Different rRNA gene expression in primary and adventitious roots of Allium cepa L.

Sequentially used silver staining and in situ hybridization allowed to estimate the number of rDNA loci and their activity in meristematic cells of Allium cepa roots. In primary roots, obtained from germinated seeds, the rDNA probe hybridized with four chromosomes and showed four strong sites of hybridization. All of them displayed very clear positive silver staining. In cells of adventitious roots, from bulbs, only one pair of rRNA gene loci was active and after in situ hybridization showed strong signals while two other sites were very weak. The results indicate different transcriptional rRNA gene activity in meristematic cells of roots of different developmental origin. The reduction of the number of active rRNA loci can be the result of DNA methylation but the reduction mechanism of in situ hybridization sites in adventitious roots of Allium cepa remains an open question.