Real time qPCR expression analysis of some stress related genes in leaf tissue of Pyrus communis cv. Conference after infection with Erwinia amylovora

Hydrogen peroxide plays a critical role in the expression of disease resistance in several plant/pathogen interactions. It serves as a substrate for oxidative cross-linking of various plant cell wall components leading to the reinforcement of the cell structure, as a direct toxin against the pathogen and as a signal molecule for the induction of defence-related genes in the adjacent, still healthy tissues. In plant cells, enzymes and redox metabolites act in synergy to carry out the detoxification of hydrogen peroxide and other reactive oxygen species (ROS). Superoxide dismutase (SOD) catalyses the dismutation of superoxide to hydrogen peroxide, catalase (CAT) dismutates hydrogen peroxide to oxygen and water, and ascorbate peroxidase (APX) reduces hydrogen peroxide to water by utilising ascorbate as specific electron donor. These are considered some of the main enzymatic systems for protecting cells against oxidative damage. These enzymes are present in various isozyme forms in several cell compartments and their expression is genetically controlled and regulated both by developmental and environmental stimuli, according to the necessity to remove ROS produced in cells. The aim of this study was to determine the possible role of these antioxidants in the defence mechanism of Pyrus communis cv. Conference leaf tissue after an infection with Erwinia amylovora. Shoots of 2-year-old pear trees cv. Conference were infected with E. amylovora strain SGB 225/12, were mock infected or left untreated. To account for structural changes, not only a difference was made between control, infected and mock-infected leaves, but we also included a distinction between young and old leaves, because it is known that older leaves are less susceptible for fire blight infections. Leaf samples were taken at specific time points after infection and the expression pattern of not necrotic tissue close to the infection site was analysed for their diverse isoforms of SOD, APX and CAT by using real time qPCR. In this study, no striking differences in transcription patterns of these enzymes between control, mock infected and E. amylovora infected leaves were observed. However, a significant difference between the expression levels of some genes in young and old leaves was noticed. These differences could partially explain the different progression rate by which E. amylovora infects, invades and causes necrosis in young and old leaves.