Amplification of ribosomal cistrons during the maturation of metaxylem in the root ofAllium cepa

Summary A mixture of 18 S and 25 S³H-rRNA fractions was used for cytological hybridization with DNA of immature metaxylem cells ofAllium cepa in different stages of development. Labelling by³H-rRNA was detected over nucleolus associated DNA of almost all investigated stages and in correspondence of DNA bodies of one stage. Utilizingin vitro DNA-rRNA hybridization technique it was shown that DNA, extracted from root portions where extra synthesis of DNA occurs in metaxylem cells, contains approximately 6-fold rDNA than DNA extracted from meristems. Since the frequency of metaxylem cells among root cells is very low, the detected difference demonstrates a conspicuos amplification process. To our knowledge this is the first report on the quantitative estimation of amplification of ribosomal cistrons in plants.Publication no. 65 from the Laboratorio di Mutagenesi e Differenziamento, Consiglio Nazionale della Ricerche, Pisa.This work was supported by research contract (69/1850/14/11/3) between the Laboratorio di Mutagenesi e Differenziamento, Pisa, and the Institute of Botany, Pisa.