Chapter 39:
Control Systems in Plants
Question


Do plants sense and respond to their
environment ?
Yes - By adjusting their pattern of growth and
development.
In Dark
In Light
Comment


Plants can’t “move” away from a stimulus,
but can change their growth response.
Result – plant bodies are more “flexible” in
morphology than animals.
Classical Example


Phototropism - plant growth response to
unilateral light.
Observation – plants bend or grow towards
the light.
Phototropism Experimenters



Darwins: late 1800's.
Boysen & Jenson: early 1900's.
F.W. Went: 1926
Went Experiments
Mechanism of Phototropism


Cells on the dark side elongate faster than the
cells on the light side.
The uneven growth rate causes the bending of
the stem toward the light.
Question


What is the adaptive value of phototropism?
It tilts the leaves toward the light source for
more efficient photosynthesis.
Cause of Phototropism


Chemical messenger from the tip caused the
growth response in the stem.
The distribution of the chemical changes in
the unequal light, resulting in unequal cell
elongation.
Hormone


Chemical signal produced in one location,
transported, has effect in another location.
Phototropism is caused by a plant hormone.
Plant Hormones


Are produced in small quantities.
Effects may reflect balance between several
hormones.
Mechanism
Plant Hormones
1. Auxins
2. Cytokinins
3. Gibberellins
4. Abscisic Acid
5. Ethylene
Auxins



Named by Went in 1926.
First plant hormone described.
Ex: IAA (natural)
2,4-D (synthetic)
Major Functions




Stimulates cell elongation.
Fruit development.
Apical Dominance.
Tropism responses.
Apical Dominance
Where Produced



Apical Meristems.
Young leaves.
Embryos.
Cytokinins



Isolated from coconut "milk" (endosperm) in
the 1940’s.
Named because they stimulate cell division.
Ex: Zeatin
Major Effects




Stimulates cell division.
Delays senescence.
Root growth and differentiation.
Where Produced - roots
Auxin/Cytokinin Ratios

Control shoot or root
differentiation in tissue
cultures.
Gibberellins


Found from the "Foolish Seedling" disease in
rice.
Ex: GA3
70 types known
Foolish Seedlings
Major Effects
Extra GA3




Internode elongation.
Seed/Bud germination.
Flowering (some species).
Fruit development.
No GA3
Have GA3
Lack GA3
Where Produced



Apical Meristems.
Young leaves.
Embryos.
Abscisic Acid


Slows or inhibits plant growth.
"Stress" hormone produced under unfavorable
conditions.
Major Effects




Inhibits growth
Seed/Bud dormancy.
Stomata closure.
Leaf drop – produces abscission layer.

Abscission
Layer
Where Produced



Leaves
Stems
Green fruit
Ethylene


Gaseous hormone (fast diffusion rates).
Often interacts with Auxin.
Major Effects



Fruit ripening.
Accelerates Senescence.
Stem/Root Elongation (+ or -).
Where Produced



Ripening fruits.
Senescent tissue.
Nodes.
New Hormones


Oligosaccharins – short chains of sugars released
from the cell wall.
Function:



Pathogen responses
Cell differentiation
Flowering
New Hormones


Brassinosteroids – steroid hormones similar to
animal sex hormones.
Function:

Needed for normal growth and development.
Commercial Applications of Plant Hormones




Weed killers
Seedless fruit
Rooting of cuttings
Tissue culture
Plant Movements
1. Tropisms
2. Circadian Rhythms
Tropisms



Growth responses in response to external
stimuli.
+ toward a stimulus
- away from stimulus
Examples
1. Phototropism
2. Gravitropism
Phototropism

Response to light (blue).
Movie
Gravitropism


Response to gravity.
Stems are – gravitropic and roots are +
gravitropic.
Gravitropism - mechanism

Statolith movement
may be the receptor for
the stimulus.
Thigmotropism



Response to touch.
A series of 5 genes are involved.
Ex: Tendrils
Climbing stems
Wind direction response
of stems.
Turgor Movements

Movement caused by turgor pressure
differences in certain cells.
Types
1. Rapid Leaf Movement
Ex: Mimosa
2. Sleep Movements
Ex: Bean Leaves
Prayer Plant
Sleep Movements
Day
Night
Mimosa
Rapid Leaf
Movement
Circadian Rhythms


A physiological cycle about 24 hours long.
Ex: Stomata opening
Sleep movements
Causes


Synthesis of a transcription factor protein that
regulates is own manufacturing through
feedback control.
Gene is believed to be common in most
eukaryotic organisms.
Photoperiodism


A physiological response to changing day
lengths.
Used to detect and direct growth responses to
seasonal changes.
Advantages


Match growth responses to proper season.
Ex: Leaf drop in fall
Flowering
Flowering Types
1. Short - Day Plants
2. Long - Day Plants
3. Day - Neutral Plants
Short-Day Plants


Flower when days are shorter than a critical
period (long nights).
Ex: Mums
Poinsettias
Long-Day Plants


Flower when days are longer than a critical
period (short nights).
Ex: Spinach
Iris
Lettuce
Day-Neutral Plants


Flower whenever they have enough energy.
Ex: Roses
African Violets
Night Length


Actually controls flowering response, not day
length.
Proof – experiments show that if you interrupt
the dark period, you reset the “clock”.
Comment

Length of night not absolute, but relative for
the response to be triggered.
Question


What detects day/night length changes?
Phytochrome - plant pigment involved with
photoperiodism.
Phytochrome Forms


Pr - responds to
660nm (red light).
Pfr - responds to
730nm
(far red).
Phytochrome


Changes between the two forms.
Ratio or accumulation of enough Pfr triggers
the responses


In Red light:
Pr  Pfr
Far-red light or darkness:
Pfr  Pr
Photoperiodism


Very sensitive
(1 minute
difference).
Sets clocks for plant
responses.
Other Effects



Seed Germination
Stomatal Opening
Leaf Drop
Lettuce Germination
Responses to Stress

Stress – an environmental condition that can
have an adverse effect on a plant’s growth,
reproduction and survival.
Plant Response
1. Developmental changes
2. Physiological changes
Water Deficit



During high Ts, guard cells may close.
Young leaves may slow expansion.
Leaves may roll to reduce surface area.
Oxygen Deprivation


Common in roots in water-logged soils.
Air tubes in roots may bring oxygen to the
cells.
Salt Stress


Damages the plant through unfavorable soil
water potentials and toxic ions.
Some plants can concentrate and excrete salt
through salt glands (ex. halophytes).
Heat and Cold Stress



Heat - use heat-shock proteins to protect
other proteins from denaturing.
Cold – lipid shifts to keep lipid bilayers
“liquid”.
Cold – solute changes to lower freezing
point.
Herbivores



Plants have many physical and chemical
defenses against herbivores.
Physical – thorns
Chemical – crystals, tannins and other toxic
compounds.
Herbivores

Often trigger a plant to release chemicals to
attract predators or to warn other plants to
increase their production of toxins.
Pathogens


First Defense – epidermis
Second Defense – chemical events to restrict
or kill the invader.
SAR


Systemic Acquired Resistance: chemicals that
spread the “alarm” of an infection to other
parts of the plant.
Possible Candidate: salicylic acid
Summary




Know the general plant hormones and their
effects.
Know tropisms.
Know photoperiodism.
Know general ideas about how plants respond
to stress.