Differential DNA Replication in Pisum sativum L. Seedlings at the Onset of Germination

Abstract

The main characteristics of newly synthesized and total DNA in the elongating pea epicotyl and differentiating plumula (0—3 d) were studied. Boyant density ( δ = 1.695 g/cm 3 ) and thermal denaturation studies (T m = 86.6 to 87.0°C in 0.12 M phosphate buffer, p H 6.8) reveal identical features for both types of DNA. The reassociation of epicotyl and plumula DNA was followed in the spectrophotometer up to C 0 t 10, a value nearly corresponding to the reassociation of heterogeneous repetitive DNA. Reassociation curves of both types of DNA coincide up to C 0 t 0.1; beyond this C 0 t value both curves diverge, representing 40 and 48% reassociation at C 0 t 10 for the epicotyl and plumula DNA respectively. Reassociation characteristics of newly synthesized DNA, labelled by radioactive precursor incorporation, were studied by hydroxylapatite (HAP) column chromatography. By this technique a moderate underreplication of repetitive DNA in elongating epicotyl cortex cells and an intensive underreplication of redundant sequences in the plumula cells was demonstrated. HAP reassociation data further confirm the spectrophotometrically detected difference between the epicotyl and plumula DNA. Data obtained by biochemical methods were accomplished by extensive hostofluorometric measurements on epicotyl and plumula tissues at different stages of germination. Tihe conclusions concerning differential DNA replication at the onset of germination were represented in a scheme.