Biological clocks

Biological clocks - Chronobiology
Chronos = the time , It was first discovered in plants that some developments
are controlled by the changing lengths of the days (Erwin Bunning 1967). The
plant has an endogenous rhythm that is about 24 h (= circadian) & which
can be used to compare with what is going on outside.
Bean leaf position reflects
physiological clock at night
(“sleep”) & at daytime
Light-Dark coordinated leaf
movements continue also without
“zeitgeber” or trigger
stimuli.Although, now they are
slightly changed & circadian!
Emergence of insects from their
pupae, hibernation, gonade growth &
wakening in hamsters, luminescense
of unicellular alga Gonyaulax,
photosynthesis, abscission, flowering
Erwin Bunning (1967) The physiological clock. Springer New York
Photoperiodism and Thermoperiodism
In nature animals and plants are well aware of circadian changes in
illumination (light-dark Î light intensity, light quality Î color or
wavelength) & temperature (relative day to night temperatures required, in a
to induce growth, reproduction, or flowering of plants or animals). However
the ratio of duration of the phases as day : night & warm : cold increases
from winter to summer and then declines again. The duration of the dark or
cold periods is easily measured by comparison of an internal rhythmic process
– a biological clock
Maintaining the biological clock and adapting it to the prevailing photoand thermo-periods of the season depends both on red light
(phytochrome)and blue light (cryptochrome)
Plants & Time: Waiting for the Spring
Thermoperiodism
In some organisms thermoderiods play an important role in the phase setting of
circadian rhythmicity. Plants; such as, chrysanthemum and tomato respond to
alternating periods of low and high temperatures.& will flower earlier if
subjected to low night and high day temperatures. Diurnal temperature
differences influence internode length, plant height, leaf orientation, shoot
orientation, chlorophyll content, lateral branching and petiole and flower stalk
elongation (Moe et. al. 1995).
Hyanzinth bulbs grown at home
or in the green house do not
stretch the peduncle but flower
close to the bulb. They require
cold to grow like this Î
Thermoperiodism ÍÎ Photoperiodism
Vernalization is a
process in which cold
stimulates ...
It is often required
before the photoperiodic
system is activated.
This makes sense both
increases in day temps
and day length suggest
that spring is coming
Henbane Hyoscyamus niger
needs thermal stimulus
(cold) before responding to
longer days to flower
The opening of Flowers Î Flower Clock
Linnaeus' flower clock was a
garden plan hypothesized by
Carolus Linnaeus that would
show plants that open or close
their flowers at particular times
of the day to accurately predict
the time. He called it specifically
the Horologium Florae (lit.
"flower clock"), and proposed
the concept in the 1751
publication Philosophia
3 a.m. Tragopogon pratensis Goat's-Beard
Botanica.
4-5 a.m. Cichorium intybus L. Chicory
He may never have planted such
a garden, but the idea was
attempted by several botanical
gardens in the early 19th
century, with mixed success
5 a.m. - 12 p.m. Taraxacum officinale Dandelion
8-9 a.m. Goat's beard Convolvulus. Morning Glory
12 AM Lactuca sativa L. Garden Lettuce
1-3 p.m. Nymphaea alba L. White Waterlily
3 p.m. Calendula officinalis L.Pot marigold
3-4 p.m. Alyssum alyssoides L.
Papaver
nudicaule L. Iceland poppy
Flower Opening
Opening of flowers is interesting 7
gets lots of interest .
However it is not related to
photoperiodism – the induction of
flowers by changing day lengths.
Flower Induction
The Shoot Apical Meristem can change into stage 3 (sexual maturity) under
the influence of internal (food, age, stress) and external influences
(photoperiod, cold, warmth, thermoperiodism)
Flower Induction
The switching of the SAM form making vegetative shoots to flowers must be
really complicated, right? =Î wrong
it needs only 3 genes to switch on the production of sepals, pistils and stamens
Flower Induction by day-length changes?
Garner & Allard (1920) found a new mutant of tobacco
called “Maryland Mammoth” that would not flower in
Maryland even when grown in the greenhouse
see mutant right and normal tobacco plant on the left Î
Later attempts with cuttings showed the MM could
flower in December in a warm greenhouse
Î
MM is a “Short Day Plant” ??? Other
SDPs were found in chrysanthemum and Poinsettia
Î concept of photoperiodism developed for
MM applied to other plants & later adapted to
animals like hamsters & humans
Depending on their geographic origin, plants flower either
(1) When days get longer = spring in Northern hemisphere Î Long-Day Plants
or
(2) When days get shorter = in some warmer and dryer regions this is when rains
stop and it is the best time to survive the dry season Î Short-Day Plants
Flower Induction by day-length changes?
SDPs and LDPs are coming from different areas of the world and their demands
are almost opposite. While LDPs demand a short night or one that is interrupted
by light, SDPs hate this to the point of being sterile
Flower Induction
long-day plants typically flower in the northern hemisphere during late spring or early
summer as days are getting longer (longest day of the year is 21 June (solstice). After
that days grow shorter (i.e. nights grow longer) until 21 December (solstice). This
situation is reversed in the southern hemisphere (i.e. longest day is 21 December and
shortest day is 21 June).
Obligate LD plants are:
•Carnation (Dianthus)
•Henbane (Hyoscyamus)
•Oat (Avena)
•Ryegrass (Lolium)
•Clover (Trifolium)
•Bellflower (Campanula carpatica)
•Oat, Ryegrass
•Spinach
•Dill, clover
Some long-day facultative plants are:
•Pea (Pisum sativum)
•Barley (Hordeum vulgare)
•Arabidopsis thaliana (model organism)
LDP are de facto
short night plants Î
do not like a long dark period
Flower Induction
Short-day plants flower when the night is longer than a critical length. They require a
consolidated period of darkness before floral development can begin. short-day plants
typically flower in the Southern hemisphere during late summer or fall as days are
getting shorter (shortest day down under is 21 June, longest day is 21 December).
