The Occurrence of a Ca2+-Dependent Period in the Red Light-Induced Late G1 Phase of Germinating Adiantum Spores

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Abstract
The first cell cycle of Adiantum spores, which is induced by red light (R), was studied with regard to its temporal dependence on Pfr (the active form of phytochrome) and Ca2+. Responses were quantified with increases of the spore content of DNA, thus limiting the investigation to the reactions taking place before the S phase of the cell cycle (i.e. during G0/G1/S transitions). Spores hydrated for more than 2 days in the standard medium (includes 1 millimolar free Ca2+) showed, after stimulation with a saturating R pulse, an increase of DNA beginning at about 25 hours and saturating at about 55 hours. Reversal by far-red light of the inductive effect of R was used to examine the temporal requirement for Pfr. Spores became dependent on the supply of external Ca2+ when incubated in a low Ca medium (32 nanomolar free Ca2+ with 10 millimolar EGTA); this culture condition was used, after observing that the DNA increase occurs similarly if Ca2+ is supplied after the R pulse, to examine the temporal requirement for external Ca2+. It was concluded that the G1 phase of the spore is separated into three subphases: (a) the Pfr-dependent period which immediately follows the R pulse and varies among individual spores from a few to about 25 hours, (b) the Ca2+-dependent period (about 10 hours) which occurs immediately before the S phase, and (c) a gap (15-20 hours) between the two periods. In the Ca2+-dependent period, spores require the presence of extracellular Ca2+. This period occurs only during a determined time after the R pulse, and the competence of spores to enter the S phase is lost sharply if external Ca2+ is not available continuously during this period.