Introduction to Plant Hormones
Plants respond to their environment in many different ways. They can respond to stresses such as lack of water, herbivore or insect attack and can grow in response to environmental cues such as light and gravity.Plants do not have a rapid nervous system but they are able to send messages between different cells in the organism or between different plants. These are coordinated responses, often in response to the environment. Plants have hormones that are produced in one part of the plant and transported to another part of the plant to have a response there. (nb you do not need to learn the structures of these hormones for A level Biology).
Here is a short video lecture by Patrick Roisen of Stanford University describing in brief the roles of plant hormones. Click on the link below.
https://www.brightstorm.com/science/biology/plants/plant-hormones/.
Specific examples of the roles of Plant Hormones.
Hormones role in leaf loss in deciduous plants
Plants that grow in temperate regions (where there is great changes in temperature and day length over the course of a year) need to adjust to these fluctuations. Making the most of good conditions in spring whilst protecting themselves from the harsh conditions of winter.
As day lengths become shorter and light intensity decreases, there will be less photosynthesis occurring in the leaves. When the amount of glucose being produced is lower than the amount needed in respiration to maintain the leaves and stop the leaves from freezing, it is inefficient for the plant to keep the leaves alive. This is why deciduous plants lose their leaves over winter and don't grow them again until spring (when days lengthen and temperatures increase).
Auxin plays a role in the abscission of leaves and fruits.
Young leaves and fruits produce auxin and so long as they do so, they remain attached to the stem. When the level of auxin declines, a special layer of cells — the abscission layer — forms at the base of the petiole or fruit stalk. Soon the petiole or fruit stalk breaks free at this point and the leaf or fruit falls to the ground.
The lengthening of the dark night in autumn triggers changes in the deciduous tree - including abscission (leaf fall) and dormancy of the plant.
1. Falling light levels result in falling auxin levels.
2. Falling auxin levels result in rising ethene levels.
3. The cells in the abscission layer are sensitive to ethene. When they detect ethene, genes for digestive enzymes in these cells are turned on. The enzymes produced weaken the cell walls in the outer layer of the abscission zone.
4. Vascular bundles carrying materials into and out of the leaf are closed off. Fatty material is deposited on the stem side of the separation layer (this prevents a vulnerable opening on the plant for pathogens to enter).
5. Cells inside the separation zone respond to the hormonal cues by retaining water and swelling - thus putting pressure on the already weakened outer layer.
6. The leaves are then eventually blown off by autumn winds or fall from the weight of frost, when the strain on the seperation zone becomes too much.
As day lengths become shorter and light intensity decreases, there will be less photosynthesis occurring in the leaves. When the amount of glucose being produced is lower than the amount needed in respiration to maintain the leaves and stop the leaves from freezing, it is inefficient for the plant to keep the leaves alive. This is why deciduous plants lose their leaves over winter and don't grow them again until spring (when days lengthen and temperatures increase).
Auxin plays a role in the abscission of leaves and fruits. Young leaves and fruits produce auxin and so long as they do so, they remain attached to the stem. When the level of auxin declines, a special layer of cells — the abscission layer — forms at the base of the petiole or fruit stalk. Soon the petiole or fruit stalk breaks free at this point and the leaf or fruit falls to the ground.
The lengthening of the dark night in autumn triggers changes in the deciduous tree - including abscission (leaf fall) and dormancy of the plant.
1. Falling light levels result in falling auxin levels.
2. Falling auxin levels result in rising ethene levels.
3. The cells in the abscission layer are sensitive to ethene. When they detect ethene, genes for digestive enzymes in these cells are turned on. The enzymes produced weaken the cell walls in the outer layer of the abscission zone.
4. Vascular bundles carrying materials into and out of the leaf are closed off. Fatty material is deposited on the stem side of the separation layer (this prevents a vulnerable opening on the plant for pathogens to enter).
5. Cells inside the separation zone respond to the hormonal cues by retaining water and swelling - thus putting pressure on the already weakened outer layer.
6. The leaves are then eventually blown off by autumn winds or fall from the weight of frost, when the strain on the seperation zone becomes too much.
