Copper treatment of peach leaves causes lesion formation similar to the biotic stress response

Peach (Prunus persica (L.) Batsch) leaves are sensitive to copper (Cu) exposure. The symptoms of Cu exposure are similar to those of bacterial spot disease; however, the mechanism underlying lesion formation caused by Cu exposure is not clear. Here, we investigated whether lesion formation caused by Cu exposure was related to the mechanism underlying plant resistance to microbial pathogens. When Cu was applied to the centre of a pinhole on peach leaves, a two-step process was observed. A pale green section in the shape of a doughnut, located far from a Cu treatment point, first appeared on a leaf treated with 2 mM CuSO4. Next, a yellow–white section gradually spread from the Cu treatment point to the pale green section. Finally, a gap was formed in the middle of the pale green section. The inner part of the pale green section contained 96% of the Cu applied, indicating that Cu is retained in the lesion area. Real-time PCR analysis of the expression of genes encoding pathogenesis-related proteins and enzymes involved in phytoalexin synthesis revealed that three genes (encoding chitinase, pathogenesis-related protein 4, and β-1,3-glucanase-3) of the eight tested were upregulated by Cu treatment. Furthermore, treatment with caspase-1 inhibitors reduced lesion formation. These results show that Cu treatment of peach leaves causes cell death similar to that occurring during the biotic stress response.