Tansley Review No. 37 Circadian rhythms: their origin and control

Area of Science:

• Plant physiology
• Biochemistry
• Chronobiology

Background:

• Circadian rhythms are endogenous biological processes that regulate daily physiological and biochemical activities.
• Crassulacean plants, like Bryophyllum, exhibit unique carbon dioxide metabolism patterns due to their adaptation to arid environments.
• Understanding the molecular mechanisms underlying these rhythms is crucial for plant adaptation and productivity.

Purpose of the Study:

• To review and elucidate the circadian rhythm of carbon dioxide metabolism in Bryophyllum (Kalanchoë) fedtsckenkoi leaves.
• To identify the key enzymes and molecular events responsible for generating and regulating this rhythm.
• To explore similarities with circadian rhythms in other organisms and discuss potential genetic insights.

Main Methods:

• Review of existing literature on circadian rhythms in Bryophyllum and other organisms.
• Analysis of physiological and biochemical data related to carbon dioxide metabolism.
• Hypothesizing molecular mechanisms based on enzyme activity, allosteric inhibition, and post-translational modifications.

Main Results:

• The circadian rhythm in Bryophyllum leaves is primarily driven by the periodic activity of phosphoenolpyruvate carboxylase (PEPc).
• This rhythm persists under continuous darkness or light, and CO2-free or normal air conditions.
• Malate accumulation and periodic phosphorylation/dephosphorylation of PEPc are key to rhythm generation and regulation.
• Environmental factors like light and temperature influence phase shifts through malate transport and enzyme regulation.

Conclusions:

• The circadian rhythm in Bryophyllum's CO2 metabolism is a complex process involving PEPc activity, malate dynamics, and enzyme phosphorylation.
• Environmental signals modulate this rhythm by affecting malate levels and enzyme regulation, particularly in stomatal guard cells.
• Further research using techniques like anti-sense nucleic acid technology could illuminate the core circadian oscillator in various organisms.