2. Many other light controlled plant responses other than
photosynthesis, collectively called photo-morphogenesis,
are the effects of phytochrome action.
Light plays a crucial role in various aspects of plant
development including seed germination
3. Light and Plant Development
• Plants detect parts of the light spectrum that are relevant for
photosynthesis.
• Classes of major plant photoreceptors:
• 1) Phytochromes: detect red light
• 2) Cryptochromes: detect blue light
• 3) Phytotropins: detect blue light
4. Wavelength (nm)
400 500 600 700
0
20
40
60
80
100
chlorophyll b
chlorophyll a
Percent
of
light
absorbed
Light wavelengths detected by plant light receptors
Phytochromes
Cryptochromes and
Phytotropins
Red light
(630 to 680 nm)
far-red light
(710 to 740 nm)
5. Phytochromes
(Phyto: Plant, Chromes: Pigment)
Phytochrome is a blue protein pigment responsible for the
perception of light in photo-physiological processes.
Detect Red Light
6. Seed germination of many plant species is influenced
by light. Of the various photoreceptor systems, phytochrome plays an especially
important role in seed germination.
1. PHYA (Important under far-red light)
2. PHYB (important under red light)
3. PHYC
4. PHYD
5. PHYE
PHYD is a recent duplicated copy, most closely related
to PHYB
Clades of Phytochromes
PHYA and PHYB regulate germination responses to light
PHYE has been shown
to contribute to
germination in
continuous far-red light
PHYD, in combination with
PHYB, play a role in
inhibiting the PHYA-
mediated very low-fluence
response in far-red light
7. Red Light
• To maximize photosynthesis
Phytochromes :
1) promote seed germination
2) promote de-etiolation
3) control shade avoidance
4) control circadian entrainment
5) control flowering
• Etiolated plants are
plants that have been
growing in complete
darkness and are usually
long, skinny and white
• De-etiolation is the
process by which plants
change or convert their
etioplasts, plastids,
within its cells into
chloroplasts.
8. History of Phytochrome in Seed Germination
• In 1932, Beltsville research group of the USDA headed by Borthwick
and Hendricks showed that red light (630 to 680 nm) elicits the
germination of lettuce seeds, whereas far-red light (710 to 740 nm)
inhibits the process.
• It was further observed that when lettuce seeds were exposed to
alternating red and far-red light, almost all seeds that received red light
as the final treatment germinated, whereas the seeds receiving far-red
light as the final treatment did not germinate.
9. Phytochrome Promote Seed Germination
Phytochrome was also
shown to control the
germination of seeds.
Red light (activates the
receptor) promotes
seed germination and
far red light suppresses
the red light effect.
Phytochromes play important role in promoting the synthesis of gibberellins (important stimulants for
germination)
Phytochromes have also been shown to be involved in the degradation of abscisic acid
10. The predicted properties of the receptor
Irradiation is the process by which an object is exposed to radiation
Phytochrome
red light
Phytochrome
far-red light
Interconvertible
11. A protein linked to a chromophore
The chromophore (a tetrapyrrole compound) allows phytochrome to
change in response to red or far-red light.
13. Active version of
Phytochrome:
Promotes seed
germination,
shade avoidance,
and controls circadian
entrainment, flowering,
etc…
Inactive version of
Phytochrome
Active Phytochrome
14. Phytochrome promotes de-etiolation
Seedlings grown in the dark display an etiolated growth
pattern:
1) yellow unexpanded cotyledons
2) apical hook
3) Long hypocotyl
Seedlings grown in red light (or white light) display a de-
etiolated growth pattern (opposite to etiolated):
• Green expanded cotyledons
• No apical hook
• Short hypocotyl
Red light promotes chloroplast development and leaf
expansion. Leaves (cotyledons) are also growing in
upright position, allowing optimal light impact. Active
phytochrome promotes seedling development that is
optimal for photosynthesis.
15. Phytochrome controls shade avoidance
Seedlings that are shaded by larger (taller)
plants that grow above them will show a
shade avoidance response.
A shade avoidance response involves
increased elongation growth (stems and
petioles) and inhibition of leaf expansion.
As a result, the seedling will grow “above”
of what causes the shade and will now be
able to perform more efficient
photosynthesis.
As soon as the seedling is not anymore
shaded, shade-avoidance growth stops.
16. The shade avoidance response is controlled by Phytochromes
and results from changes in the ratio of red to far-red light.
Chlorophyl from plants that grow above the shaded seedling
absorb blue and red light (but not far red light). The result is a
lower ratio of red to far-red light received by the shaded plant.
Lower levels of red light compared to far-red light means a
lower level of active Phytochrome (Pfr) compared to inactive
Phytochrome (Pr).
Lower level of active Phytochrome will lead to more
elongation growth and less leaf expansion.
Phytochrome controls shade avoidance
Due to high level
Phytochrome (Pfr)
18. Germination is the process by which the seed embryo begins
growth. A seed is considered to have germinated when the
embryonic root emerges from the seed coat. Many important
crops are grown from seed. Wheat, rice, cotton, and vegetables are
started from seeds. Seed germination is a complex process that
begins when conditions are favorable for growth.
Some plants produce seeds, which germinate immediately
once they are released. Others produce seeds that have
internal dormancy mechanisms and remain dormant until
conditions are favorable before the seed can germinate.
19.
20. Stages of Germination
Stage 1:
Germination begins with the seed’s
absorption of water. Most dormant
seeds have 5–10% moisture content.
When conditions are right, water is
absorbed very rapidly. Most water is
absorbed through the micropyle. As
the cells hydrate, they swell and
become turgid or rigid. The moisture
triggers an increase in cellular
respiration. Oxygen must be present
for cellular respiration.
Stage 2:
In stage two, metabolic activity surges.
Proteins are synthesized. Gibberellins
stimulate the production of enzymes. The
enzyme amylase converts stored starches
to sugars. The enzyme protease breaks
down stored proteins into amino acids.
The sugars and amino acids are directed
towards cell division, growth, and
differentiation sites at the root and shoot
meristems or tips.
There are three major stages in the germination process.
21. Stage 3:
Metabolic processes
increase in the third
phase of germination.
The swelling of cells
causes the seed coat to
rupture. The primary
root or radicle emerges
downward, and the
stem grows upwards.
The shoot begins
manufacturing food
through
photosynthesis. The
roots absorb water and
nutrients.
22. Conditions are Required for Seed
Germination begins when favorable conditions exist for
the survival of the developing plant.
The conditions for germination include moisture, air,
optimal temperatures, and possibly light or darkness.
– Water triggers germination processes and is necessary as the
embryo grows and develops.
All seeds need oxygen to germinate. Oxygen is required for cellular
respiration, a process necessary for converting stored food into energy.
Seeds germinate at a wide range of temperatures ranging from 0°C
to 40.6°C. However, the optimum temperature for most seeds lies
between 18.3°C and 26.7°C. Temperature influences the speed of
metabolic activities. Metabolism is faster when temperatures are
warm than when temperatures are cool.
Seeds of some plants need
exposure to light before they
will germinate. Seeds of other
plants require darkness in order
to germinate, and there are
those that are not influenced by
light or darkness. Seeds that are
light sensitive have a
photoreceptor pigment, called
phytochrome, found in the
seed coat. This pigment sends
messages to the seed
instructing it to initiate or to
stop germination.
