Download Auxins

Control of Growth and Responses
in Plants
Outline
•
Chapter 27
•
•
•
Mader: Biology 8th Ed.
Tropisms
Nastic Movements
Plant Hormones
– Auxins
– Gibberellins
– Cytokinins
– Abscisic Acid
– Ethylene
Photoperiodism
Mader: Biology 8th Ed.
1
2
Plant Responses
•
Phototropism
Tropisms
– Plant growth toward or away from a
unidirectional stimulus is called a tropism.
! Positive is towards stimulus.
! Negative is away from stimulus.
– Phototropism - Light
– Gravitropism - Gravity
– Thigmotropism - Touch
•
Mader: Biology 8th Ed.
Positive phototropism occurs because cells
on the shady side of the stem elongate.
– Believed a pigment related to riboflavin
acts as a photoreceptor for light when
phototropism occurs.
! Auxin migrates to shady side of stem.
" Shady sides elongate faster than
bright side.
Mader: Biology 8th Ed.
3
4
Phototropism
Gravitropism
•
•
•
When a plant is placed on its side, the stem
grows upward, opposite of the pull of
gravity.
Stems with root caps grow downward.
– Root cells contain statoliths.
Auxin is responsible for both positive
gravitropism of roots, and negative
gravitropism of shoots.
Mader: Biology 8th Ed.
Mader: Biology 8th Ed.
5
6
Gravitropism
Fig. 27.2
Mader: Biology 8th Ed.
7
Mader: Biology 8th Ed.
8
Thigmotropism
•
•
Unusual growth due to contact with solid
objects is called thigmotropism.
– Coiling of tendrils.
Thigmomorphogenesis occurs when the
entire plant responds to the presence of
environmental stimuli.
– Wind
– Rain
Fig. 27.2c
Mader: Biology 8th Ed.
Mader: Biology 8th Ed.
9
10
Thigmotropism
Nastic Movements
•
Mader: Biology 8th Ed.
Nastic movements do not involve growth
and are not dependent on the stimulus
direction.
– Seismonastic movement results from
touch, shaking, or thermal stimulation.
– Sleep movement occurs daily in response
to light and dark changes.
! Prayer Plant
Mader: Biology 8th Ed.
11
12
Seismonastic Movement
Sleep Movement
Mader: Biology 8th Ed.
Mader: Biology 8th Ed.
13
14
Circadian Rhythms
•
Plant Hormones
A circadian rhythm is a biological rhythm
with a 24-hour cycle.
– Tends to be persistent.
– Rhythm is maintained in the absence of
environmental stimuli via a biological
clock.
•
Mader: Biology 8th Ed.
Almost all communication on a plant is done
by hormones.
– Synthesized in one part of the plant, but
travels within phloem in response to the
appropriate stimulus.
Mader: Biology 8th Ed.
15
16
Auxins
•
Apical Dominance
Auxin is produced in shoot and apical
meristem and is found in young leaves and in
flowers and fruits.
– Apically produced auxin prevents the
growth of axillary buds.
! Apical dominance.
– Weak solution of auxin applied to woody
cutting causes rapid growth of adventitious
roots.
! Promotes fruit growth.
Mader: Biology 8th Ed.
Mader: Biology 8th Ed.
17
18
How Auxins Work
•
•
•
Auxins bind to plasma membrane receptors.
Activated proton pumps H+ out of cell.
Cell wall loosens and turgor pressure
increases due to the entry of water.
– Cell enlarges.
Mader: Biology 8th Ed.
Mader: Biology 8th Ed.
19
20
Auxin Mode of Action
Gibberellins
•
Mader: Biology 8th Ed.
Gibberellins are growth promoting
hormones that bring about elongation of the
resulting cells.
– Gibberellic Acid
! Stem Elongation
! Breaking of Dormancy
Mader: Biology 8th Ed.
21
22
Fig. 27.9
Mader: Biology 8th Ed.
Mader: Biology 8th Ed.
23
24
Cytokinins
•
Cytokinins are a class of plant hormones
that promote cell division.
– Derivatives of adenine.
! Prevent senescence
! Initiate growth
Mader: Biology 8th Ed.
Mader: Biology 8th Ed.
25
26
Abscisic Acid
•
Ethylene
Abscisic acid (ABA) initiates and maintains
seed and bud dormancy and brings about
stomata closure.
– Produced by any “green tissue” with
chloroplasts, monocot endosperm, and
roots.
•
Mader: Biology 8th Ed.
Ethylene is involved in abscission.
– Once abscission has begun, ethylene
stimulates certain enzymes which cause
leaf, fruit, or flower to drop.
– Ripens fruit.
! Increase activity of enzymes that soften
fruit.
Mader: Biology 8th Ed.
27
28
Functions of Ethylene
Fig. 27.14c
Mader: Biology 8th Ed.
Mader: Biology 8th Ed.
29
30
Photoperiodism
•
•
Phytochrome
Photoperiodism refers to any physiological
response prompted by changes in day or
night length.
– Short-day plants
– Long-day plants
– Day-neutral plants
Some plants may require a specific
sequence of day lengths in order to flower.
•
Phytochrome is a blue-green leaf pigment
that alternately exists in two forms.
– Phytochrome red (P )
r
–
Phytochrome far-red (Pfr)
!
Mader: Biology 8th Ed.
Conversion of forms allows a plant to
detect photoperiod changes.
" Also promotes seed germination and
inhibits stem elongation.
Mader: Biology 8th Ed.
31
32
Phytochrome Conversion Cycle
Mader: Biology 8th Ed.
Fig. 27.15
Mader: Biology 8th Ed.
33
34
Fig. 27.17
Review
•
•
•
•
Mader: Biology 8th Ed.
Tropisms
Nastic Movements
Plant Hormones
– Auxins
– Gibberellins
– Cytokinins
– Abscisic Acid
– Ethylene
Photoperiodism
Mader: Biology 8th Ed.
35
36
Mader: Biology 8th Ed.
37