The invention relates to transgenic plants comprising an inverted-repeat construct which triggers post-transcriptional gene silencing of an endogenous visual reporter gene driven by a tissue-specific promoter wherein said tissue is relevant for pathogen entry, propagation or replication and their uses for screening natural or synthetic molecules, microorganisms or extracts from micro- or macro-organisms for their potential ability to inhibit pathogen entry, propagation or replication in plants by enhancing PTGS or for characterizing the mode of action of natural or synthetic molecules that are known to enhance plant disease resistance through an ill-defined mode of action.