Challenge by a nonadapted powdery mildew fungal pathogen leads to the formation of a local cell-wall apposition (papilla) beneath the point of attempted penetration. Several plasma membrane (PM) proteins with opposing roles in powdery mildew infection, including Arabidopsis thaliana PENETRATION1 (PEN1) and barley (Hordeum vulgare) MILDEW RESISTANCE LOCUS O (MLO), are localized to the site of powdery mildew attack. PEN1 contributes to penetration resistance to nonadapted powdery mildews, whereas MLO is a susceptibility factor required by adapted powdery mildew pathogens for host cell entry. Our previous studies have demonstrated that the vesicle and endosomal trafficking inhibitors, brefeldin A and wortmannin, have opposite effects on the penetration rates of adapted and nonadapted powdery mildews on grapevine. These findings prompted us to study the pathogen-induced intracellular trafficking of grapevine variants of MLO and PEN1. We first identified grapevine (Vitis vinifera) VvPEN1 and VvMLO orthologs that rescue Arabidopsis Atpen1 and Atmlo2 mlo6 mlo12 null mutants, respectively. By using endomembrane trafficking inhibitors in combination with fluorescence microscopy, we demonstrate that VvMLO3/VvMLO4 and VvPEN1 are co-trafficked together from the PM to the site of powdery mildew challenge. This focal accumulation of VvMLO3/VvMLO4 and VvPEN1 to the site of attack seems to be required for their opposing functions during powdery mildew attack, because their subcellular localization is correlated with the outcome of attempted powdery mildew penetration. ; A. Feechan, A. M. Jermakow, A. Ivancevic, D. Godfrey, H. Pak, R. Panstruga, and I. B. Dry