Charles L. Niblett

Development of a dot-immunobinding assay for detection of citrus tristeza virus

The dot-immunobinding assay (DIBA) was adapted for detection of citrus tristeza virus (CTV) and compared with DAS-ELISA and DAS-indirect ELISA. DIBA was easy to perform and as sensitive as either ELISA procedure for CTV diagnosis. The entire test could be performed in 2-3 h using polyclonal antibodies, with minimal laboratory equipment. Three different polyclonal antibodies gave a strong positive reaction with 12 selected CTV isolates; however, each serum had to be cross-absorbed with sap from healthy plants before use. The broad spectrum 3DF1 monoclonal antibody reacted with most of the CTV isolates. The MCA-13 strain-specific monoclonal antibody was specific for most severe CTV isolates. As blocking agents, 3% bovine serum albumin (BSA), 3% gelatin, 0.5% non-fat dry milk or 5% Triton X-100 gave an adequate white background on the nitrocellulose membranes and permitted discrimination between infected and healthy samples. However, 3% gelatin gave the best contrast between green for the healthy samples, and purple color for infected samples.

Asymmetric PCR ELISA: increased sensitivity and reduced costs for the detection of plant viruses

PCR ELISA is the immunodetection of the products of a polymerase chain reaction (PCR). It is effective for detecting and differentiating plant viral nucleic acids, but as currently performed, it is laborious and expensive. The procedure has been modified and simplified by using asymmetric PCR. This eliminated the need to denature and neutralize samples prior to hybridization. It also increased the relative concentration of the target DNA species, making PCR ELISA more sensitive than TaqMan™, a fluorescence-based detection method. Reducing the reaction volumes to half and the concentration of the dNTPs and the digoxigenin label by tenfold significantly reduced the costs of PCR ELISA without reducing its sensitivity. The usefulness of these modifications was demonstrated for the detection of Citrus tristeza virus and Rupestris stem pitting-associated virus. We expect that with only minor modifications asymmetric PCR ELISA could be used effectively for the detection of most nucleic acid molecules of interest.

Differentiation of "Citrus tristeza virus" (CTV) Isolates by Cleavase Fragment Lenght Polymorphism (CFLP) Analysis of the Major Coat Protein Gene

A panel of Citrus tristeza virus (CTV, genus Closterovirus, family Closteroviridae) isolates of different origins and with different biological properties were compared for polymorphisms in the major coat protein (CP) gene by cleavase fragment length polymorphism (CFLP) and single stranded conformation polymorphism (SSCP) analysis. The similarity between the CFLP patterns, which consisted of 15 to 20 bands, was estimated by the Pearson coefficient. The clustering patterns from the CFLP data were very similar to those from sequence data in an experiment with 16 cloned standards of the CP gene. By SSCP analysis on the other hand, most of the clones were not clustered in the same way. To assess the ability of CFLP to analyse biological samples, which may consist of a mixture of genomic variants, the CP gene of 12 CTV isolates was obtained directly from infected plants by immunocapture/ RT-PCR and analysed. With few exceptions, the isolates were correctly clustered according to the sequences of the variants composing the isolates. In artificial mixed infections of mild and severe isolates the patterns obtained were more closely related to the severe isolate. Thus the CFLP method was an accurate method for the identification, typing and clustering of CTV isolates. The usefulness of this technique as an alternative to SSCP analysis is suggested and discussed.