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