Peroxidase gene expression and antioxidant enzymes activity in three citrus genotypes in challenging with bacterial blast agent Pesudomonas syringae pv. Syringae

Citrus blast is a widespread disease citrus in most citrus growing areas of the world except in tropical regions. The disease is predominantly incited by strains of Pseudomonas syringae pv. syringae. The reaction of the plants to infection with pathogen is manifested by metabolic changes in tissues exemplified by generation of reactive oxygen species and expression of genes involved in systemic acquired resistance (SAR). In the present study the expression of some defense genes including catalase, peroxidase and ascorbate oxidase was determined in three species and cultivar of citrus (Okitso Satsuma mandarin, Sour orange and limequat) using real-time PCR at different time intervals following injection-inoculation of the plant leaves. Expression of peroxidase peaked at 24 hours post inoculation (hpi) in satsuma mandarin and sour orange. Whereas in limquat it peaked at 48 hpi. The peroxidase level also showed an elevating trend, albeit more slowly, peaking 48 hpi in Okitso satsuma and sour orange and 12 hpi in limequat. The trend with catalase and ascorbate oxidase was the revers and elevated only in limequat plants. Therefore, the level of peroxidase in the resistant citrus species was higher than the susceptible limquat plant, whereas catalase and ascorbate oxidase levels was lower in the resistant species leading to build up of hydrogen peroxidase in the tissues appearance of the resistant phenotype.