Light
q
All life on earth depends directly or
indirectly on radiant energy from the sun.
q
Each year 3.5 million kilowatts of electrical
energy strike the earth.
q
Light regulates the earths temperature.
q
Light provides the energy for rain, wind and
ocean currents.
q
Light travels 93 million miles from the sun to
earth at a speed of 186,000 miles per second.
Introduction
Light is an especially important factor
on the lives of plants.
In addition to being required for
photosynthesis, light also cues many key events in plant growth and
development.
These effects of light on plant
morphology are what plant biologists call photomorphogenesis.
Light reception is also important in allowing
plants to measure the passage of days and seasons.
Plants detect the direction, intensity,
and wavelengths of light.
For example, the measure of physiological
response
to light wavelength, the action spectrum, of photosynthesis has two
peaks, one in the red and
one in the blue.
These match the absorption peaks of
chlorophyll.
Action spectra can be useful in the study
of any process that depends on light.
A close correspondence between an action
spectrum of a plant response and the absorption spectrum of a purified pigment
suggests that the pigment may be the photoreceptor involved in mediating the
response.
Action spectra reveal that red and blue
light are the most important colors regulating a plants photomorphogenesis.
These observations led researchers to two
major classes of light receptors: a heterogeneous group of blue-light
photoreceptors and a family of photoreceptors called phytochromes
that absorb mostly red light.
1. Blue-light photoreceptors are a heterogeneous group of
pigments
The action spectra of
many plant processes
demonstrate that
blue light is most
effective in
initiating a
diversity of
responses.
The biochemical identity of blue-light
photoreceptors was so elusive that they were called cryptochromes.
In the 1990s, molecular biologists
analyzing Arabidopsis mutants found three completely different types of
pigments that detect blue light.
These are cryptochromes
(for the inhibition of hypocotyl elongation), phototropin (for phototropism), and a carotenoid-based photoreceptor called zeaxanthin
(for stomatal opening).
The Pigment That Controls Growth and Flowering In Many Plants
Non - Photoperiodic Effects
Involving Phytochrome
Photoperiodic Effects
Involving Phytochrome
2. Phytochromes function as
photoreceptors in many plant responses to light
Phytochromes
were discovered from studies of seed germination.
Because of their limited food resources,
successful sprouting of many types of small seeds, such as lettuce, requires
that they germinate only when conditions, especially light conditions, are near
optimal.
Such seeds often remain dormant for many
years until a change in light conditions.
For example, the death of a shading tree
or the plowing of a field may create a favorable light environment.
In the 1930s, scientists at the U.S.
Department of Agriculture determined the action spectrum for light-induced
germination of lettuce seeds.
They exposed seeds to a few minutes of
monochromatic light of various wavelengths and stored them in the dark for two
days and recorded the number of seeds that had germinated under each light
regimen.
While red light increased germination,
far red light inhibited it and the response depended on the last
flash.
The photoreceptor responsible for these
opposing effects of red and far-red light is a phytochrome.
It consists of a protein (a kinase) covalently bonded to a nonprotein
part that functions as a chromophore, the light
absorbing part of the molecule.
The chromophore
reverts back and forth between two isomeric forms with one (Pr)
absorbing red light and becoming (Pfr),
and the other (Pfr) absorbing far-red
light and becoming
(Pr).
This interconversion
between isomers acts as a switching mechanism that controls various
light-induced events in the life of the plant.
The Pfr
form triggers many of the plants developmental responses to light.
Exposure to far-red light inhibits the
germination response.
Plants synthesize phytochrome
as Pr and if seeds are kept in the dark the pigment remains almost
entirely in the Pr form.
If the seeds are illuminated with
sunlight, the phytochrome is exposed to red light
(along with other wavelengths) and much of the Pr is converted to (Pfr), triggering germination.
The phytochrome
system also provides plants with information about the quality of light.
During the day, with the mix of both red
and far-red radiation, the Pr <=>Pfr
photoreversion reaches a dynamic equilibrium.
Plants can use the ratio of these two
forms to monitor and adapt to changes in light conditions.
For example, changes in this equilibrium
might be used by a tree that requires high light intensity as a way to assess
appropriate growth strategies.
