Characterization of Citrus tristeza virus strains from southern China based on analysis of restriction patterns and sequences of their coat protein genes

Citrus tristeza virus (CTV) isolates collected from southern China were characterized by biological indexing on citrus indicators, restriction fragment length polymorphism (RFLP), and bi-directional reverse transcription-polymerase chain reaction (BD-PCR) analysis of their coat protein (CP) genes. Of the 30 isolates, only two isolates, N3 and N4, did not induce visible symptoms. Twenty-eight other isolates induced stem pitting and vein clearing, plant stunting, and leaf yellowing symptoms on Mexican lime, Duncan grapefruit, and sour orange seedlings. In BD-PCR analysis, a 392-bp fragment specific for the mild strains was amplified from isolates N3 and N4, and a 320-bp fragment specific for the severe strains was produced from the other 28 isolates. The RFLP analysis for RT-PCR products of the CP gene with restriction enzyme HinfI identified seven groups representing groups I-VI and a new group, which was not involved in the seven groups defined by Gillings (J Virol Methods 44:305-317, 1993). The sequences of the CP genes from 12 Chinese CTV isolates showed a high divergence, with 91.5-99.7% identities at the nucleotide level and 94.2-99.6% identities at the amino acid level. Our results suggest that the composition of CTV populations from China has a high genetic diversity in the CP gene.