Lecture 14: Control Systems in Plants
1. Plant Responses to
the Environment
At every stage in the life of a plant,
sensitivity to the environment and coordination of responses are evident.
One part of a plant can send signals to other
parts.
Plants can sense gravity and the direction of
light.
A plants morphology and physiology are
constantly tuned to its variable surroundings by complex interactions between
environmental stimuli and internal signals
At the organismal
level, plants and animals respond to environmental stimuli by very different
means
Animals, being mobile, respond mainly by
behavioral mechanisms, moving toward positive stimuli and away from negative
stimuli.
Rooted in one location for life, a plant
generally responds to environmental cues by adjusting its pattern of growth and
development.
Plants of the same species vary in body form
much more than do animals of the same species.
At the cellular level, plants and all other
eukaryotes are surprisingly similar in their signaling mechanisms.
All organisms, including plants, have the
ability to receive specific environmental and internal signals and respond to
them in ways that enhance survival and reproductive success.
Like animals, plants have cellular receptors
that they use to detect important changes in their environment.
These changes may be an increase in the
concentration of a growth hormone, an injury from a caterpillar munching on
leaves, or a decrease in day length as winter approaches.
In order for an internal or external stimulus to
elicit a physiological response, certain cells in the organism must possess an
appropriate receptor, a molecule that is sensitive to and affected by the
specific stimulus.
Upon receiving a stimulus, a receptor initiates
a specific series of biochemical steps, a signal transduction pathway.
This couples reception of the stimulus to the
response of the organism.
Plants are sensitive to a wide range of internal
and external stimuli, and each of these initiates a specific signal
transduction pathway.
2. Signal transduction pathways link internal and
environmental signals to cellular responses
Signals, whether internal or external, are first
detected by receptors, proteins that change shape in response to a specific
stimulus.
The receptor for greening in plants is called a phytochrome, which consists of a light-absorbing pigment
attached to a specific protein.
Unlike many receptors, which are in the plasma
membrane, this phytochrome is in the cytoplasm.
Ultimately, a signal-transduction pathway leads
to the regulation of one or more cellular activities.
In most cases, these responses to stimulation
involve the increased activity of certain enzymes.
This occurs through two mechanisms: stimulating
transcription of mRNA for the enzyme or by activating existing enzyme molecules
(post-translational modification).
In transcriptional regulation,
transcription factors bind directly to specific regions of DNA and control the
transcription of specific genes.
In the case of phytochrome-induced
greening, several transcription factors are activated by phosphorylation,
some through the cyclic GMP pathway, and others through the Ca2+-calmodulin
pathway.
Some of the activated transcription factors increase
transcription of specific genes, others deactivate
negative transcription factors which decrease transcription.
3. Plant hormones
help coordinate growth, development, and responses to environmental stimuli
Found in all multicellular
organisms, hormones are chemical signals that are produced in one part
of the body, transported to other parts, bind to specific receptors, and
trigger responses in targets cells and tissues.
Only minute quantities of hormones are necessary
to induce substantial change in an organism.
Often the response of a plant is governed by the
interaction of two or more hormones.
In general, plant hormones control plant growth
and development by affecting the division, elongation, and differentiation of
cells.
Some hormones also mediate shorter-term
physiological responses of plants to environmental stimuli.
Each hormone has multiple effects, depending on
its site of action, its concentration, and the developmental stage of the
plant.
include auxin, cytokinins, gibberellins, abscisic acid, ethylene, and brassinosteroids.
Many molecules that function in plant defense against
pathogens are probably plant hormones as well.
Plant hormones tend to be relatively small
molecules that are transported from cell to cell across cells walls, a pathway
that blocks the movement of large molecules.
Plant hormones are produced at very low
concentrations.
Signal transduction pathways amplify the
hormonal signal many fold and connect it to a cells specific responses.
These include altering the expression of genes,
by affecting the activity of existing enzymes, or changing the properties of
membranes.
