Steps Linking the Photosynthetic Light Reactions to the Biological Clock Require Calcium

Abstract

The effect of modifying calcium concentration on the expression of the photosynthesis circadian rhythm was examined in Euglena gracilis, Klebs strain Z. Expression of the oxygen evolution rhythm required the presence of both extracellular and intracellular calcium. Several treatments were found to uncouple the rate of the light reactions from the biological clock. In the presence of these chemical agents, the rate of oxygen evolution increased steadily throughout the light portion of the light/dark cycle, instead of showing a peak of activity in the middle of the light cycle. Oxygen evolution was uncoupled from the biological clock when extracellular calcium concentrations were altered by the presence of EGTA or LaCI3. Uncoupling was also observed when intracellular calcium concentrations were disrupted by the use of Ca2+ channel blockers, the intracellular Ca2 antagonist 8-(diethylamino)-octyl-3,4,5-trimethoxybenzoate, or by disrupting expression of the inositol trisphosphate system. Uncoupling was also observed when the diacylglycerol signaling system, which activates kinase C, was inhibited by acndine orange. The inhibition was reversed by the presence of phorbol esters which activate the kinase. It was concluded that both the inositol trisphosphate and diacylglycerol signaling systems were required for the expression of the oxygen evolution rhythm generated by the biological clock.The biological clock is a timing mechanism used by eucaryotic cells to control the rates of some biochemical and physiological processes. The cell can thereby alter the rate of these reactions to match predictable daily changes in the environment such as the duration of the light cycle. Even though attempts have been made to study the biological clock in numerous systems, the molecular mechanism(s) used by cells to generate 24 h rhythms in such processes as cell division or photosynthesis has not yet been elucidated. It is generally accepted that the clock can be described as a limit cycle consisting of several steps (27). It has been determined that protein synthesis affects the expression of the clock (22), and membrane properties may also be involved (4, 18).One approach that has not been used to study the biological clock is the identification of the coupling or transducing steps that connect the biological clock to the processes controlled by the clock. Steps connecting the various rhythmic processes to the clock must exist because it has been shown that although there are rhythms in photosynthetic rate and the timing of mitosis, neither of these processes are part of the actual clock. Disruption of the cell division or photosynthesis rhythms does not stop other clock-controlled reactions in Euglena (1 1, 13). Theoretically, it might be possible to trace the transducing steps backward for a process controlled by the clock, and thus approach and possibly identify the step(s) interfacing with the clock. This approach might prove useful if the transducing steps for several rhythms in the same organism were examined and co...

Ca2+ and Calmodulin Dynamics during Photopolarization in Fucus serratus Zygotes

“…However, concentrations in excess of 1 mM LaC1, resulted in zygotes becoming unstuck from the culture dishes, which made accurate scoring of polarization impossible and casts doubt on the specificity of La3+ at these high concentrations. Indeed, La3+ is usually active at micromolar concentrations, three orders of magnitude less than in the experiments described here (Lonergan, 1990;Natawa, 1992;Herrmann and Felle, 1995;Sinclair et al, 1996). Nifedipine, bepridil, and verapamil each inhibited photopolarization at a similar concentration range and in a dose-dependent manner (Fig.…”

“…Ca2+ channels were directly inhibited during photopolarization with bepridil, nifedipine, and verapamil (Lee and Tsien, 1983;Schramm et al, 1983;Lehtonen, 1984;Reiss and Herth, 1985;Lonergan, 1990). Each compound reduced the degree of photopolarization by approximately 40%.…”

“…This might be due to severa1 types of Ca2+ channels being involved in Ca2+ import and/or to Ca2+ being mobilized from intracellular stores during photopolarization. A requirement for intracellular Ca2+ release during polar axis formation was shown by the application of TMB-8 (Saunders and Hepler, 1983;Lonergan, 1990), which inhibited photopolarization in approximately 70% of the zygote population. The effect was reduced in the presence of extracellular Ca2+, indicating that Ca2+ influx from the extracellular medium may compensate for the inhibition of intracellular Ca2+ release.…”

Spatial characteristics to calcium signalling; the calcium wave as a basic unit in plant cell calcium signalling

“…Light-induced oscillations in [Ca 2+ ] c have also been reported. The oscillations may account for an apparent coupling of [Ca 2+ ] c to circadian variations in photosynthetic reactions and shape changes in Euglena (Lonergan 1990). …”

Circadian Rhythms of Chemotaxis to Ammonium and of Methylammonium Uptake in Chlamydomonas

“…Because light stimulates methylammonium uptake, we wondered whether the methylammonium uptake rhythm reflected an underlying rhythm of photosynthetic capacity, such as has been observed in other algae (13,21). Therefore, we measured photosynthetic oxygen evolution under saturating light conditions and subsaturating/saturating bicarbonate conditions from cells tested at various circadian phases in dim LL.…”