physcion against the barley powdery mildew pathogen was studied using bioassay and microarray methods.

Results: Bioassay indicated that physcion did not directly affect conidial germination of Blumeria graminis Speer f. sp. hordei Marchal, but significantly inhibited conidial germination in vivo. Challenge inoculation indicated that physcion induced localised resistance rather than systemic resistance against powdery mildew. Gene expression profiling of physcion-treated barley leaves detected four upregulated and five downregulated genes (ratio >or= 2.0 and P-value < 0.05) by using an Affymetrix Barley GeneChip. The five upregulated probe sequences blasted to the same barley leaf-specific thionin gene, with significant changes varying from 4.26 to 19.91-fold. All downregulated genes were defence-related, linked to peroxidase, oxalate oxidase, bsi1 protein and a pathogenesis-related protein. These changes varied from - 2.34 to - 2.96. Quantitative real-time PCR data confirmed that physcion enhanced the gene expression of leaf-specific thionin of barley.

Conclusion: Results indicated that physcion controls powdery mildew mainly through changing the expression of defence-related genes, and especially enhancing expression of leaf-specific thionin in barley leaves.