Effect of Citrus tristeza virus on the expression of isozymes and sour pathogenesis-related proteins in citrus plants.

Citrus tristeza virus (CTV) is one of the most severe pathogens of citrus. In this study,the activities of the peroxi-dase (POD) and polyphenol oxidase (PPO) in citrus leaves infected by CTV mild and severe strains were examined,which will provide some useful information on the biochemical changes in hosts during the interaction between the virus and host plants. Results showed that the effect of CTV infection on activities of these enzymes was depended on the combinations of citrus species and CTV strains. The activity of POD was decreased in leaves of Mexican lime (Citrus aurantifolia)infected by both CTV serve and mild strains,increased in leaves of sweet orange (C. sinensis) infected by CTV severe strains. However,in Poncirus trifoliate,it was not affected by any analyzed CTV strains. Meanwhile,the PPO activity was increased in leaves of Mexican lime infected by a CTV severe strain N25 and decreased in CTV-infected Poncirus trifoliata and not affected in sweet orange. The isozyme spectra of POD,PPO and esterase (EST)were also analyzed in CTV-infected sweet orange and Poncirus trifoliate. Two isozyme bands of EST were induced by a CTV mild strain in Poncirus trifoliata and no induced isozyme spectrum was detected in other inoculated citrus plants. The sour pathogenesis-related proteins (PRs) were extracted from leaves of sweet orange and P. trifoliate inoculated with CTV severe strain N25 and mild strain N4. It was found that the infection of CTV strain N25 induced expression of 6 sour PRs.