DNA fiber replication during a morphogenetic switch in the shoot meristematic cells of a higher plant*1

Activation of replicon origins as a possible target for cytokinins in shoot meristems of Sinapis

“…1. The corresponding histograms for untreated control meristems were published elsewhere (Jacqmard and Houssa 1988). F r o m these data R was f o u n d to decrease from 15 to 7.5 g m after application o f BA (Table 1, Fig.…”

“…The parameters of DNA replication were determined using pulses of [3H]TdR of 2, 3, 4 and 5 h and the technique of DNA-fibre autoradiography adapted to the shoot meristem of mustard (Van't Hof 1976; Jacqmard and Houssa 1988). For each pulse duration, 440-1370 labelled DNA segments from five to nine meristems were measured.…”

“…Fork rate (F) was calculated from the mean length values, X, obtained with 2-and 3-h pulses. The size of most replicons (R) was the mid-value of the modal class of the midpoint-to-midpoint distances between pairs of labelled DNA segments after the shortest pulse (Ormrod and Francis 1986;Jacqmard and Houssa 1988). The time taken for a replicon to replicate its alloted DNA (R,) was calculated as the R/2F ratio ( Van't Hof et al 1978).…”

“…The chromosomal DNA of eucaryotes, including higher plants, is organized for the purpose of replication during S phase into multiple discrete units, the replicons ( Van't Hof 1985;Campbell 1986), Each replicon has an origin where replication starts and two replication forks that diverge bidirectionally from the origin while replicating nascent DNA chains. The shortening of the S phase in the transitional meristem of mustard was found to be essentially attributable to the halving of the replicon size (Jacqmard and Houssa 1988). On the other hand, analyses of the changes in the cytokinin activity in the xylem and phloem saps during the floral transition in mustard indicated that these changes in the parameters of DNA replication could be caused by an increase of the cytokinin supply for the meristem Bernier et al 1989).…”

Plant and organ development

“…In Sinapis, the shortened cell cycle occupies much of the period of evocation, including the 36 h or so before the start of floral morphogenesis itself. During this period, secondary initiation sites of DNA replication are switched on [43], S-phase shortens [34] and the cell cycle becomes synchronous [8 ]. In Sinapis, the activation of secondary origins is a response to a component of the floral stimulus which may be a cytokinin [40].…”