Circadian rhythms are endogenous 24-hour cycles that regulate biological processes in plants, animals, fungi and cyanobacteria. They are driven by a circadian clock and allow organisms to anticipate and adapt to daily changes in light and temperature. In plants, circadian rhythms control behaviors like leaf movement, growth, flowering and photosynthesis. The first observations of circadian rhythms were made in the 18th century, and they have since been widely studied. In Arabidopsis, the central circadian oscillator consists of interacting morning and evening feedback loops of gene expression. Circadian rhythms are temperature compensated and allow organisms to keep accurate 24-hour time even in changing conditions.

1. TOPIC :CIRCADIAN RHYTHM

2. Circadian rhythms:
• A circadian rhythm is any biological process that displays an
endogenous, entrainable oscillation of about 24 hours.
• These 24-hour rhythms are driven by a circadian clock, and they
have been widely observed in plants, animals, fungi, and
cyanobacteria.
• The term circadian comes from the Latin circa, meaning "around" (or
"approximately"), and diēm, meaning "day".

3. • Circadian : approximately day
• Rhythms : regular repeated pattern of something
• 24 hours cyclic rhythm that synchronizes with presence and absence
of light ( solar light).
• The formal study of biological temporal rhythms, such as daily, tidal,
weekly, seasonal, and annual rhythms, is called chronobiology

5. Plant circadian rhythms:
• 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.

6. History:
• The first writings, at least in the western canon, to recognize diurnal
rhythms come from the fourth century BC.
• Androsthenes described the observation of daily leaf movements of
the tamarind tree, Tamarindus indicus, that were observed on the
island of Tylos (now Bahrein) in the Persian Gulf during the marches
of Alexander the Great (Bretzl, 1903).

7. History :
• The first recorded observation of an endogenous circadian oscillation
was by the French scientist Jean-Jacques d'Ortous de Mairan in 1729.
• He noted that 24-hour patterns in the movement of the leaves of the
plant Mimosa pudica continued even when the plants were kept in
constant darkness .
• These movements were controlled by an internal mechanism and
were therefore endogenous.

8. Conti….
• Then many distinguished biologists have investigated have
investigated and confirmed this phenomenon including the famous
plant physiologist Wilhem Pfeffer and Charles and Francis Darwin
who devoted a full book to “The Power of Movements in
Plants”(1880).
• Erwin Bunning (1971) who studied circadian leaf movements for over
four decades.

9. In plants:
• The central oscillator generates a self-sustaining rhythm and is driven
by two interacting feedback loops that are active at different times of
day.
• The morning loop consists of CCA1 and LHY which encode closely
related MYB transcription factors that regulate circadian rhythms in
Arabidopsis, as well as PRR 7 and 9 .
• The evening loop consists of GI (Gigantea) and ELF4, both involved in
regulation of flowering time genes.

10. Conti…
• When CCA1 and LHY are overexpressd plants become arrhythmic, and
mRNA signals reduce, contributing to a negative feedback loop.
• Gene expression of CCA1 and LHY oscillates and peaks in the early
morning, whereas TOC1 gene expression oscillates and peaks in the early
evening.
• These three genes model a negative feedback loop in which over-expressed
CCA1 and LHY repress TOC1 and over-expressed TOC1 is a positive
regulator of CCA1 and LHY .
• it was shown in 2012 by Andrew Millar and others that TOC1 in fact serves
as a repressor not only of CCA1, LHY, and PRR7 and 9 in the morning loop
but also of GI and ELF4 in the evening loop.

12. Criteria :
• The rhythm has an endogenous free-running period that lasts
approximately 24 hours. The rhythm persists in constant conditions,
(i.e., constant darkness) with a period of about 24 hours. The period
of the rhythm in constant conditions is called the free-running period
and is denoted by the Greek letter τ (tau).
• The rhythms are entrainable. The rhythm can be reset by exposure to
external stimuli (such as light and heat), a process called entrainment.
The external stimulus used to entrain a rhythm is called the
Zeitgeber, or "time giver".

13. conti…
• The rhythms exhibit temperature compensation.
• In other words, they maintain circadian periodicity over a range of
physiological temperatures.
• Many organisms live at a broad range of temperatures, and
differences in thermal energy will affect the kinetics of all molecular
processes in their cell.

14. • What to do and at what time, synchronize body at day and night with
different functions.

15. In plants :
• Bryophytes and gymnosperms does not show the pattern of
circadian rhythms.
• Other plants such as algae,fungi,pteridophytes and angiosperms
shows the pattern of circadian rhythm activities in their species at
different time of their life cycle.

16. Conti….
• Besides circadian rhythms the organisms have also developed many
other endogenous rhythemicities during their long evolutionary
history.
• Circalunar rhythms (period 29 days)
• Circatidal rhythms (period 12.4 or 24.8 hrs)
• Circaannual rhythms (period a year)
• Circa semilunar rhythms (period 14.7 days)
• Ultradian rhythm

17. Biological clock :
• The plants showing endogenous circadian rhythms have time measuring
system or biological clock inside their cells which measures the passage of
time .
• Molecular mechanism in hypothalamus .
• Inputs –SCN – outputs
• The nature and function is
not clearly understood.

18. How plants tell time:
• Linnaeous designed the flower clock .
• Flowers bloom during day and close at specific time(night).
• Light and temperature triggers the photosynthetic reaction.
• The cells in stem, leaves, flower contain phytochrome that detect
light.
• Also can detect the diference in wavelengths.