Experimental evidence.
The figure below shows a nice demonstration of the role of auxin in abscission.
If the blade of the leaf is removed, as shown in the figure, the petiole remains attached to the stem for a few more days. The removal of the blade seems to be the trigger as an undamaged leaf at the same node of the stem remains on the plant much longer, in fact, the normal length of time.
If, however, auxin is applied to the cut end of the petiole, abscission of the petiole is greatly delayed.
Fruit growers often apply auxin sprays to cut down the loss of fruit from premature dropping.
The role of hormones in Seed Germination
Why do seeds go from being dormant packets of potential to being actively growing little plants?!
2. Evidence suggests that Gibberellins switch on the genes that code for amylases and proteases.
3. Proteases break down proteins into amino acids which are then used to build up the enzyme amylase.
4. a) Amylases break down the starch in the food store (aka cotyledon) into Maltose.
b) Maltases break the maltose into glucose.
c) Glucose can now be used in respiration to produce the ATP required to allow the embryo to grow and break out of the seed coat.
Another hormone ABA seems to interfere with gibberellin (stopping it from working) if there is too much ABA around.
Experimental evidence
1. Mutant varieties of seeds without a gibberellin gene do not germinate, but if gibberellin is applied to the seed externally, it will germinate.
2. If wild type seeds (that can make gibberellin) have inhibitors given to them that will stop the biosynthesis of gibberellin, the seeds to not germinate. If these inhibitors are removed, germination occurs. If they are not removed but gibberellin is externally added, germination occurs.
The role of hormones in stomatal control
Remember this from year 1?
Stomata can be closed on hot days to prevent water loss by transpiration.
When leaves are under stress due to lack of water, they release a hormone called ABA. Scientists also think roots release this hormone when they detect a low amount of water in the soil and it travels through the plant to the leaves.
Leave guard cells have ABA receptors on their membranes. This binding changes the ionic concentration in the guard cells which in turn reduces the water potential (as it moves out of the guard cells) and these cells become flaccid and so close.
Figure Changes in water potential, stomatal resistance, and ABA content in corn in response to water stress (source: Taiz L., Zeiger E., 2010
Auxins
We have already met auxins in GCSE. They were known to exist and move around the plant thanks to some elegant experiments carried out by some pretty eminent scientists! Remind yourself by watching the experiments below.
- Indolacetic acid (IAA) is an example of an auxin.
- Growth stimulants made in the cells at the tips of the roots and shoots and in the meristems.
- Auxins can be transported into different areas of a plant via cell to cell transport or in the transport tissues.
- Auxins have different effects depending on the concentration that is present.
- They have a number of major roles in plant growth.
Auxins stimulate the growth of the apical (main) shoot as they allow the cell walls to become more flexible - the cells
can elongate.
The activated proton pump moves protons out of the cytoplasm and into the cell wall. The pH of the cell wall lowers to about pH5 - a pH optimum for the action of the enzymes that break down the cross-linking in the cells wall of the cellulose and the microfibrils. Once these cross links are broken, the cell walls are now able to expand. as the vacuole fills with water, the cell is able to elongate.
Auxin is only made in young tissue, as the cells mature, the auxin is destroyed meaning the cell wall becomes rigid again. Cross links reform but do not get broken as the pH is no longer at an optimum for the relevant enzymes to be active. The cells can no longer expand and grow.
High concentrations of auxin results in apical dominance.
Auxin released from the apical cells diffuses down the stem. Where there are high concentrations of auxin, the lateral shoots are inhibited and they do not grow.
The cells further down the stem do not receive such a high dose of auxin and this concentration is not high enough to inhibit the lateral buds. They are able to grow.
Below are examples of experimental evidence for the role of auxins in apical dominance.
Auxins at low concentration promote root growth but at higher concentrations (those which promote shoot growth) root growth is inhibited.
This explains why auxins are able to induce geotropisms in the roots and phototropisms in the shoots.
Gibberellins
We have already seen the role of gibberellins in germination. They are also important in the growth of plants - particularly in the elongation of the stems.
