Plant hormones and growth in plants

16.1 PLANT RESPONSES

​16.1 Plant hormones and growth in plants

p440-444

Chemical coordination

Plants are often thought of as passive organisms that simply grow and occassionally flower.

However, plants are coordinated organisms that show clear responses to their environment, communication between cells and even communication between different plants & other organisms.

Whilst plants lack a nervous system and responses are slower than animals, they still respond as a result of complex chemical interactions.

Plants have evolved a system of hormones that can be transported through their transport tissues and from cell-to-cell.

Plant hormones

​Hormone	Roles​
​auxins	​control cell elongation, prevent leaf fall (abscission), maintain apical dominance, involved in tropisms, stimulate the release of ethene, involved in fruit ripening.
gibberellin​	​cause stem elongation, trigger the mobilisation of food stores in a seed at germination, stimulate pollen tube growth (fertilisation).
ethene​	​cause fruit ripening, promotes abscission in deciduous trees (a tree that loses leaves in autumn and grows new ones in spring).
ABA (abscisic acid)​	​maintains dormancy of seeds and buds, stimulates cold protective responses, (e.g. antifreeze production), stimulates stomatal closing

Chemical coordination

The details of many plant reponses is still unclear. Plant hormones work at very low concentrations so isolating specific hormones and measuring changes in concentrations is difficult.

There are also multiple interactions between different control systems, which makes it difficult to isolate the role of a specific hormone.

Plant hormones and seed germination

When a seed absorbs water, the embryo is activated and begins to produce gibberellins. They switch on genes for the production of enzymes (such as amylases and proteases) that break down the food stores for growth.

Plant hormones and seed germination

In dicot seeds, the food store is the cotyledons (a tissue that forms the seed leaves).

In monocot seeds the food store is the endosperm (a tissue that acts as a food store).

Dicot seeds form 2 seed leaves and monocot seeds form 1 seed leaf.

Plant hormones and seed germination

The embryo uses these food stores to make ATP, which it uses to produce the building materials it needs to grow and break out through the seed coat.

Plant hormones and seed germination

There is evidence to suggest that ABA acts as an antagonist to gibberellins, and that it is the relative levels of both hormones which determines when a seed germinates.

Experimental evidence

• Mutant seeds that lack the gene which allows them to make gibberellins do not germinate. If gibberellins are applied externally, they germinate.

• When gibberellin biosynthesis inhibitors are applied to seeds, they do not germinate. If inhibition is removed or gibberellins applied, seeds germinate.

Plant hormones, growth and apical dominance

Auxins

Auxins (e.g. indoleacetic acid (IAA)) are growth stimulants. They are made in cells at the tip of roots and shoots, and in the meristems.

They can move down the stem and up from the roots in the transport tissue or from cell to cell.

Functions of auxins - cell stretchiness

Auxins stimulate the production of the main, apical shoot.

Auxins affect the plasticity of the cell wall - they make it stretch more easily. They do this by binding to specific recepter sites in the cell membrane causing the pH in that region to fall to about 5. This is the optimum pH of the enzymes that keep the walls flexible and plastic.

Functions of auxins

As cells mature, auxin is destroyed, so the pH increases and the enzymes become inactive.

As a result, the wall becomes rigid and more fixed in shape and size so they can no longer expand and grow.

Functions of auxins - apical dominance

High concentrations of auxins suppress the growth of lateral shoots, resulting in apical dominance.

Auxins produced at the tip stimulate it to grow quickly. The lateral shoots are inhibited by the hormone that moves back down the stem.

Further down, auxin concentration is lower, so lateral shoots grow more strongly.

Evidence for apical dominance

• If the apical shoot is removed, lateral shoots grow faster.

• If auxin is applied to the cut shoot, lateral growth is suppressed.

Functions of auxins - root growth

Auxin is produced by the root tips and also reaches the roots in low concentrations from the shoot tips.

Low concentrations of auxins promote root growth. As the concentration of auxin increases, root growth increases, up to a point. High auxin concetrations inhibit root growth.

Evidence - root growth

• If the apical shoot is removed, root growth slows and stops.

• Replacing the auxin artificially at the cut apical shoot restores root growth.

• High concentrations inhibit root growth.

Gibberellins

We know that gibberellins are involved in the germination of seeds. They are also involved in the elongation of plant stems during growth.

Gibberellins affect the length of the internodes (the region between the leaves on a stem).

Gibberellins were initially discovered because they are produced by a fungus of the genus Gibberella, which affects rice.

Gibberellins

The infected seedlings grew extremely tall and thin.

Scientists isolated the chemicals (gibberellins), which when applied, produced the same spindly growth in the plants.

It was also found that plants with short stems produce few or no gibberellins.

Gibberellins

Scientists have since discovered over a hundred naturally occuring gibberellins and have bred many dwarf varieties of plants by interrupting the gibberellin synthesis pathway.

This reduces waste and makes the plants less vulnerable to damage by weather and harvesting.

Plant hormones - synergism and antagonism

Synergism and antagonism

Few plants hormones work on their own, often they interact to produce an overall response.

If different hormones work together, complementing each other and giving a greater response than they would on their own, the interaction is known as synergism.

If the hormones have opposite effects, e.g. one promotes growth and one inhibits it, the interaction is known as antagonism.

Multiple Choice

The seed embryo is activated when the seed absorbs what?

• oxygen

• carbon dioxide

• water

• light

Multiple Choice

Which type of seed produces two seed leaves?

• monocot

• dicot

• tricot

Multiple Choice

What is used as the food store in dicot seeds?

• cotyledons

• endosperm

• seed coat

• radicle

Multiple Choice

What is used as the food store in monocot seeds?

• cotyledons

• endosperm

• seed coat

• radicle

Multiple Choice

Where are auxins made?

• shoot tips only

• root tips only

• shoot and root tips

Multiple Choice

When the embryo is activated, it begins to produce which hormone?

• auxins

• gibberellins

• ethene

• ABA

Multiple Choice

Which hormone affects the length of the internodes of a plant?

• ethene

• gibberellins

Multiple Choice

Plants that produce higher quantities of gibberellins have...

• long stems

• short stems

Multiple Choice

Which hormone acts as an antagonist to gibberellins?

• auxins

• ethene

• ABA

Multiple Select

Which of the following seeds failed to germinate?

• seeds which lacked the gene for making gibberellins

• seeds that had gibberellins applied externaly

• seeds applied with gibberellin biosynthesis inhibitors

Multiple Choice

Lateral growth of shoots is inhibited by...

• high concentrations of auxin

• low concentrations of auxin

Multiple Choice

When auxins bind to the membrane of plant cells, they cause the pH to...

• ...increase

• ...fall

Multiple Choice

High concentrations of auxin has what effect on root growth?

• it stimulates it

• it inhibits it

Multiple Choice

If two hormones complement each other, the ineraction is...

• antagonistic

• synergistic