OCR UNIT 215 PLANT RESPONSESSpecification details:

Explain why plants need to respond to their environment in terms of the need to avoid predation and abiotic stress

Define the term tropism

Explain how plant responses to environmental changes are co-ordinated by hormones, with reference to responding to changes in light direction

Evaluate the experimental evidence for the role of auxins in the control of apical dominance and gibberellin in the control of stem elongation

Outline the role of hormones in leaf loss in deciduous plants

Describe how plant hormones are used commercially

Why Plants need to respond to their Environment

Plants can avoid abiotic stress factors such as drought and high temperatures. They use plant hormones to induce stomatal closure in order to reduce transpiration

Plant shoots grow towards light in order to maximise light absorption for increased photosynthesis. Shoots are positively phototropic and bend towards a light source

Plant roots grow into the soil, towards the pull of gravity, in order to maximise the absorption of water and mineral ions for increased photosynthesis

In order to avoid competition, plants will grow taller to out-compete surrounding plants for light. Their growth is often spindly referred to as etiolation. They will grow deeper roots to absorb water and mineral ions from soil where there is less competition. These growth responses require auxins and cytokinins

Some plants avoid grazing by herbivores. Some plants produce toxins. Others have spines on their leaves

Some plants produce chemicals such as phenols that are leached into the soil inhibiting the germination of seeds of other plants. This interspecific effect is called allelopathy

By using plant hormones, the plant ensures that its seeds only germinate in suitable conditions

Tropisms

A tropism is a growth response to a stimulus in plants

The direction of growth is always related to the direction of the stimulus

Shoots are positively phototropic – they grow towards a light source, to absorb light for photosynthesis

Roots are positively geotropic – they grow towards the pull of gravity. This anchors them in the soil and enables the roots to absorb water and mineral ions from the soil

However, shoots are negatively geotropic and grow away from the direction of gravity

Plant Growth Substances/Plant Hormones

1

Plant responses to external stimuli are coordinated by plant hormones

Plant hormones may affect the cells that produce them or they may be transported to other target cells where they have an effect

Plant hormones are not produced by endocrine glands - unlike animal hormones

Plant hormones bind to specific receptor molecules on the plasma membranes of target cells. These receptor molecules have a complementary shape to that of the plant hormone - this is the same mechanism as animal hormones

After binding to receptors on target cells, plant hormones cause a response - a sequence of events to occur within the cells, just like animal hormones

Plant hormones may be transported by mass flow in xylem vessels or phloem sieve tubes

They may move into target cells by diffusion or active transport

Two or more hormones may have the same effects on plant growth and together have an enhanced effect. This is called synergy

Two hormones may have opposite effects on plant growth. This is called antagonism

Types of Plant Growth Substances

There are five hormone types that control physiological processes in plants:

Auxins - the parent molecule is indole acetic acid (IAA)

Cytokinins

Gibberellins

Abscisic acid

Ethene – the only gaseous hormone

The Role of Auxin in the Phototropic Response

Shoots are positively phototropic, meaning that they grow (by bending) towards a light source

If the light source is unidirectional (shining from one side), cell elongation in the shoot, occurs at a faster rate on the shaded side of the shoot and this produces the bending growth towards the light source

Auxins cause cell elongation of cells in the zone of elongation, just behind the shoot apex as seen in the diagram below

Coleoptiles like the one shown above, were used by various scientists to study the effect of light on shoot growth in germinating seeds

2

Auxins are produced in cells at the shoot apex (this is where mitosis occurs)

When a shoot is subjected to all round lighting, the auxins move down from the apex to the cells in the zone of cell elongation. The auxins are probably, actively transported from cell to cell

When the shoot is subjected to one sided lighting, the auxins are transported away from the light source and accumulate in the cells on the shaded side

Auxin Effect on Cell Elongation of Shoots subjected to All Round Lighting

Auxin Effect on Cell Elongation of Shoots subjected to Unidirectional Light

3

Auxins cause the cell walls of cells to be less rigid – they increase the stretchiness of the cell walls. This allows the newly produced cells (without vacuoles) to take in more water and produce a large, permanent central vacuole containing cell sap. When the cell has done this, it is elongated

Auxins bind to receptors in the plasma membranes of plant cells. Auxins then stimulate the active transport of H+ from the cytoplasm into the cell wall. This decreases the cell wall pH which activates expansin enzymes in the cell wall. Expansins hydrolyse glycosidic bonds in the cellulose molecules and the H+ disrupts some of the hydrogen bonds between the cellulose molecules. These effects lead to the cell walls becoming less rigid

