A non-isotopic hybridisation procedure was developed to differentiate isolates of citrus tristeza virus (CTV) using digoxigenin (DIG)-labelled cDNA probes and different kinds of target RNA. Hybridisation of DIG-probes with purified double-stranded RNA (dsRNA) or concentrated total RNA extracts spotted on nylon membranes allowed detection of CTV nucleic acid equivalent to as little as 0.1-1 mg infected tissue, when the reaction was developed with a chemiluminiscent substrate. This sensitivity was similar to or slightly better than that obtained by hybridisation with a 32P-labelled probe. CTV was also detectable by hybridisation of DIG-probes with tissue prints from freshly cut young citrus shoots. Hybridisation of tissue prints with DIG-probes under stringent conditions (60 degrees C and 50% formamide) could differentiate CTV isolates in citrus, whether grown in the greenhouse or in the field. This rapid and sensitive procedure can easily be applied to many samples, even under field conditions, and opens the way to monitoring spatio-temporal movement of specific CTV strains (or groups of strains) in epidemiological studies.
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