Immunogold silver staining associated with epi-fluorescence for cucumber mosaic virus localisation on semi-thin sections of banana tissues

The immunogold-silver staining (IGSS) technique in combination with epi-fluorescence detection was used to localise cucumber mosaic virus (CMV) particles within banana infected tissues. For this purpose, tissue samples (2 mm3) were excised from CMV-infected and highly proliferating meristem cultures of Williams BSJ banana (ITC. 0570, AAA, Cavendish subgroup). These samples were immediately fixed in a 2% paraformaldehyde/0.25% glutaraldehyde mixture, dehydrated in ethanol, and finally embedded in L.R.White resin. Semi-thin sections were cut, mounted on clean treated glass slides and immunostained for CMV particles using gold-labelled secondary antibodies and silver enhancement. Sections were counterstained with basic fuchsin and examined using laser scanning confocal microscopy. Negative controls included immuno-stained samples excised from non-virus infected material as well as infected material on which primary or secondary antibodies were not applied. Images of autofluorescence (in red) and of epi-reflectance of silver-enhanced immunogold particles (in green) were recorded separately and merged, allowing the specific localisation of CMV particles at the cellular level on semi-thin sections of aldehyde-fixed banana tissues. The main advantage of this analytical approach compared to previously published protocols is that it combines a fast staining procedure, stable preparation, a high resolution, and a narrow plane of focus with the flexibility in generation, processing and analysis of images offered by laser scanning confocal microscopy. Finally, the presence of numerous CMV particles within banana meristems constitutes a clear explanation of the very low CMV elimination efficiency when using meristem- tip culture alone.