DNA probe and labelling
The rDNA probes used for fluorescent in situ hybridization were the following: (i) a 2.3-kb ClaI fragment of the 25S rRNA gene isolated from A. thaliana (Unfried and Gruendler 1990), labelled with digoxigenin-11-dUTP (Roche) with the use of a nick translation kit (Roche) according to the manufacturer’s protocol and immunodetected;and (ii) 5S rDNA (pTa794): a 410-bp clone isolated
from Triticum aestivum (Gerlach and Dyer 1980) and labelled with rhodamine-5-dUTP (Roche) in a polymerase chain reaction