Transcript lecture 12 Plant growth_ Environmental cues

Environmental cues govern plant
growth
Zivuku M
Objectives
• Discuss environmental cues governing plant
growth
• Outline the role of phytochrome in phototropism.
• Describe geotropism, thigmotropism
• Explain how plants response to water and
temperature response
• Hormones and sensory response
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How Do Seeds Germinate?
• Germination is the resumption of growth after
a time of arrested embryonic development
• Environmental factors influence germination
– Spring rains provide the water amounts
necessary to swell and rupture the seed coat
(taking in water is imbibition)
– Oxygen moves in and allows the embryo to
switch to aerobic metabolism
– Increase temperatures and number of daylight
hours
Genetic Programs, Environmental
Cues
• Patterns of germination and development
have a heritable basis dictated by a plant’s
genes
• Early cell divisions may result in unequal
distribution of cytoplasm
– Cytoplasmic differences trigger variable gene
expression, which may results in variations in
hormone synthesis
– Even though all cells have the same genes, it
is the selective expression of those genes that
results in cell differentiation.
Growth and Development
• Growth and development are necessary
for plants to survive
– Growth is defined as an increase in the
number, size, and volume of cells
– Development is the emergence of specialized,
morphologically different body parts
Responses to Light
• Pigments not used in photosynthesis
• Detect light and mediate the plant’s
response to it
• Photomorphogenesis
– Nondirectional, light-triggered development
• Phototropisms
– Directional growth responses to light
• Both compensate for inability to move
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Responses to Light
• Phytochrome molecule exists in 2 forms
– Pr – absorbs red light at 660 nm
• Biologically inactive
• Converted to Pfr when red photons available
– Pfr – absorbs far-red light at 730 nm
• Active form
• Converted back to Pr when far-red photons
available
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Responses to Light
• Phytochrome (P) consists of two parts:
– Chromophore which is light-receptive
– Apoprotein which facilitates expression of lightresponse genes
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Chromophore
Apoprotein
H2N
COOH
Serine
Protein-binding site
Protein kinase
Ubiquitin-binding site
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Responses to Light
• Phytochromes are involved in many
signaling pathways that lead to gene
expression
– Pr is found in the cytoplasm
– When it is converted to Pfr it enters the
nucleus
– Pfr binds with other proteins that form a
transcription complex, leading to the
expression of light-regulated genes
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Responses to Light
• Phytochrome also works through protein
kinase-signaling pathways
• When Pr is converted to Pfr, its protein
kinase domain causes
autophosphorylation or phosphorylation of
another protein
• This initiates a signaling cascade that
activates transcription factors leading to
expression of light-regulated genes
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Responses to Light
• Amount of Pfr is also regulated by
degradation
• Ubiquitin tags Pfr for transport to the
proteasome
• Process of tagging and recycling Pfr is
precisely regulated to maintain needed
amounts of phytochrome in the cell
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Responses to Light
• Phytochrome is involved in
– Seed germination
• Inhibited by far-red light and stimulated by red light
in many plants
• Only germinate when exposed to direct sunlight
– Shoot elongation
• Etiolation is caused by a lack of red light
– Detection of plant spacing
• Plants measure the amount of far-red light
bounced back from neighboring plants
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Phototropisms
• Directional growth responses
• Connect environmental signal with cellular perception of
the signal, transduction into biochemical pathways, and
ultimately an altered growth response
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Phototropisms
• Blue-light receptor phototropin 1 (PHOT1)
– 2 light-sensing regions – change
conformation in response to blue light
– Stimulates the kinase region of PHOT1 to
autophosphorylate
– Triggers signal transduction
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Circadian Clocks
• ~ 24-hour rhythms are particularly
common among eukaryotes
• Have four characteristics:
1. Continue in absence of external inputs
2. Must be about 24 hours in duration
3. Cycle can be reset or entrained
•
Phytochrome action
4. Clock can compensate for differences in
temperature
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Gravitropism
• Response of a plant to the gravitational
field of the Earth
• Shoots exhibit negative gravitropism
• Roots have a positive gravitropic response
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Responses to Gravity
• Four general steps lead to a gravitropic
response:
1. Gravity is perceived by the cell