If other trees shade this tree, its phytochrome ratio will shift in favor of Pr
because the canopy screens out more red light than far-red light.
The tree could use this information to
indicate that it should allocate resources to growing taller.
If the target tree is in direct sunlight,
then the proportion of Pfr will increase,
which stimulates branching and inhibits vertical growth.
Phytochrome
Introducing
Phytochrome
You have already learned much about
pigments, especially chlorophyll. You know that a pigment absorbs light and is
altered electronically at least for a instant. This
alteration results in a change in other chemicals in the immediate environment
to drive photosynthesis. Chlorophyll is not the only important pigment, you
learned about antenna pigments too. Today we are examining a pigment that
fundamentally alters plant behavior. It is phytochrome.
Just as in botanical history, you knew
about phytochrome effects before we discussed the
identity and functions of this critical pigment
Had we done the seed germination laboratory exercise
with the red and far red light with lettuce we would see the classic phytochrome effect.
The seeds germinate better in red light and
fail in far-red light compared to control seeds in kept in darkness.
Here is a figure to extend what you
observed in lab...
Lettuce seeds kept in the dark germinate
at low frequency.
Seeds kept in the dark but briefly
exposed, after imbibing water, to red light results in considerable
germination.
Seeds kept in the dark but briefly
exposed, after imbibing water, to far-red light results in virtually no
germination.
Seeds kept in the dark but briefly
exposed, after imbibing water, to red light and then briefly exposed to far-red
light results in virtually no germination. The FR exposure appears to reverse
the R response.
Seeds kept in the dark but briefly
exposed, after imbibing water, to far-red light and then briefly exposed to red
light results in considerable germination.
The R exposure appears to reverse the FR
response.
Lettuce seeds kept in the dark but exposed,
after imbibing water, to any sequence of red and far-red light ending in FR,
results in very low germination.
Lettuce seeds kept in the dark but exposed,
after imbibing water, to any sequence of red and far-red light ending in R,
results in considerable germination.
Phytochrome
exists in two interconvertible forms
Unlike other pigments you have met so
far, phytochrome has two different chemical
structures that are inter-convertible.
The forms are named by the color of light
that they absorb maximally: Pr is a blue form that absorbs red light (660 nm)
and Pfr is a blue-green form that absorbs far-red
light (730 nm).
What is strange about these pigments is that
when they DO absorb these photons, they change chemically into the OTHER form.
This is shown in the following diagram
which you should commit to memory:
After a seed germinates, the hypocotyl lifts the cotyledons above the soil in some
species (epigeous).
This growth is rapid until the plant
penetrates the soil and is exposed to light.
This rapid water-uptake growth of a
seedling is called etiolated growth.
The seedling has evolved to include a
mechanism to ensure that it rapidly penetrates soil before it runs out of
stored nutrients in the seeds.
From the diagram below, can you tell
which color of light (R or FR) is inhibiting hypocotyl
growth?
So
which form of phytochrome (Pr or Pfr)
appears to be active in this case?
So now we look at the corresponding
growth differences between responses of two genotypes of plants.
Sun plants are defined here as those with
higher light compensation points and requiring greater photon flux density to
grow and reproduce.
Shade plants are defined here as those
with lower light compensation points and requiring lower photon flux density to
grow and reproduce.
We could think of these as canopy and
forest floor species, respectively.
Their different responses are shown in
the graph below.
Plant Responses to Signals
Photomorphogenesis
Circadian
Rhythms
Gravitropism
Signal Transduction
general
Molecular Switch
looking for a photoreceptor,
Absorption vs. Action Spectra
looking for a photoreceptor,
Phytochrome
photoreceptor molecule
Phytochrome
photoreceptor molecule
Phytochrome
Location
Phytochrome
has multiple functions,
Seed Germination,
Flowering
time (photoperiodism),
Entraining
(setting) the biological clock,
End of day,
Stem elongation,
Leaf Expansion,
Pigment
synthesis.
Photoperiodism
flowering times,
Phytochrome
photoperiodism,
Phytochrome
Cryptochromes
Phototropins
mediate phototropism,
3. Biological clocks control circadian rhythms in plants and
other eukaryotes
Many plant processes, such as
transpiration and synthesis of certain enzymes, oscillate during the day.