Senescence (ageing) and Abscission (shedding) of Deciduous Leaves

In temperate climates (as in the UK), deciduous plants lose their leaves in the Autumn, to reduce transpiration in the Winter, when water may freeze in the soil

Leaf senescence is the ageing of leaves – they change colour as the chlorophyll breaks down

Cytokinins are plant hormones that prevent leaf senescence by maintaining a nutrient supply in the leaves. When the cytokinin levels decrease, the leaves have less nutrients and they start to die

Leaf senescence is followed by leaf abscission. This is when the leaves are shed from the plant

At least three different plant hormones control leaf abscission – auxin, ethene and possibly abscisic acid

Usually auxin (produced at the leaf tip) inhibits leaf abscission

Leaf senescence causes a decrease in the production of auxin at the leaf tip

A decrease in auxin concentration causes an increase in ethene production

The change in the proportion of auxin to ethene – less auxin and more ethene, results in the development of an abscission layer at the base of the leaf stalk

The abscission layer is made of thin walled cells. Ethene stimulates the synthesis of cellulase enzymes in these cells that hydrolyse the cellulose molecules in the cell walls. Eventually, this layer of cells is so weak that the petiole breaks and the leaf falls. Ethene therefore, causes the shedding of leaves

Before the leaf falls, the tree grows a layer of protective tissue with suberin in the cell walls, below the abscission layer. This protective layer forms a scar to prevent the entry of pathogens

Although abscisic acid was named because of its involvement in leaf abscission, its role is now less clear. It may control the production or activity of ethene

Apical Dominance – the Experimental Evidence for the Role of Auxins

4

Definition - Apical dominance is when the apical bud, growing at the tip of a shoot inhibits growth of lateral buds further down the shoot

The specification does not just require a summary of the plant growth substances involved in apical dominance. It requires an understanding of the interpretation of the experimental findings that led to our current understanding of the process

When the tip of a shoot (the apical meristem) is cut, the lateral buds will develop and side branches will grow. The plant becomes bushy

5

Since auxin is produced at the tip of the shoot in the apical meristem, it was assumed that removing the auxin supply, allowed the side branches to grow and that apical dominance was the effect of auxin inhibiting the growth of side branches

To test this assumption, auxin paste was applied to the cut end left after cutting off the shoot tip, and again, the side branches did not grow

However, the information observed so far, does not exclude the involvement of other factors eg the cut end is exposed to oxygen. Perhaps this exposure stimulates the production of another hormone that causes side branches to grow

Thimann and Skoog then applied a ring of auxin transport inhibitor below the intact shoot apex. The side branches grew suggesting even more strongly, that normal auxin concentrations inhibit side branch growth

Several years later, Gocal disproved a direct causative link between auxin concentration and and lateral bud growth inhibition. He found that removal of the apical meristem actually caused an increase in auxin concentration in the lateral buds

It is now thought that two other plant hormones are also involved

Abscisic acid inhibits the growth of lateral buds. High concentrations of auxin produced in the intact apical meristem may maintain abscisic acid concentrations at a high level to inhibit lateral bud growth. When the shoot tip is cut, the concentration of auxin decreases causing the abscisic acid levels to fall. The lateral buds then grow

Cytokinins promote lateral bud growth. High concentrations of auxin at the shoot tip make the tip a sink for cytokinins produced in the roots. When the shoot tips are intact, most of the cytokinin moves to the tips. When the tip is removed, the cytokinin is distributed more evenly around the plant and its presence in the lateral buds stimulates their growth

Gibberellins and Stem Elongation

In Japan, rice plants grow very tall when infected by a fungus. The fungal compounds responsible were isolated and identified as gibberellins. One of these was gibberellic acid GA3

Gibberellic acid was then tested on dwarf varieties of different plants such as peas and maize. These plants grew taller when gibberellic acid was spread over the stems

6

These experiments did not prove that in nature, gibberellic acid causes stem growth. It is important to test the low concentrations of gibberellins found naturally in plants and to test the effect on plant cells that gibberellins would normally reach

Researchers analysed the levels of GA1 (another gibberellin) in tall pea plants (homozygous for the dominant allele Le Le) and dwarf pea plants (homozygous for the recessive allele le le). The taller plants had higher GA1 concentrations in their stem cells

To show that GA1 causes stem growth directly, researches looked at the metabolic pathway to produce GA1 and used plants with mutations

The complex metabolic pathway for gibberellin synthesis is shown below:

Ent kaurene Site of mutation - plants only grow 1cm tall GA12 aldehyde

GA12

GA53

GA44

GA19

GA20

Enzyme produced by the Le allele acts here

GA1

A shoot of a mutant plant that could not produce any gibberellins (see site of mutation above) was grafted onto a dwarf pea plant, homozygous recessive for (le le)

The dwarf pea plant could not produce GA1 because of its inability to produce the enzyme to convert GA20 to GA1

However, the dwarf pea plant did produce GA20 and the mutant grafted shoot was able to convert GA20 to GA1 (it produced the enzyme required since it had at least one dominant Le allele). The shoot grew tall confirming that GA1 caused stem elongation

7

Further work has shown that gibberellins cause stem growth by causing loosening of the cell walls and cell elongation. They also promote cell division.Commercial Use of Plant Hormones

Auxins

Commercial Use Explanation Taking plant cuttings to produce new plants

Artificial vegetative propagation. Auxins are an ingredient in hormone rooting powder, bought from garden centres. The ends of stem cuttings are dipped into the rooting powder and then planted in compost. The auxins stimulate root growth at the end of the cutting

Production of seedless fruit eg grapes

Unpollinated (and therefore unfertilised) flowers are sprayed with auxins. The auxins stimulate fruit development. Since the ovary is not fertilised, no seeds will be present in the fruit

Selective weedkillers (herbicides)

Artificial auxins are sprayed onto broadleaved weeds eg dandelion. The auxins are transported in phloem and are not broken down by the enzymes that usually break down natural auxins (the shape of the artificial auxin does not fit the enzyme active sites). These auxins promote excessive rapid shoot growth. The stem cannot support itself and the plant collapses and dies.Narrow leaved plants such as grasses and cereals are not killed by these weedkillers. These weedkillers are used to kill weeds in lawns and in fields of cereal crops

Gibberellins

Commercial Use ExplanationDelays senescence in citrus fruit. Fruits can be left unpicked to extend the availability time in shops

Citrus fruits are sprayed with gibberellins

Causes elongation of grape stalks, giving more room for the grapes to grow

Developing fruits are sprayed with gibberellins

Brewing Industry – gibberellins stimulate the germination of barley seeds to produce malted barley

When barley seeds germinate they take up water. Gibberellins stimulate the synthesis of amylase in the aleurone layer of the seed. The amylase hydrolyses the starch in the seed endosperm to produce respiratory substrates for ATP synthesis in the seed during its growth. The germination process of the seeds is allowed to continue for a few days to produce maltose and glucose respiratory substrates. The product is then dried and ground up to form malted barley

Beer is made when yeast cells use the sugars in the malted barley as substrates for their respiration

Sugar production from sugar cane – gibberellins increase the length of the sugar cane internodes

Spraying the internodes of sugar cane stems with gibberellins increases the number of cells resulting in stem elongation. Since the sucrose sugar is contained in these stem cells, the more cells, the higher the sugar yield

Promoting seed production in some plants – promote sexual

Spraying young conifer plants with gibberellins can promote seed formation (conifer plants take a long time to become

8

maturity in conifers reproductively active).

Biennials plants (that produce seeds in the second year of life) can be induced to produce seeds in the first year by spraying with gibberellins

Production of short stocky plants

Spraying plants with gibberellin synthesis inhibitors can inhibit the growth promoted by gibberellins. This process is used to produce short poinsettias for the Christmas market

Cytokinins

Commercial uses Explanation Used in tissue culture to mass produce genetically identical plants

Cytokinins promote bud and shoot growth from small pieces of tissue taken from the parent plant. This produces a short shoot with lots of side branches. One plant can be divided into several small plants and each is grown separately

Sprayed onto lettuce leaves after picking to prevent yellowing

Cytokinins delay leaf senescence which involves the yellowing of the leaves

Ethene

Ethene is a gas and cannot be sprayed. Scientists have developed 2-chloroethylphosphonic acid which can be sprayed in solution onto plants. This compound is easily absorbed and releases ethene inside the plant.

Commercial uses Explanation Stimulates ripening of apples, tomatoes and citrus fruits

Ethene naturally causes ripening of fruits. Commercially the fruits indicated will be sprayed with the alkene

Promotes fruit drop in cherry and walnut trees

This process may be carried out either to reduce the number of fruits so that those left will grow bigger or to harvest the fruits all at the same time

Promotes female sex expression in cucumber flowers

If cucumber flowers are pollinated, the fruits tend to be bitter. Spraying the flowers prevents bitter fruit

9