Control of Growth
and Responses in
Plants
Chapter 27
Plant Responses
Tropism
● Plant growth away or toward a
unidirectional stimulus
o
due to differential growth
There are three well known types of tropism.
Phototropism
● Positive
o
cells on the shady side of a stem elongate
● Negative
o
roots curve away from light
Riboflavin- acts as a photoreceptor
Auxin- migrates to the shady side of a stem
from the bright side.
Gravitropism
● Positive
o roots grow in the direction of gravity
 if root caps are removed, roots no longer respond to gravity
●

contain statoliths- starch grains within amyloplasts
o amyloplasts come in contact with ER and lead to pumping of auxin
into the cell
auxin inhibits the growth of root cells and the upper surface
elongates
● Negative
o stems grow opposite the pull of gravity
 auxin stimulates the growth of stem cells and the lower
surface elongates
Thigmotropism
Unequal growth due to contact with solid
objects
● Cells on the object grow less, while the
other side elongates
Auxin and ethylene can induce curvature
Thigmomorphogensis
Touch response that the entire plant
responds to environmental stimuli.
Nastic Movements
Do not involve growth and are not dependent
on the direction of stimulus
A single stimulus can cause all leaves to
respond. This indicates a form of
communication within the plant.
Sleep Movement
Type of nastic movement
Response to changes in light (day vs. night)
Circadian Rhythm
- biological rhythm with 24 hour cycle
-
may persist without appropriate environmental
cues
- Biological clock maintains circadian
rhythm
-
CAM plants
Plants secrete nectar at the same time
Flowers opening during the day, closing at night
Plant Hormones
Almost all communication in a plant is done
by hormones.
Hormones are made or stored in one part of
a plant. They travel through phloem or from
cell to cell in response to stimuli.
Auxins
● There are different types of auxin
o
The most common is Indoeacetic acid (IAA)
 It is produced in shoot apical meristems and in
young leaves, flowers, and fruits.
● Auxins affect many aspects of plant
growth and development
o
May be applied to plant cuttings to stimulate root
formation
Effects of Auxin
● Apical dominance- apically produced
auxin prevents the growth of lateral buds
● Can cause woody plants to grow
adventitious roots quickly
● Production by seeds promotes fruit growth
● Synthetic auxin can be sprayed on plants
to cause fruit to grow without fertilization
(seedless tomatoes, watermelon grapes)
Effects of Auxin
● Agent Orange
● Involved in phototropism and gravitropism
How Auxins Work
1. Auxin binds to a plasma membrane receptor
2. Second messengers:
a. activate an ATP driven H+ pump. As hydrogen ions are pumped
out of the cell, the cell becomes acidic, loosening the cell wall, and
enabling elongation
b. stimulate the golgi to discharge transport vesicles carrying cell wall
materials
c. stimulate DNA binding protein that enters the nucleus and
activates a gene to produce growth factors
Gibberellins
Growth-promoting hormones that bring about
internode elongation of stem cells.
There are at least 70 different gibberellins.
The most common is gibberellic acid, GA3.
Gibberellins Use
●
●
●
●
●
Break dormancy of buds and seeds
Bring onset of flowering in plants
Increase flower size
produce larger seedless grapes
improve rice production
Cytokinins
Class of hormones that promote cell division
May also prolong the life of flower cuttings
and vegetables in storage.
Senescence
Aging process when a leaf loses its color.
During senescence, large molecules are broken down
and transported to other parts of the plants. This does
not always affect the entire plant
Cytokinin cans prevent leaf death and initiate their
growth.
Abscisic Acid (ABA)
Stress hormone
- Initiates and maintains seed and bud
dormancy
- Brings about the closure of stomata
Ethylene
- Involved in abscission
- Stimulates enzymes, like cellulase, to
cause leaf, fruit, or flower drop
- Used in agriculture to
-
hasten ripening
increase female flowers
Photoperiodism
Photoperiodism is a physiological response
prompted by changes in the length of day or
night.
It may induce flowering in the spring or
flowering in the fall.
Types of plants
Short-Day Plants
- Flower when the day length is shorter than critical
length
Long-Day Plants
- Flower when the day length is longer than a
critical length
Day-Neutral Plants
- Flowering is not dependent on day length
Phytochrome
Blue-green leaf pigment that exists in 2 forms
- Phytochrome red
- Phytochrome far-red
Far-red is converted to red during the night.
This conversion may be the first step in a
pathway that results in flowering.
Phytochrome Functions
Red to Far-red conversion controls growth
functions.
Far-red promotes seed generation and
inhibits stem elongation. It also leads to the
activation of regulatory proteins in the
cytoplasm.