Inhibition of prepenetration processes of the powdery mildew Golovinomyces orontii on host inflorescence stems is reduced in the Arabidopsis cuticular mutant cer3 but not in cer1

The obligate biotrophic fungus Golovinomyces orontii causes powdery mildew (PM) disease on its host Arabidopsis thaliana. Most research on the G. orontii-Arabidopsis pathosystem uses rosette leaves as experimental materials, so little is known about the behavior of G. orontii on other Arabidopsis organs. We thus conducted microscopic analyses of the PM infection process on leaves, stems, fruits and roots of Arabidopsis. Adaxial and abaxial surfaces of mature rosette leaves supported G. orontii infection, but growth was somewhat suppressed on cauline leaves. Prepenetration processes (germination and appressorium formation) were strongly inhibited on stems, fruits and roots. To test the effect of alterations in the Arabidopsis cuticle on the inhibition of prepenetration processes on stems, we used cuticle mutants of Arabidopsis eceriferum (cer) cer1 and cer3. Both mutants had been characterized for reduction in cuticular wax, but our chlorophyll leaching assay revealed increased cuticle permeability only in cer3. Mature rosette leaves and stems of cer1 and cer3 were inoculated, and prepenetration inhibition on stems was found to be significantly reduced in cer3 but not in cer1. We discuss the function of the cuticle in prepenetration inhibition, as well as our finding on suppression of G. orontii growth and reproduction on cer3 rosette leaves.