Differentiation of metaxylem cell line in the root ofAllium cepa

Summary DNA was extracted from three root segments ofAllium cepa: i) an apical portion 500 m long from the tip (meristem); ii) a second portion 4 mm long (I root segment containing metaxylem cells in the initial stages of differentiation); iii) a third portion 6 mm long (II root segment containing metaxylem cells in further stages of differentiation). A mixture of homologous 18 S and 25 S³H-rRNA was used for invitro DNA-rRNA hybridization. The following percent saturation values were detected in the three samples: 0.08 in meristem DNA (samplea), 0.129 in I root segment DNA (sampleb), and 0.105 in II root segment DNA (samplec).Thermal denaturation of DNA and the derivative curves of the melting profiles evidenced five DNA families which were differently represented in the three DNA samples. DNA elution by thermal chromatography on hydroxyapatite followed by hybridization with³H-rRNA, revealed that ribosomal cistrons melt between 90 and 91 °C, corresponding to a G-C content of 50.7%. Moreover, the amount of the DNA family containing ribosomal cistrons was greater in sampleb andc, in sampleb to a greater extent, as compared with samplea. On the other hand, one DNA family melting at a higher temperature (92–93 °C) was drastically increased in samplec.Buoyant density profiles of unsonicated DNA showed no peaks in the three DNA samples. Upon somcation, a heavy shoulder was observed in the profile of sampleb. As the density of ribosomal cistrons and that of shoulder were very similar, it seems possible that the two fractions contain many DNA sequences in common.The present studies demonstrate that the proportion of ribosomal cistrons and other DNA families does not keep constant during the development of the metaxylem cell line.