Question

how the circadian rhythm works at a mechanistic level in plants 

Answer 1

Circadian rhythms in plant.

Definition

Circadian rhythms are observable biological oscillations that occur with a 24-hour periodicity. They are based on an endogenous transcriptional clock, which itself is reinforced by environmental cues such as variations in light and temperature.

Example: Plant circadian rhythms tell the plant what season it is and when to flower for the best chance of attracting pollinators. Behaviors showing rhythms include leaf movement, growth, germination, stomatal/gas exchange, enzyme activity, photosynthetic activity, and fragrance emission, among others.

INTRODUCTION

The innate ability to gauge time of day juxtaposed with the perception of environmental light represents one of the most widespread and closely coupled forms of cellular, tissue, and organismal regulation across a broad range of taxonomic groups. Changes in light fluence and wavelength are detected with the harvesting of photons by chromophore-binding photoreceptive molecules allowing immediate detection of important environmental changes. Biological rhythms provide organisms with the ability to anticipate environmental changes arising from the Earth’s rotation. The physiological and molecular mechanisms of daily biological rhythms, called circadian rhythms, have been the focus of study for more than a century, but the past quarter century has witnessed a wealth of information explaining the basis of these clocks, with microbial systems playing a key role in many regards. All circadian clocks are cellular in nature, a property first described in microbes . More recently, the filamentous fungus Neurospora crassa and the fruit fly Drosophila melanogaster have pioneered the description of feedback loops as the molecular basis of circadian rhythmicity.

Genetic, molecular, and biochemical analyses of the Neurospora clock have led to truly extraordinary advances in our understanding of much of the organism’s circadian properties, including the generation and sustainability of rhythmicity, phase resetting by light and temperature, and the means by which the clock controls metabolism and behavior.

OUTPUT FROM THE CLOCK IS THE NATURE OF TEMPORAL INFORMATION

The utilitarian and therefore most important aspect of all clocks, often referred to as output, is the ability to invoke “time of dayness” onto the organism, such that it can predict daily changes in the environment to regulate its own changing metabolic needs over the course of the diurnal cycle. Most outputs may occur in a linear fashion and be distinct from the clock mechanism.

Neurospora has been developed as a paradigmatic system for understanding the physiological changes governed by the clock, including development of the hypothesis that changes in transcription would be a universal means by which the clock mechanism could regulate clock output. Testing this hypothesis led to the first genome-wide screens, using subtractive hybridization.

Light input to the circadian clock

Light forms the dominant signal in resetting the clock and a close link between the circadian photoreceptors and the clock itself has been demonstrated for several systems . In fact, the distinction between input, oscillator and output is becoming more and more blurred as more is discovered about the mechanism itself.

In resetting, the clock exhibits a change in phase. That is, the hands of the circadian clock are moved forward or backwards. In terms of the molecular mechanism of the clock this would represent a change in the level or activity of a clock component to a level or activity that would normally be found at a different point in the cycle . The clock then continues as before. For most organisms the period of the circadian cycle is not quite 24 h and they show a slight resetting with each dawn and/or dusk. This plasticity allows an organism to adjust continually to changing daylength as the seasons of the year progress. The response of the clock to light is different at different times of day. The onset of light prior to the expected dawn will generally cause an advance in the phase of the rhythm whilst extension of the light period after the expected dusk will generally cause a delay in the phase of the rhythm. It is possible to produce a ‘phase response curve’ (PRC) illustrating this effect .The effects of light pulses at different times of day on the phase of the rhythm are examined for organisms otherwise maintained in darkness. An example of a typical phase response curve is shown in Fig. 2. The curve shows a strong phase‐advance response around subjective dawn and a strong phase‐delay response around subjective dusk. The response to light is greatly reduced during the subjective day, as might be expected, when the organism will normally be ‘seeing’ light. For many organisms the PRC exhibits a ‘dead‐zone’ during the subjective day where no response at all is seen to light .

Entrainment of the circadian clock by such pulses of light is known as non‐parametric entrainment as opposed to parametric entrainment to day/night cycles. Although the clock is sensitive to pulses of light for resetting, it is important that the clock should not be unduly influenced by aberrations in the environment. It must not, for example, be reset by a flash of lightning at night. In general, a prolonged pulse of irradiation is required to reset the clock .

Oscillator:

Circadian oscillators are networks of biochemical feedback loops that generate 24-hour rhythms in organisms from bacteria to animals. These periodic rhythms result from a complex interplay among clock components that are specific to the organism, but share molecular mechanisms across kingdoms. A full understanding of these processes requires detailed knowledge, not only of the biochemical properties of clock proteins and their interactions, but also of the three-dimensional structure of clockwork components. Posttranslational modifications and protein–protein interactions have become a recent focus, in particular the complex interactions mediated by the phosphorylation of clock proteins and the formation of multimeric protein complexes that regulate clock genes at transcriptional and translational levels.