Main physiological mechanisms of maize resistance to white spot pathogen infection and the response patterns of glutathione peroxidase related genes

【Objective】To study the effects of maize white spot disease on the photosynthetic traits and physiological indexes of maize leaves, and to explore the expression pattern of glutathione peroxidase related genes（GSH-Px） under the stress of white spot pathogen, which could provide theoretical basis for the study of the function and regulatory mechanism of maize under the stress of white spot pathogen. 【Method】Artificial inoculation of white spot pathogen（Epicoccum sorghinum） was carried out on 2 maize inbred lines with different resistances to white spot disease（highly resistant ZHL908 and highly susceptible QB2816） during the tasseling and silking stage. Two treatments were set up:inoculated with sterile distilled water（CK） and inoculated with pathogenic bacteria（T）. Photosynthetic related traits were analyzed at 0, 48, 72 and 168 h after inoculation. The activities of catalase（CAT） and peroxidase（POD） as well as the content of glutathione（GSH）, were determined. The expression patterns of 6 members（GPX1, GPX2, GPX3, GPX4, GPX6, GPX7 genes） of the ZmGSH-Px gene family were detected by real-time fluorescence quantitative PCR, and their bioinformatics analysis was conducted. 【Result】The infection of white spot pathogen led to obvious decrease in relative chlorophyll content（SPAD value）, maximum fluorescence（F_m）, potential photochemical quantum efficiency（F_v/F_m）, nonphotochemical quenching coefficient（NPQ）, but obvious increase in intercellular CO₂ concentration（C_i）, GSH content, CAT activity and POD activity in maize leaves. Notably, the response times of GSH content and POD activity in the infected leaves of the highly resistant and highly susceptible inbred lines to the white spot pathogen were different, and showed more obvious response at 72 and 48 h after inoculation respectively. After 72 h of white spot pathogen infection, net photosynthetic rate（P_n）, stomatal conductance（G_s） and transpiration rate（T_r） in both inbred lines significantly（P＜0.05） or extremely significantly（P＜0.01） decreased compared to CK. The results of real-time fluorescence quantitative PCR showed that the 6 genes exhibited different response patterns after pathogen inoculation. The expression patterns and expression peak timings of GPX1, GPX3 and GPX4 genes were generally consistent, while the expression patterns of GPX2, GPX6 and GPX7 genes in the 2 inbred lines were greatly different. The correlation between relative expression and physiological indexes of the 6 genes in the 2 inbred lines were different. Among them, P_n, C_i and NPQ had high correlation coefficients with different genes in the 2 inbred lines respectively. The bioinformatics analysis results showed that the 6 genes were distributed on 6 different chromosomes of maize and had different physicochemical properties. 【Conclusion】White spot disease causes damage to the photosynthetic performance of maize, and the highly susceptible inbred line shows damage earlier than the highly resistant inbred line. Related ROS scavenging enzymes and small-molecule antioxidants also respond earlier. GSH and POD may play key roles in different resistance responses of maize to white spot disease. The 6 members of the ZmGSH-Px gene family show different degrees of sensitivity to white spot pathogen stress. It is speculated that GPX1, GPX3 and GPX4 genes are the genes involved in the systemic acquired resistance of maize to white spot disease, while GPX2, GPX6 and GPX7 genes are the genes involved in the specific disease resistance of maize.