The part of the plant that the gibberellin affects is the internode. These are the regions between the leaves on a stem.
There is a fungal disease in Japanese rice called Bakanae - meaning 'foolish seedling disease'. Plants that are infected grow taller and thinner than their uninfected counterparts.
Scientists isolated the chemicals that were produced by the fungus and called them gibberellins.
If the scientists gave the isolated gibberellins to uninfected rice, these plants grew with a spindly growth.
It was then discovered that the plants were also able to produce these chemicals and that application of gibberellins to a dwarf plant caused it to bolt.
But just because the application of a gibberellin to a dwarf plant can cause it to have longer internodes, does the gibberellin actually have this role in nature?
We need to do some tests using naturally occurring gibberellins in the correct concentrations and made in the correct places of the plant to find out if this is actually the natural function of gibberellin.
The part of the plant that the gibberellin affects is the internode. These are the regions between the leaves on a stem.
Discovery of gibberellin
There is a fungal disease in Japanese rice called Bakanae - meaning 'foolish seedling disease'. Plants that are infected grow taller and thinner than their uninfected counterparts.
Scientists isolated the chemicals that were produced by the fungus and called them gibberellins.
If the scientists gave the isolated gibberellins to uninfected rice, these plants grew with a spindly growth.
It was then discovered that the plants were also able to produce these chemicals and that application of gibberellins to a dwarf plant caused it to bolt.
But just because the application of a gibberellin to a dwarf plant can cause it to have longer internodes, does the gibberellin actually have this role in nature?
We need to do some tests using naturally occurring gibberellins in the correct concentrations and made in the correct places of the plant to find out if this is actually the natural function of gibberellin.
1st - prove that dwarf plants lack gibberellins while taller plants have them - and this is the only difference.
Researchers were able to do this by looking at mutants of the enzymes involved in making gibberellins.
Click here if you want a refresher on how biochemical pathways are catalysed by enzymes.
In gibberellins case, it's really long and complicated, the figure below only shows a short section!! (Please don't learn this off by heart! Life's too short and you are young! enjoy life!)
Hopefully you can see that at each step, there are only a few more atoms added on or changed around. Step by step the biochemical is built up until it becomes a functional working gibberellin. Each of these building steps is catalysed by an enzyme.
Here below I have a list of all the enzymes involved. Enzymes of course are proteins and proteins are coded for by genes. So the list below does not only show you what enzymes are involved in catalysing each step, but what genes are involved in the biosynthesis of each step too.
Q- What would happen if just one of these genes was mutated, producing a non-functional enzyme....
In gibberellins case, it's really long and complicated, the figure below only shows a short section!! (Please don't learn this off by heart! Life's too short and you are young! enjoy life!)
Here below I have a list of all the enzymes involved. Enzymes of course are proteins and proteins are coded for by genes. So the list below does not only show you what enzymes are involved in catalysing each step, but what genes are involved in the biosynthesis of each step too.
Q- What would happen if just one of these genes was mutated, producing a non-functional enzyme....
We will be looking at the gene for Le (protein is called LE). Tall plants, Le Le or Le le, did produce more GA1 than the mutant le le.
Well if Le gene for example was mutated, then GA20 would never become the closely related but not identical GA1. The le genotype is a mutated copy of the Le gene.
So they now knew that GA1 was involved in making a plant grow as tall as it should but how do they know it is directly involved?
To investigate this, scientists looked at another gene on the pathway Na.
SO na na cannot make GA12 or any gibberellins after it.
Na le - can make GA12 but cannot convert GA20 into GA1.
Na Le - Can make both enzymes so is able to make enough gibberellins to be tall.
Q - what would happen if you cut off a little of the na na plant and transplanted it onto the le le plant?
While part of the shoot would not produce NA so could not make any GA20, GA20 from the bottom le le mutant would be accumulating as it can't become GA1. The na na is not a le le mutant so it is still producing functional LE enzyme, which could move down through the plant and be used to turn the GA20 into GA1 and the plant can grow naturally tall.
This happened showing that both of these enzymes were important to the gibberellin pathway and the increase of internodes in plant growth.
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