Activation of Latent Origins of DNA Replication in Forally 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 foral 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\ud followed by 1 d of CL, the last day of which coincides\ud with determination of the sepal, petal and stamen\ud whorls, or given one of two independent non-inductive\ud treatments: 48 h dark interrupted by red light (R) + 1 d\ud of CL, or 8 d of CL. Following these treatments, each\ud batch of plants was exposed to tritiated [methyl-3H]thy-\ud midine for 30, 60, 90 or 120 min. Apical domes were\ud dissected, nuclei lysed and prepared as fibre autoradio-\ud graphs from which replicon size was recorded. In S. coeli-rosa, replicon size was in the range 10±15 um\ud in SD (non-inductive) and 0±5 um in LD (inductive)\ud while in P. nil it was 10±15 um in the 48 h dark\ud interrupted by R, 5±10 um in CL (both non-inductive)\ud but was reduced to 0±5 um in the 48 h dark treatment\ud (inductive). Therefore, the recruitment of additional\ud initiation points for DNA replication occurred in both a\ud LD and a SD plant immediately before the appearance\ud of floral organs. The data are consistent in showing that\ud a shortening of S-phase, which is a characteristic feature\ud of florally determined shoot meristems for both species,\ud is brought about by the activation of latent origins of\ud DNA replication