Characterization of Citrus tristeza virus Isolates by Indicators and Molecular Biology Methods

Molecular and biological characterization of a novel citrus tristeza virus isolate that causes severe symptoms in Citrus junos cv. Ziyangxiangcheng

The complete genomic sequence of a novel citrus tristeza virus (CTV) isolate, CT91-A1, from Orah tangor grafted on Citrus junos cv. Ziyangxiangcheng rootstock in China was determined by transcriptome sequencing. Sequence alignments showed that isolate CT91-A1 shared 83.3 to 95.5% nucleotide sequence identity with extant CTV genotypes at the whole-genome level, with the highest similarity to the S1 genotype. Phylogenetic analysis revealed that CT91-A1 clustered in a unique subclade with the S1 genotype. Isolate CT91-A1 induced severe stem pitting in Mexican lime and C. junos cv. Ziyangxiangcheng and moderate stem pitting in Guanximiyou pummelo and Duncan grapefruit. It was successfully transmitted by Aphis citricidus, and it can potentially cause significant damage to the citrus industry in China.

Application of biological and single-strand conformation polymorphism assays for characterizing potential mild isolates of Citrus tristeza virus for cross protection

Citrus tristeza virus (CTV) by killing millions of citrus cultivars grown on sour orange rootstock worldwide has become one of the most dangerous viral pathogen. Characterization of 12 CTV isolates was analyzed by biological indexing. Infected samples of citrus were collected from citrus growing areas of Pakistan and CTV was detected by symptoms on indicator plants and confirmed by direct tissue blot immunoassay (DTBIA). CTV positive samples were graft inoculated on six biological indicator hosts in the study. A standardized protocol was deployed to study biological characteristics of these isolates. All biological indicators induced mild and from mild to moderate reactions against all of the CTV isolates tested. About two isolates produced stem-pitting symptoms from moderate to severe on Mexican lime. CTV strains were further characterized and confirmed by the analysis of p25 gene of CTV isolates using single-strand conformation polymorphism (SSCP) assay. SSCP analysis revealed that most isolates confined only one predominant sequence variant. SSCP profiles of PCR amplified products from CTV isolates showed bands patterns corresponding to mild and sever strain. Three isolates (4MF, 8KBS and 10GS) from different regions and cultivars were identified as potential source of mild strains for cross protection. These results are the best base for mild strain cross protection (MSCP) in the country.

Population structure and diversity of citrus tristeza virus (CTV) isolates in Hunan province, China

Stem-pitting (SP) is the main type of citrus tristeza virus (CTV) that causes severe damage to citrus trees, especially those of sweet orange, in Hunan province, China. Understanding the local CTV population structure should provide clues for effective mild strain cross-protection (MSCP) of the SP strain of CTV. In this study, markers for the p23 gene, multiple molecular markers (MMMs), and sequence analysis of the three silencing suppressor genes (p20, p23 and p25) were employed to analyze the genetic diversity and genotype composition of the CTV population based on 51 CTV-positive samples collected from 14 citrus orchards scattered around six major citrus-growing areas of Hunan. The results indicated that the CTV population structure was extremely complex and that infection was highly mixed. In total, p23 gene markers resulted in six profiles, and MMMs demonstrated 25 profiles. The severe VT and T3 types appeared to be predominantly associated with SP, while the mild T30 and RB types were related to asymptomatic samples. Based on phylogenetic analysis of the amino acid sequences of p20, p23 and p25, 19 representative CTV samples were classified into seven recently established CTV groups and a potentially novel one. A high level of genetic diversity, as well as potential recombination, was revealed among different CTV isolates. Five pure SP severe and two pure mild strains were identified by genotype composition analysis. Taken together, the results update the genetic diversity of CTV in Hunan with the detection of one possible novel strain, and this information might be applicable for the selection of appropriate mild CTV strains for controlling citrus SP disease through cross-protection.

Monoclonal antibody-based serological methods for detecting Citrus tristeza virus in citrus groves

Citrus tristeza virus (CTV) is one of the most economically important citrus viruses and harms the citrus industry worldwide. To develop reliable and effective serological detection assays of CTV, the major capsid protein (CP) gene of CTV was expressed in Escherichia coli BL21 (DE3) using the expression vector pET-28a and purified through Ni+-NTA affinity chromatography. The recombinant protein was used to immunize BALB/c mice. Four hybridoma cell lines (14B10, 14H11, 20D5, and 20G12) secreting monoclonal antibodies (MAbs) against CTV were obtained through conventional hybridoma technology. The titers of MAb-containing ascitic fluids secreted by the four hybridoma lines ranged from 10(-6) to 10(-7) in indirect enzyme-linked immunosorbent assay (ELISA). Western blots showed that all four MAbs could specifically react with CTV CP. Using the prepared MAbs, dot-ELISA, Tissue print-ELISA, and triple antibody sandwich (TAS)-ELISA were developed to detect CTV in tree nurseries and epidemiological studies. The developed dot-ELISA and TAS-ELISA methods could detect CTV in crude extracts of infected citrus leaves with dilutions of 1:2560 and 1:10, 240 (w/v, g/mL), respectively. Tissue print-ELISA was particularly useful for large-scale field sample detection, mainly owing to its simplicity and lack of sample preparation requirements. The field survey revealed that CTV is prevalent on citrus trees in the Chongqing Municipality, Jiangxi Province, and Zhejiang Province of China. The coincidence rate of serological and RT-PCR test results reached more than 99.5%. The prepared MAbs against CTV and established sensitive and specific serological assays have a significant role in the detection and prevention and control of CTV in our country.

Genetic diversity and evolution of two capsid protein genes of citrus tristeza virus isolates from China

The genetic diversity and population structure of citrus tristeza virus (CTV) isolates from China were investigated based on partial sequences spanning the C-terminal end of p61 and the complete sequences of the CPm and CP genes. Phylogenetic analysis revealed five known groups (RB, T30, T36, HA and VT) and one new group (VI) consisting of only Chinese CTV isolates. Incongruent phylogenetic trees coupled with recombination analysis suggested several recombination events in the CPm gene. Positive selection was detected at codon 9 of CPm and codons 31, 41 and 68 of CP. The widespread CTV subpopulation AT-1 found in China has a unique amino acid insertion at the C-terminus of p61, which could increase CTV population complexity with implications for the evolutionary history of the virus. Our results suggest relevant roles for gene flow, purifying selection and recombination in shaping the CTV population in China.

The epitope structure of Citrus tristeza virus coat protein mapped by recombinant proteins and monoclonal antibodies

It has been known that there exists serological differentiation among Citrus tristeza virus (CTV) isolates. The present study reports three linear epitopes (aa 48-63, 97-104, and 114-125) identified by using bacterially expressed truncated coat proteins and ten monoclonal antibodies against the native virions of CTV-S4. Site-directed mutagenesis analysis demonstrated that the mutation D98G within the newly identified epitope (97)DDDSTGIT(104) abolished its reaction to MAbs 1, 4, and 10, and the presence of G98 in HB1-CP also resulted in its failure to recognize the three MAbs. Our results suggest that the conformational differences in the epitope I (48)LGTQQNAALNRDLFLT(63) between the CPs of isolates S4 and HB1 might contribute to the different reactions of two isolates to MAbs 5 and 6. This study provides new information for the antigenic structures of CTV, and will extend the understanding of the processes required for antibody binding and aid the development of epitope-based diagnostic tools.