Activation of latent origins of DNA replication in florally determined shoot meristems of long-day and short-day plants: Silene coeli-rosa and Pharbitis nil

Replicon spacing was measured during the S-phase of the cell cycle in shoot meristems of Silene coeli-rosa L., a long-day (LD) plant, and Pharbitis nil Chois, a short-day (SD) plant to examine the hypothesis that activation of latent origins of DNA replication is a feature of floral determination. Silene coeli-rosa was germinated and grown in SD for 28 d and then exposed to either a florally inductive combination of 7 LD + 2 SD, the last day of which coincides with determination of the sepal and stamen whorls, or was germinated and grown in 37 non-inductive SD. Pharbitis nil was germinated and grown in continuous light (CL) for 5 d and then given either 48 h of inductive darkness followed by 1 d of CL, the last day of which coincides with determination of the sepal, petal and stamen whorls, or given one of two independent non-inductive treatments: 48 h dark interrupted by red light (R) + 1 d of CL, or 8 d of CL. Following these treatments, each batch of plants was exposed to tritiated methyl-³H]thymidine for 30, 60, 90 or 120 min. Apical domes were dissected, nuclei lysed and prepared as fibre autoradiographs from which replicon size was recorded. In S. coeli-rosa, replicon size was in the range 10–15 μm in SD (non-inductive) and 0–5 μm in LD (inductive) while in P. nil it was 10–15 μm in the 48 h dark interrupted by R, 5–10 μm in CL (both non-inductive) but was reduced to 0–5 μm in the 48 h dark treatment (inductive). Therefore, the recruitment of additional initiation points for DNA replication occurred in both a LD and a SD plant immediately before the appearance of floral organs. The data are consistent in showing that a shortening of S-phase, which is a characteristic feature of florally determined shoot meristems for both species, is brought about by the activation of latent origins of DNA replication. Received: 14 May 1998 / Accepted: 20 August 1998