Spot hybridization : application to viroid identification

Summary A cloned 32 P-labelled cDNA probe to potato spindle tuber viroid (PSTV) was used to investigate the possibility of applying the spot hybridization technique to the identification of 2 virois PSTV and chrysanthemum stunt viroid (CSV). The best conditions for obtaining the highest sensitivity consisted of spotting heat-denatured samples onto the nitro-cellulose membrane, then hybridizing at 42 or 55°C in the presence of 50% formamide. The technique led to detection of 100–250 pg of purified viroid RNA and to identification of the viroid in a plant extract corresponding to 0.1 mg of infected leaves. These values demonstrated a clear advantage of the hybridization technique over the electrophoretic assay. A 3 H-labelled probe could be substituted for the 32 P probe, but it exhibited poor sensitivity due to the initially low specific radioactivity of the 3 H probe. Despite large sequence homologies, the PSTV 32 P-labelled probe hybridized only with CSV in purified solutions under non-stringent conditions, e. g. low temperature (42°C) and in the presence of relatively high amounts of CSV (50 mg). Therefore, spot hybridization appeared to be a more sensitive diagnostic technique than the electrophoretic assay, but with a high degree of specificity even with apparently closely related viroids.