Identification of citrus APX gene family and their response to CYVCV infection.

Ascorbate peroxidase (APX) is one of the most important antioxidant enzymes in the reactive oxygen metabolic pathway of plants. The role of APX under biotic and abiotic stress conditions has been explored, but the response pattern of APX under biotic stresses is relatively less known. In this study, seven CsAPXs gene family members were identified based on the sweet orange (Citrus sinensis) genome and subjected to evolutionary and structural analysis using bioinformatics software. The APX genes of lemon (ClAPXs) were cloned and showed a high conservation to CsAPXs by sequences alignment. In citrus yellow vein clearing virus (CYVCV)-infected Eureka lemons (C. limon) at 30th day post inoculation, APX activity and accumulation of hydrogen peroxide (H2O2) and malondialdehyde were measured to be 3.63, 2.29, and 1.73 times to that of the healthy control. The expression levels of 7 ClAPX genes in different periods of CYVCV-infected Eureka lemon were analyzed. Notably, ClAPX1, ClAPX5, and ClAPX7 showed higher expression levels compared to healthy plants, while ClAPX2, ClAPX3, and ClAPX4 showed lower expression levels. Functional identification of ClAPX1 in Nicotiana benthamiana showed that increasing the expression of ClAPX1 could significantly reduce the accumulation of H2O2, and it was verified that ClAPX1 is located in the plasma membrane of the cell. The present study provided information on the evolution and function of citrus APXs and revealed for the first time their response pattern to CYVCV infection.