Probable disease resistance (R) protein. Expressed in leaves after powdery mildew infection (e.g. Erysiphe cichoracearum UCSC1) (PubMed:15155802). Accumulates upon oviposition by pierid butterflies (e.g. Pieris brassicae) (PubMed:17142483). Induced in seedlings by the beneficial symbiotic fungus Trichoderma atroviride and, by the plant growth-promoting rhizobacterium (PGPR) Pseudomonas fluorescens after a transient repression (PubMed:22942755). Expressed after infection by the bacterial pathogenic Pseudomonas syringae pv. tomato DC3000 (PubMed:22942755). Triggered by various phytohormones such as ethylene (ET), salicylic acid (SA) and methyl jasmonate (MeJA) (PubMed:22942755). Ecotypes susceptible to Erysiphe cichoracearum, such as cv. Columbia, are lacking RPW8.1 and RPW8.2 but contain in place HR4. Belongs to the plant RPW8 protein family.