<p>One of the promising directions for protecting plants against diseases is to induce their mechanisms of resistance by plant resistance inducers (PRIs). In addition to assessing the effectiveness of such a strategy, it is important to elucidate the molecular basis of changes occurring in the plant after the use of the studied inducer. In this study, three products of natural origin: protein harpin derived from <i>Erwinia amylovora</i>, a causal agent of fire blight, commercialized as Messenger® and ethanol extracts from ivy (<i>Hedera helix</i>) and mistletoe (<i>Viscum album</i>) were examined for their potential to induce defense-related genes in apple (<i>Malus</i> × <i>domestica</i>) cv. ‘Ligol’. Their potential usefulness in protecting plants against diseases has been confirmed by other authors. For expression profiling, two approaches, cDNA-AFLP and quantitative real-time PCR (qRT-PCR) were used. Thirty-eight differentially expressed sequence tags (dESTs) were identified in leaves of plants sprayed with each Messenger® formulation and ivy extract and 37 dESTs – after treatment with mistletoe extract. Most of the genes were up-regulated in response to treatments. The majority of identified dESTs were involved in signalling pathways, defence and stress response, cell cycle and morphogenesis, as well as protein processing or transport. The qRT-PCR analysis of selected defence-related genes revealed, that PR genes were strongly up-regulated in response to treatments, while expression of genes encoding antioxidant enzymes and enzymes of phenylpropanoid pathway were only little or not at all affected. The expression of PR genes, considered as markers of systemic acquired resistance (SAR), was most strongly induced in leaves of apple ‘Ligol’ in response to spraying with mistletoe extract.</p>