VARIATIONS OF PATHOGENICITY AND COMPOSITION OF TWO CITRUS TRISTEZA VIRUS ISOLATES INDUCED BY HOST PASSAGE
Y. Zhou, Z.A. Li, X.F. Wang, K.H. Liu, T. S. Li, C.Y. Zhou
Citrus tristeza virus (CTV) exists as a group of distinct variants in citrus. Mild strain-induced cross protection as a strategy to control severe variants of CTV requires profound understanding of the influence of citrus cultivars on the composition of CTV isolates. In this study, 36 progenies of the severe CTV isolate CT33 and the protective isolate CT40 were obtained by grafting from Jincheng-Beibei 447 to five citrus cultivars. Restriction fragment length polymorphism (RFLP) assay on coat protein gene (CPG) of the original and progeny isolates indicated that CPG/HinfI group composition of CT33 was modified from cultivar to cultivar through its transfer by grafting. Further analysis by real-time PCR showed that T36 genotype was a minor type of CTV in the plants. Although T36 genotype was suppressed markedly in the progenies of CT33 and CT40 on Yanxiwanlu ponkan and Guanxi-you pummelo, the relative expression level of T36 genotype could be restored gradually after the progenies were grafted back to Jincheng- Beibei 447. With the decrease of the relative expression levels of the T36 genotype, the progenies of CT33 showed weaker seedling yellows in the indicators. Furthermore, CTV isolates only produced mild symptoms on the indicators when T30 genotype was dominant.