This is often in response to changes in
light levels, temperature, and relative humidity that accompany the 24-hour
cycle of day and night.
Even under constant conditions in a
growth chamber, many physiological processes in plants, such as opening and
closing stomata and the production of photosynthetic enzymes, continue to
oscillate with a frequency of about 24 hours.
For example, many legumes lower their
leaves in the evening and raise them in the morning.
These movements will be continued even if
plants are kept in constant light or constant darkness.
Such physiological cycles
with a frequency of about
24 hours and that are not
directly paced by any
known environmental
variable are called
circadian rhythms.
These rhythms are
ubiquitous features
of eukaryotic life.
Because organisms continue their rhythms
even when placed in the deepest mine shafts or when orbited in satellites, they
do not appear to be triggered by some subtle but pervasive environmental
signal.
All research thus far indicates that the
oscillator for circadian rhythms is endogenous (internal).
This internal clock, however, is
entrained (set) to a period of precisely 24 hours by daily signals from the
environment.
If an organism is kept in a constant
environment, its circadian rhythms deviate from a 24-hour period, with
free-running periods ranging from 21 to 27 hours.
Deviations of
the free-running period from 24 hours does not
mean that the biological clocks drift erratically, but that they are not
synchronized with the outside world.
In considering biological clocks, we need
to distinguish between the oscillator (clock) and the rhythmic processes it
controls.
For example, if we were to restrain the
leaves of a bean plant so that they cannot move, they will rush to the
appropriate position for that time of day when we release them.
We can interfere with a biological
rhythm, but the clockwork goes right on ticking off the time.
A leading hypothesis for the molecular
mechanisms underlying biological timekeeping is that it depends on synthesis of
a protein that regulates its own production through feedback control.
This protein may be a transcription
factor that inhibits transcription of the gene that encodes for the
transcription factor itself.
The concentration of this transcription
factor may accumulate during the first half of the circadian cycle, and then it
declines during the second half, due to self-inhibition of its own production.
Researchers have recently used a novel
technique to identify clock mutants in Arabidopsis.
Molecular biologists spliced the gene for
luciferase to the promotor
of a certain photosynthesis-related genes that show circadian rhythms in
transcription.
Luciferase
is the enzyme responsible for bioluminescence in fireflies.
When the biological clock turned on the promotor of the photosynthesis genes in Arabidopsis,
it also stimulated production of luciferase and the
plant glowed.
This enabled researchers to screen plants
for clock mutations, several of which are defects in proteins that normally
bind photoreceptors.
4. Light entrains the biological clock
Because the free running period of many
circadian rhythms is greater than or less than the 24 hour daily cycle, they
eventually become desynchronized with the natural environment when denied
environmental cues.
Humans experience this type of desynchronization when we cross several times zone in an
airplane, leading to the phenomenon we call jetlag.
Eventually, our circadian rhythms become
resynchronized with the external environment.
Plants are also capable of
re-establishing (entraining) their circadian synchronization.
Both phytochrome
and blue-light photoreceptors can entrain circadian rhythms of plants.
The phytochrome
system involves turning cellular responses off and on by means of the Pr
<=> Pfr switch.
In darkness, the phytochrome
ratio shifts gradually in favor of the Pr form, in part from
synthesis of new Pr molecules and, in some species, by slow
biochemical conversion of Pfr to Pr.
When the sun rises, the Pfr level suddenly increases by rapid photoconversion of Pr.
This sudden increase in Pfr each day at dawn resets the biological
clock.
5. Photoperiodism synchronizes many
plant responses to changes of season
The appropriate appearance
of seasonal events are of critical importance in the life cycles of most
plants.
These seasonal events include seed
germination, flowering, and the onset and breaking of bud dormancy.
The environmental stimulus that plants
use most often to detect the time of year is the photoperiod, the relative
lengths of night and day.
A physiological response to photoperiod,
such as flowering, is called photoperiodism.
One of the earliest clues to how plants
detect the progress of the seasons came from a mutant variety of tobacco
studied by W.W. Garner and H.A. Allard in 1920.
This variety, Maryland Mammoth, does not
flower in summer like normal tobacco plants, but in winter.