Secondly, they originate form regions where cold is not crucial but drought and rain are
Short-day plants
•Chrysanthemum
•Coffee
•Poinsettia
•Strawberry
•Tobacco, var. Maryland Mammouth
•Common duckweed, (Lemna minor)
•Cocklebur (Xanthium)
• Cosmos
•Violet
Some short-day facultative plants are:
•Hemp (Cannabis)
•Cotton (Gossypium)
•Rice
•Sugar cane
Flower Induction
Day-neutral plants
do not initiate flowering based on photoperiodism at all; they flower regardless of the
night length. They may initiate flowering after attaining a certain overall developmental
stage or age, or in response to alternative environmental stimuli, such as vernalisation (a
period of low temperature), thermoperiodism, age rather than to photoperiod
Day-neutral plants
Cucumbers
roses
tomatoes,
Blue grass
Potatoes
Viburnum
Azalea
Flower types
Where does the signal for flowering come from?
Experiments show clearly that the signal for photoperiodic stimulation comes
from the leaves and transfers from there to the shoot apical meristem.
What is the photoreceptor?
Where does the signal for flowering come from?
The photoreceptor for the production of the flower stimulus in the leaves is a
pigment absorbing red light between 600 & 700 nm Î phytochrome
What is the nature of the flower signal?
Grafting experiments by Chailakhyan (1936) showed clearly that the signal was
a chemical substance that he called Î florigen = anthesin + gibberellin.
This substance was active & hence identical in LDP, SDP & day-neutral plants
Flower Induction
Flowering is the third
stage of maturation of
the shot apical
meristem.
Flower-promoting
substance or signal is
moving from one plant
to to the draft partner.
This substance/signal is
transported by the
phloem from the leaves
of the flower-inducted
plant to the one that is
not.
Hyanzinth bulbs grown at home
or in the green house do not
stretch the peduncle but flower
In addition
closeto
to the bulb. They require
photoperiodism
(1),like this Î
cold to grow
thermoperiodism there
is also the effects of
vernalization (2, cold)
and Giberellic acid (3)
that stimulate flowering
Flower Induction Florigen is coming of age after 70 years
Jan Zeevart in the Plant Cell 18: 1783-1789 (2006)
Julius Sachs (1865) developed concept of a “flower hormone” produced by … ……..
…
illuminated leaves of Tropaeolum majus going to dark-kept SAM
Garner & Allard 1920 Flowering depends on day length Î Photoperiodism
Knott 1934
Day length is perceived by leaves while flower formation occurs in
...
the stem implies long-distance transport of hormone
Chailakhyan 1936 introduced “florigen” = Anthesin + Gibberellin after grafting . . .
.
between LDP and SDP Î florigen is universal in plants
1980ies
so-called molecular genetics approach of studying mutant plants
. .. .
with deficiencies in flowering ignorant of “florigen” concept
2000s
network of 4 pathways controlling flowering in Arabidopsis . .
... .
involve photoperiod, gibberellins, Vernalization & internal stimuli
An et al 2004
speculated that the Flowering Locus T gene or protein (FT) might
..
be a mobile signal
Huang et al. 2005 FT mRNA is the long-sought florigen was considered .. . . ..
…
#3 breakthrough of 1905 by journal Science
Bohlenius et al 2006
Constans CO protein is mediating between leaf-perceived
……… .
shortening of day length and stem-located induction of bud dormancy
Other Photoperiodic Effects
In addition to
photoperiodism (1),
thermoperiodism there
is also the effects of
vernalization (2, cold)
and Giberellic acid (3)
that stimulate flowering
Photoperiodism of Pseudotsuga menziesii
Plants us photoreceptors as
phytochrome or cryptochrome
to sense seasonal changes in night
length (photoperiod) to flower, grow,
or drop their leaves & go dormant.
The coincidence of the active forms of
phytochrome or cryptochrome, created
by light during the daytime, with the
rhythm of the circadian clock allows
plants to measure the length of the
night.
Other than flowering,
photoperiodism in plants includes
(1) the growth of stems or roots
(2) the loss of leaves (leaf abscission)
(3) dormancy of buds see Î
Other Photoperiodic Effects
Other Photoperiodic effects not associated with flowering is the leaf
abscission occurring in the fall due to shortening of day length.
Leaf abscission is therefore a SD effect.
Other Photoperiodic Effects - animals
In addition to
photoperiodism (1),
thermoperiodism there
is also the effects of
vernalization (2, cold)
and Giberellic acid (3)
that stimulate flowering
Photoperiodism
At a latitude of 0 (i.e. equator) or the arctic you should not expect plants with
PP. At a latitude of 40 N (Chicago) you should. Note that the altering day
lengths is a particularity of our moderate climate zone
Basics of Photoperiods
LDPs requiring > 16 h day are at the limit because the onset of the cold would prevent
successful fruiting & seeding. SDPs requiring > 12 h darkness are not adopted to the
Northern climate since they flower later than September and that is no good.
Is flowering unhealthy for plants?
In annuals and biennials as well as some perennials like agaves and bamboo it is
flowering rather than age that causes sudden senescence and death of the individual
plant. These plants cannot
Perennial shrubs & trees
Spinach is a dioecious plant
meaning what? …………
It shows that the male plants with
pistillate flowers and no fruit
senesce and die as fast as
unfertilized staminate plants
Comparison of flowering and fruiting
plants shows that both undergo
senescence and death Î so…..
Flowers provide the death signal no
matter what. What chemical is it ?
The death of spinach Spinacia oleracea
(amaranthaceae)