According to the acid growth hypothesis, in a
shoots region of elongation, auxin stimulates plasma
membrane proton pumps, increasing the voltage across the membrane and lowering
the pH in the cell wall.
Lowering the pH activates expansin
enzymes that break the cross-links between cellulose microfibrils.
Increasing the voltage enhances ion uptake into
the cell, which causes the osmotic uptake of water
Uptake of water with looser walls elongates the
cell.
A change in the balance of ethylene and auxin controls abscission.
An aged leaf produces less and less auxin and this makes the cells of the abscission layer more
sensitive to ethylene.
As the influence of ethylene prevails, the cells
in the abscission layer produce enzymes that digest the cellulose and other
components of cell walls.
A chain reaction occurs during ripening:
ethylene triggers ripening and ripening, in turn, triggers even more ethylene
production - a rare example of positive feedback on physiology.
Because ethylene is a gas, the signal to ripen
even spreads from fruit to fruit.
Fruits can be ripened quickly by storing the
fruit in a plastic bag, accumulating ethylene gas or by enhancing ethylene
levels in commercial production.
Alternatively, to prevent premature ripening, apples are stored in bins flushed with carbon dioxide, which prevents ethylene from accumulating and inhibits the synthesis of new ethylene.
4. Plant responses to
light
Light is an especially important factor on the
lives of plants.
In addition to being required for
photosynthesis, light also cues many key events in plant growth and
development.
These effects of light on plant morphology are
what plant biologists call photomorphogenesis.
Light reception is also important in allowing
plants to measure the passage of days and seasons.
Plants detect the direction, intensity, and
wavelengths of light.
The photoreceptor responsible for these opposing
effects of red and far-red light is a phytochrome.
It consists of a protein (a kinase)
covalently bonded to a nonprotein part that functions
as a chromophore, the light absorbing part of the
molecule.
The chromophore
reverts back and forth between two isomeric forms with one (Pr) absorbing red
light and becoming (Pfr), and the other (Pfr) absorbing far-red light and becoming (Pr).
This interconversion
between isomers acts as a switching mechanism that controls various
light-induced events in the life of the plant.
Long-day and short-day plants are distinguished not
by an absolute night length but by whether the critical night lengths sets a
maximum (long-day plants) or minimum (short-day plants) number of hours of
darkness required for flowering.
In both cases, the actual number of hours in the
critical night length is specific to each species of plant.
While the critical factor is night length, the terms long-day and short-day are embedded firmly in the jargon of plant physiology.
5. Plant responses to
other environmental variables
Much of the plants response to a water deficit
helps the plant conserve water by reducing transpiration.
As the deficit in a leaf rises, the guard cell lose turgor and the stomata
close.
A water deficit also stimulates increased
synthesis and release of abscisic acid in a leaf,
which also signals guard cells to close stomata.
Because cell expansion is a turgor-dependent
process, a water deficit will inhibit the growth of young leaves.
As many plants wilt, their leaves roll into a
shape that reduces transpiration by exposing less leaf surface to dry air and
wind.
These responses also reduce photosynthesis.
Plants in flooded soils can suffocate because
the soil lacks the air spaces that provide oxygen for cellular respiration in
the roots.
Some plants are adapted to very wet habitats.
Mangroves, inhabitants of coastal marshes,
produce aerial roots that provide access to oxygen.
Less specialized plants in waterlogged soils may
produce ethylene in the roots causing some cortical cells to undergo apoptosis,
which creates air tubes that function as snorkels (aerenchyma tissue which
is modified parenchyma)
Excessive heat can harm and eventually kill a
plant by denaturing its enzymes and damaging its metabolism.
Transpiration helps dissipate excess heat
through evaporative cooling, but at the cost of possibly causing a water
deficit in many plants.
Closing stomata to preserve water sacrifices evaporative cooling.
Respiration increases faster with temperature
than photosynthesis