Reactive Oxygen Species and Salicylic Acid Mediate the Responses of Pear to <i>Venturia nashicola</i> Infection

Reactive oxygen species (ROS) and salicylic acid (SA) are essential signaling molecules in plant cells that participate in responses to biotic and abiotic stresses. Changes in ROS and SA signals during interactions between pear and the pear scab pathogen <i>Venturia nashicola</i> remain unclear. Herein, we analyzed the roles of ROS in the signal transduction pathway of pear scab resistance using the highly resistant Huangguan and susceptible Xuehua cultivars of pear (<i>Pyrus bretschneideri</i> Rehd). Protoplasts, calluses, and leaves were obtained from 14-year-old pear trees and treated with <i>V. nashicola</i> for different periods. The results showed that ROS rapidly accumulated in protoplasts of both cultivars within a 120-min treatment period, but the fluorescence intensity of ROS differed between cultivars. The H₂O₂ content in fruit-derived calluses of Huangguan peaked at 48 h post-infection at levels 1.85 times higher than those in Xuehua. Induction of H₂O₂ by <i>V. nashicola</i> in Huangguan was more intense than in Xuehua over a 96-h treatment period. At 96 h post-infection, the malondialdehyde content in leaves of Huangguan was significantly lower than in Xuehua, while the activities of superoxide dismutase, peroxidase, and catalase, and the relative expression levels of <i>PbMnSOD</i>, <i>PbPOD</i>, and <i>PbCAT</i> genes were higher in Huangguan than Xuehua. <i>V. nashicola</i> infection also caused a continuous increase in the leaf SA content of Huangguan, which was 6.76 times higher than in Xuehua at 96 h post-infection, and <i>V. nashicola</i> exposure upregulated the expression of <i>PbPAL</i>, <i>PbICS</i>, <i>PbPR1</i>, and <i>PbPR5</i>. In summary, both ROS and SA participated in the responses of pear trees to <i>V. nashicola</i> infection and played vital roles in the signal transduction pathway of pear scab resistance.