2. A mechanical signal is transduced into a
physiological signal
3. Physiological signal is transduced inside the
cell and to other cells
4. Differential cell elongation occurs in the “up”
and “down” sides of root and shoot
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Responses to Gravity
• In shoots, gravity is sensed along the
length of the stem in endodermal cells
surrounding the vascular tissue
– Signaling toward the outer epidermal cells
• In roots, the cap is the site of gravity
perception
– Signaling triggers differential cell elongation
and division in the elongation zone
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Stem Response to Gravity
• Auxin accumulates on lower side of the
stem
• Results in asymmetrical cell elongation
and curvature of the stem upward
• Two Arabidopsis mutants, scarecrow (scr)
and short root (shr) do not show a normal
gravitropic response
• Due to lack of a functional endodermis and
its gravity-sensing amyloplasts
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Root Response to Gravity
• Gravity-sensing cells are located in the
root cap
• Cells that actually undergo asymmetrical
growth are in the distal elongation zone
(closest to root cap)
• Auxin may be involved
– Still occurs if auxin transport is suppressed
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Thigmomorphogenesis
• Permanent form change in response to
mechanical stresses
• Thigmotropism is directional growth of a
plant or plant part in response to contact
• Thigmonastic responses occur in same
direction independent of the stimulus
• Examples of touch responses:
– Snapping of Venus flytrap leaves
– Curling of tendrils around objects
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Responses to Mechanical Stimuli
• Some touch-induced plant movements
involve reversible changes in turgor
pressure
• If water leaves turgid cells, they may
collapse, causing plant movement
• If water enters a limp cell, it becomes
turgid and may also move
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Responses to Mechanical Stimuli
• Mimosa pudica leaves have swollen
structures called pulvini at the base of their
leaflets
– When leaves are stimulated, an electrical
signal is generated
– Triggers movement of ions to outer side of
pulvini
– Water follows by osmosis
– Decreased interior turgor pressure causes the
leaf to fold
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Responses to Mechanical Stimuli
• Some turgor movements are triggered by light
• This movement maximizes photosynthesis
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Responses to Mechanical Stimuli
• Bean leaves
– Pulvini are rigid during
the day
– But lose turgor at night
– Reduce water loss from
transpiration during the
night
– Maximize
photosynthetic surface
area during the day
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Water and Temperature Responses
• Responses can be short-term or long-term
• Dormancy results in the cessation of
growth during unfavorable conditions
– Often begins with abscission – dropping of
leaves
– Advantage is that nutrient sinks can be
discarded, conserving resources
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Water and Temperature Responses
• Abscission involves changes that occur in
an abscission zone at the petiole’s base
• Hormonal changes lead to differentiation
– Protective layer – consists of several
layers of suberin-impregnated cells
– Separation layer – consists of 1–2 layers of
swollen, gelatinous cells
• Pectins will break down middle lamellae of these
cells
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Seed Dormancy
• Seeds allow plant offspring to wait until
conditions for germination are optimal
• Triggers to break seed dormancy
– Water leaching away inhibitor; cracking seed
coat osmotically
• Favorable temperatures, day length, and
amounts of water can release buds,
underground stems and roots, and seeds
from a dormant state
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Responses to Chilling
• Lipid composition of a plant’s membranes
can help predict whether the plant will be
sensitive or resistant to chilling
– The more unsaturated the membrane lipids
are, the more resistant the plant is to chilling
• Supercooling – survive as low as –40oC
– Limits ice crystal formation to extracellular
spaces
• Antifreeze proteins
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Responses to High Temperatures
• Plants produce heat shock proteins
(HSPs) if exposed to rapid temperature
increases
– HSPs stabilize other proteins
• Plants can survive otherwise lethal
temperatures if they are gradually exposed
to increasing temperature
– Acquired thermotolerance
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Hormones and Sensory
Systems
• Hormones are chemicals produced in one
part of an organism and transported to
another part where they exert a response
• In plants, hormones are not produced by
specialized tissues
• Seven major kinds of plant hormones
– Auxin, cytokinins, gibberellins,
brassinosteroids, oligosaccharins, ethylene,
and abscisic acid
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