Transcript Life: The Science of Biology, Ninth Edition

• Plants must be able to respond to ever-changing
environment
– How is growth regulated?
– When should reproductive structures develop?
– When should germination begin?
• Plants respond to environmental cues:
– Day length, water levels, light levels
• Plants must have mechanisms to sense and
respond:
– Receptors allow plant to sense environmental cues
– Hormones mediate effects of environmental cues
• 2 types of regulators are involved in
development and growth:
– Hormones
• Chemical signals
– Photoreceptors
• Light-sensitive proteins
– Each type of photoreceptor absorbs specific wavelengths of
light
• Light induces conformation changes in photoreceptor
initiating a signal transduction cascade
• Both act through signal transduction pathways to
elicit biological response
Figure 37.4 The Effect of Gibberellins on Dwarf Plants
Figure 37.5 Gibberellin and Fruit Growth
Gibberellins and Seed Germination
• Imbibition triggers release of gibberelins
which turns on genetic expression of
amylase to mobilize food stores
Figure 37.7 How Gibberellin Works
• Auxin
– Mediates phototropism, gravitropism,
– Apical dominance
– Promotes stem elongation and root inititation
– Fruit development
Effect of Auxin due to Polar Distribution of Auxin througout Plant
High concentrations of
Auxin prevent lateral
stem growth and axillary
bud growth
Figure 37.12 Changes Occur when a Leaf Is About to Fall
Auxin is inhibits leaf
detachment
(abscission)
Figure 37.8 The Darwins’ Phototropism Experiment
• Auxin was first
discovered as
phototrophic hormone
– Phototropism = response
of plant to light (stems
bend toward light source)
• Auxin also responsible
for gravitropism
– Ability of root to grow in
direction of gravity
Figure 37.9 Went’s Experiment
Figure 37.11 Plants Respond to Light and Gravity
Function of Auxin Depends on Unidirection Transport of Hormone through Plant
• Polar transport of auxin mediates direction plant
growth:
– Auxin made in shoot apex and diffusion down shoot in
polar fashion, stimulating cell elongation
– In roots auxin moves unidirectionally towards root tip
• Lateral redistribution of auxin responsible for
phototropism and gravitropism (directional plant
growth)
– Due to active transport of auxin out one side of cell
Figure 37.13 How Auxin Affects the Cell Wall
Figure 37.15 The Cytokinin Response Pathway
• Cytokinins
– Generally induces axillary bud formation
• Promote lateral stem and root growth
• Generally produced in roots and exhibits a
root to shoot concentration gradient
– More concentrated at roots, less concentrated
at shoots
• Ratio of auxin:cytokinin determines
degrees of shoot and root development
Figure 37.15 The Cytokinin Response Pathway (Part 1)
Figure 37.15 The Cytokinin Response Pathway (Part 2)
Figure 37.17 The Signal Transduction Pathway for Ethylene
• Ethylene
– Gas; readily diffuses
into plant tissues
– Induces:
• Senescence and
abcission
• Fruit ripening
Abscisic Acid
• Abscisic acid:
– Prevents seed germination and promotes
seeds dormany
– Stress hormone
• Trigger stomata to close to prevent water loss
• Mediates various plant responses to pathogens
• Plants Have ability to respond to
environmental cues such as day length
and light intensity
• Photomorphogeneis = physiological and
developmental events in a plant controlled
by light
– Ex: flowering, seed germination, stomata
open/close; phototropism
• Plants have ability to sense
– Quality of light (wavelength)
– Quantity of light (intensity and duration)
• Photoreceptors are responsible for
detecting wavelength and intensity of light
to mediate physiological response
• Action spectrum indicates wavelength of
light involved in any particular
photoreceptor/physiological response
Figure 37.19 Action Spectrum for Phototropism
Various Known Photoreceptors
• Phototropin
– Mediates phototropic response
• Blue light at 436 nm
• Light absorption leads to change in protein shape, triggering
protein kinase cascade and stimulation of cell elongation by
auxin
• Zeaxanthin
– Mediates light-induced opening of stomata
– Blue light
• Cryptochromes
– Absorb blue and UV light, affect seedling
development
• Phytochromes
– Red and far-red light
– Mediate flower development and seed germination
Phytochromes
Phytochromes are sensitive to 2 different wavelength of red light; causing
phytochrome to switch between 2 distinct protein conformations
(~650 nm)
Seed
Germination
Flowering
Shoot
Development
(~700 nm)
Ratio of Pfr to Pr conformation determines
biological response!!!!!
Figure 37.21 Phytochrome Stimulates Gene Transcription
Figure 37.21 Phytochrome Stimulates Gene Transcription
Figure 37.20 Sensitivity of Seeds to Red and Far-Red Light
Phytochromes Mediate Flower Development
• Plants monitor photoperiod (length of
daylight) to regulate flower development
• Plants vary in response to photoperiod:
– Short-day plants
• Flower only when day is shorter than critical
maximum
• Require long-uninterrupted night!!
– Long-day plants
• Flower only when day is longer than a critical
minimum
Figure 38.12 Day Length and Flowering
• Night length is really the key photoperiodic cue
that determines flowering!!
• Plants sense night length by measuring the ratio
of Pfr to Pr isoforms of phytochrome
– During day, more red light in 650 range, therefore
more protein in Pfr conformation by end of day
– At night, Pfr can revert slowly to Pr conformation
• Longer the night, more Pr that accumulates
• Short-day plants require low Pfr:Pr ratio
• Long-day plants require high Pfr:Pr ratio
Figure 38.13 Night Length and Flowering
Figure 38.14 Interrupting the Night
Figure 37.10 Polar Transport of Auxin