In light-regulated chambers, they
discovered that this variety would only flower if the day length was 14 hours
or shorter, which explained why it would not flower during the longer days of
the summer.
Garner and Allard termed the Maryland
Mammoth a short-day plant, because it required a light period shorter
than a critical length to flower.
Other examples include chrysanthemums,
poinsettias, and some soybean varieties.
Long-day plants
will only flower when the light period is longer than a critical number
of hours.
Examples include spinach, iris, and many
cereals.
Day-neutral plants
will flower when they reach a certain stage of maturity, regardless of day
length.
Examples include tomatoes, rice, and
dandelions.
In the 1940s, researchers discovered that
it is actually night length, not day length, that
controls flowering and other responses to photoperiod.
Research demonstrated that the cocklebur,
a short-day plant, would flower if the daytime period was broken by brief
exposures to darkness, but not if the nighttime period was broken by a few
minutes of dim light.
Short-day plants are actually long-night
plants, requiring a minimum length of uninterrupted darkness.
Cocklebur is actually unresponsive to day
length, but it requires at least 8 hours of continuous darkness to
flower.
Similarly, long-day plans are actually
short-night plants.
A long-day plant grown on photoperiods of
long nights that would not normally induce flowering will flower if the period of continuous darkness are interrupted by a few
minutes of light.
Long-day
and short-day plants are distinguished not by an absolute night length
but by whether the critical night lengths sets a maximum (long-day plants) or
minimum (short-day plants) number of hours of darkness required for flowering.
In both cases, the actual number of hours
in the critical night length is specific to each species of plant.
While the critical factor is night
length, the terms long-day and short-day are embedded firmly in the jargon
of plant physiology.
Red light is the most effective color in
interrupting the nighttime portion of the photoperiod.
Action spectra and photoreversibility
experiments show that phytochrome is the active
pigment.
If a flash of red light
during the dark period is
followed immediately by
a flash of far-red light,
then the plant detects no
interruption of night
length, demonstrating
red/far-red
photoreversibility.
Plants measure night length very
accurately.
Some short-day plants will not flower if
night is even one minute shorter than the critical length.
Some plants species always flower on the
same day each year.
Humans can exploit the photoperiodic
control of flowering to produce flowers out of season.
By punctuating each long night with a
flash of light, the floriculture industry can induce chrysanthemums, normally a
short-day plant that blooms in fall, to delay their blooming until Mothers Day
in May.
The plants interpret this as not one long
night, but two short nights.
While
some plants require only a single exposure to the appropriate photoperiod to
begin flowering, other require several successive days of the appropriate
photoperiod.
Other
plants respond to photoperiod only if pretreated by another environmental stimulus.
For example, winter wheat will not flower
unless it has been exposed to several weeks of temperatures below 10oC
(called vernalization) before exposure to the
appropriate photoperiod.
While buds produce flowers, it is leaves
that detect photoperiod and trigger flowering.
If even a single leaf receives the
appropriate photoperiod, all buds on a plant can be induced to flower, even if
they have not experienced this signal.
Plants lacking leaves will not
flower, even if exposed to the
appropriate photoperiod.
Most plant physiologists believe
that the flowering signal is a
hormone or some change in the
relative concentrations of two
or more hormones.
Whatever combination of environmental
cues and internal signals is necessary for flowering to occur,
the outcome is the transition of a buds meristem
from a vegetative state to a flowering state.
This requires that meristem-identity
genes that specify that the bud will form a flower must be switched on.
Then, organ-identity genes that specify
the spatial organization of floral organs - sepals, petals, stamens, and carpels - are activated in the appropriate regions of the meristem.
Identification of the genes and the
internal and external signals that regulate them are active areas of research.
Concept Map
Circadian Rhythms
Relating to, or exhibiting approximately 24-hour
periodicity,
circa
around +
dies day.
Circadian Rhythms
Gravitropism
the gravity directed growth
processes that direct root and shoot orientation during a plants life-cycle,
Gravitropic
Set Point
Starch Statolith Hypothesis
Re-orientation
of heavy starch grains signals gravity vector.
Table
17.1-Photoreversible control of germination.
Irradiation times were Red (R) 1 min and Far Red Fr-3 min. Germination after 48h in dark at 20C
Sensory Systems in Plants
END