F215 control, genomes and environment
Module 4 – responding to the environment
Learning Outcomes
Explain why plants need to respond to their environment in terms of the need to avoid predation and abiotic stress.
Plant Responses
Plants have evolved a wide range of responses to a large variety of stimuli, this helps them to Survive long enough to reproduce Avoid stress Avoid being eaten
Sensitivity in plants
A plants responses to the external environment are mainly growth responses
Plants must respond to: Light Gravity Water Chemicals Touch
Plants communicate by plant growth regulators.
Learning Outcomes
Define the term tropism.Explain how plant responses to
environmental changes are coordinated by hormones, with reference to responding to changes in light direction.
Plant movements
Nastic Movements Usually brought about by changes in
turgidity in cells Rapid responses examples▪ Venus fly trap shutting▪ Leaves closing▪ Petals closing
Nastic Movements
Can you think of a nastic movement made by marram grass?
Describe the response and its adaptive value to the plant.
Tropisms
Slower responses resulting in directional growth
“is a directional growth response in which the direction of the response is determined by the direction of the external stimulus”
Phototropism
Phototropism is the response of plant organs to the direction of light.
A shoot shows Positive phototropism
Phototropism
This is a growth response towards or away from light
Look at the worksheet detailing some early experiments on phototropisms using oat, barley and wheat coleoptiles. Try to draw a conclusion to each
experiment.
Darwin’s experiment
Darwin’s conclusions
A growth stimulus is produced in the tip of the coleoptile
Growth stimulus is transmitted to the zone of elongation
Cells on the shaded side of the coleoptile elongate more than the cells on the other side.
Boysen-Jensen’s experiment
Boysen-Jensen’s experiment
Boysen-Jensen’s conclusions
Materials which are not permeable to water can stop the curvature response in some circumstances
Materials which are permeable to water do not interfere with the curvature response
Went’s experiment
Went’s conclusions
Went’s conclusions
Angle of curvature is related to the number of tips used
Number of tips used relates to the concentration of auxin in the agar block
Curvature response is due to a chemical which moves from the tip and affects cell elongation
Phototropin, auxin and phototropism
Phototropin, auxin and phototropism
Phototropins Proteins that act as receptors for blue
light In plasma membrane of certain cells in
plant shoots Become phosphorylated when hit by blue
light If light is directional, then the
phototropin on the side receiving the light becomes phosphorylated.
Phototropin, auxin and phototropism
Phosphorylation of phototropin brings about a sideways movement of auxin More auxin ends up on the shady side of the
shoot than on the light side Involves transporter proteins in the plasma
membranes of some cells in the shoot, these actively move auxin out of the cell
The presence of auxin stimulates cells to grow longer Where there is more auxin there is more
growth
Auxin action
Auxin binds to receptors in plasma membranes of cells in the shoot. This affects the transport of ions through
the cell membrane Build up of hydrogen ions in the cell walls The Low pH activates enzymes that break
cross-linkages between molecules in walls
Cell takes up water by osmosis, cell swell and become longer
Permanent effect
Plant growth
Plant growth occurs at meristems Apical meristem Lateral bud meristems Lateral meristems Intercalary meristems
Learning outcomes
Evaluate the experimental evidence for the role of auxins in the control of apical dominance and gibberellin in the control of stem elongation.
Why “plant growth regulators”?
Exert influence by affecting growthProduced in a region of plant
structure by unspecialised cellsSome are active at the site of
productionNot specific – can have different
effects on different tissues
The Plant growth regulators
There are five main groups Auxins Gibberellins Cytokinins Abscisic acid Ethene
Plant growth regulators
Produced in small quantitiesAre active at site of production, or
move by diffusion, active transport or mass flow.
Effects are different depending on concentration, tissues they act on and whether there is another substance present as well.
Interaction of plant growth regulators
Synergism 2 or more act together to reinforce an
effect
Antagonism Have opposing actions and inhibit
(diminish) each others effects.
Auxins
Synthesised in shoot or root tips. Most common form is IAA (indole-3-
acetic acid a.k.a. indoleacetic acid) Main effects of auxins include:
Promote stem elongation Stimulate cell division Prevent leaf fall Maintain apical dominance.
