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

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 GO/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 extemal 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 GI 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.

The Role(s) of Calcium Ions in Phytochrome Action

“…The first cell division of Adiantum spores is induced by R even in the absence of Ca2+. A R-induced DNA increase in the spores was unaffected even if a saturating concentration of Ca2' was added 15 h after R (Iino et al, 1989). Much longer retention of the capacity to respond to Ca2+ after phytochrome photoconversion was observed by Scheuerlein et al (1989) for R-induced germination of fern spores (1991) of Dryopteris paleacea.…”

“…A FR pulse (10 min, fluence 10' J m-') after R almost completely prevented the DNA increase, even if it was applied 10 h after the R pulse. After this time FR was less effective (Iino et al, 1989).…”

“…The highest increase in DNA content was found when Ca' was added to the medium immediately after the R pulse. However, R resulted in an increase in the DNA content even if the 1 mM Ca'' was delayed by 15 h. Longer delays result in the R-inductive effect being progressively lost (Iino et al, 1989). A FR pulse (10 min, fluence 10' J m-') after R almost completely prevented the DNA increase, even if it was applied 10 h after the R pulse.…”

“…These ranges of [Ca2+] are sufficient, but not optimal for phytochrome-stimulation of germination of fern spores: Onoclea (Wayne and Hepler, 1984), Dryopteris (Scheuerlein et a/. , 1989), Adiantum (Iino et a/., 1989) or Spirodela turions . During plant development Ca2+ may accumulate inside particular cells or tissues.…”

“…Independent of the delay between a R pulse and a FR pulse to remove Pfr, only a negligible response to Ca2+ addition was observed . Interaction between phytochrome and Ca2+ observed during swelling of wheat protoplasts (Bossen et al, 1988), unrolling of barley leaf sections (Viner et al, 1988), and germination of Spirodela turions ) differed from that recorded for R-induced fern spore germination (Wayne and Hepler, 1984;Iino et al, 1989;Scheuerlein et al, 1989). Wayne and Hepler (1984) studied the relationship between phytochrome photoconversion and Ca2+ transport.…”