Auxins and Apical Dominance Auxins produced
by the apical meristem
Auxin travels down the stem by diffusion or active transport
Inhibits the sideways growth from the lateral buds
Apical Dominance
Apical Dominance
Mechanism for apical dominance
Auxin made by cells in the shoot tipAuxin transported downwards cell to
cellAuxin accumulates in the nodes
beside the lateral budsPresence inhibits their activity
Evidence for mechanism (1)
If the tip is cut off of two shoots Indole-3-acetic-acid (IAA) is applied to
one of them, it continues to show apical dominance
The untreated shoot will branch out sideways
Evidence for mechanism (2)
If a growing shoot is tipped upside down Apical dominance is prevented Lateral buds start to grow out sideways
This supports the theory Auxins are transported downwards, and
can not be transported upwards against gravity
Question and reading
Suggest how apical dominance could be an advantage to a plant!
Read through Page 224 in your textbook “apical dominance”
Suggest!!
Gibberellins and stem elongation
Gibberellin (GA) increases stem length Increases the
lengths of the internodes▪ Stimulating cell
division▪ Stimulating cell
elongation
Evidence for GA and stem elongation
Dwarf beans are dwarf because they lack the gene of producing GA
Mendel’s short pea plants lacked the dominant allele that encodes for GA
Plants with higher GA concentrations are taller
Action of GA
Affects gene expression Moves through plasma membrane into cell Binds to a receptor protein, which binds to
other receptor proteins eventually breaking down DELLA protein.
DELLA proteins bind to transcription factors If DELLA protein is broken down, transcription
factor is released and transcription of the gene can begin
Gibberellins and germination of seeds Monocotyledonous plants e.g. barley
and wheat Seeds can lay dormant until
conditions are suitable for germination.
Structure of a seed Pericarp and testa Aleurone layer – protein rich Endosperm – starch store Scutellum – seed leaf Embryo
Gibberellins in the germination of barley seeds Germination need suitable conditions, this
requires presence of water, oxygen and an ideal temperature
1. Water enters seed2. GA secreted by the embryo diffuses across
endosperm to aleurone layer.3. GA activates gene coding for amylase
(transcription)4. Amylase produced in aleurone and diffuses into
the endosperm5. Amylase hydrolyses starch into maltose6. Maltose is hydrolysed into glucose, which diffuses
into the embryo.
Learning Outcomes
Outline the role of hormones in leaf loss in deciduous plants.
Leaf Abscission
Trees in temperate countries shed their leaves in autumn.
Survival advantage Reduces water loss through leaf surfaces Avoids frost damage Avoid fungal infections through damp,
cold leaf surfaces Plants have limited photosynthesis in
winter
Abscission and hormones
Three different plant hormones control abscission Auxin▪ Inhibits abscission
Ethene (gas)▪ Increase in ethene production inhibits auxin
production Abscisic Acid
Abscisic acid
Inhibits growth (antagonistic to GA and IAA)
“stress hormone” Control stomatal closure Plays a role in leaf abcission
Abscission – falling of leaves or fruit from plants.
Stages in leaf abscission
As leaves age, rate of auxin production declines
Leaf is more sensitive to ethene production
More ethene produced, inhibits auxin production
Abscission layer begins to grow at the base of the leaf stalk.
Leaf Abscission
Abscission Layer
The abscission layer is made of thin-walled cells Weakened by enzymes that hydrolyse
polysaccharides in their walls Layer is so weak that the petiole breaks Leaf falls off
Tree grows a protective layer where the leaf will break off Cell walls contain suberin Leaves a scar which prevents the entry of
pathogens
Learning Outcomes
Describe how plant hormones are used commercially.
Commercial use of AuxinsSprayed onto developing fruits to
prevent abscissionSprayed onto flowers to initiate fruit
growth without fertilisation Parthenocarpy – promotes the growth of
seedless fruitsApplied to the cut end of a shoot to
stimulate root productionSynthetic auxins are used as
selective herbicides
Commercial use of Ethene
Fruits harvested before they are ripe allows them to be transported without deteriorating, these are sprayed with ethene to promote ripening at the sale point.
E.g. bananas from the Caribbean
Commercial use of Gibberellin
Sprayed onto fruit crops to promote growth Sprayed onto citrus trees to allow fruit to
stay on the trees longer Sprayed onto sugar cane to increase the
yield of sucrose Used in brewing, where GA is sprayed onto
barley seeds to make them germinate, amylase is produced, starch is broken down into maltose, the action of yeast on the maltose produces alcohol.
Commercial use of cytokinins
Delay leaf senescence – can be sprayed on lettuce leaves to prevent them from yellowing
Can be used in tissue culture to mass produce plants