Plant Growth & Phytohormones (Lecture 7)

Question 1

How does light influence growth?

Answer 1

• Directionality
• Activating flowering
• Shade avoidance response

Question 2

Where are new cells and tissues produced?

Answer 2

Meristems: Root, Apical, Cambium

As such, the plant hormones tend do be concentrated at the meristems.

Question 3

What triggers and coordinates growth processes?

Answer 3

Plant hormones, aka plant growth regulators

Question 4

What are plant growth regulators?

Answer 4

Hormones: Signaling molecules made in one cell, acting at a distance

Question 5

What do plant growth regulators (hormones) do?

Answer 5

They regulate normal development (auxins, cytokinins, gibberellins) and rapid stress responses (abscisic acid, jasmonic acid)

Question 6

What is complex regulation?

Answer 6

The concept that any one hormone is implicated in multiple processes, and any one processe usually involves multiple hormones.

Question 7

What makes plant hormones difficult to study?

Answer 7

They are active in very low concentrations.

Question 8

How was auxin first discovered?

Answer 8

It was demonstrated by phototropism - growth towards light

Question 9

What are grass coleoptiles, and what are their characteristics?

Answer 9

Coleoptiles are covers of shoots in grass embryos.

They are very sensitive, bending towards light.

Question 10

How do grass coleoptiles undergo phototropism?

Answer 10

1. The tip senses the signal
2. The tip sends out a diffusible substance downwards
3. This increates curvature on the dark side
4. The substance on the darks side causes cell expansion

Question 11

What is a bioassay?

Answer 11

Using a biological test to measure a substance.

Question 12

Are there different auxins?

Answer 12

Yes!

Indole acetic acid is the natural auxin.
there are other natural auxins (indole-butyric acid) and artificial substances with auxin-like properties (2,4 dichlorophenoxyacetic acid, a potent herbicide).

Question 13

In what type of processes do auxins function?

Answer 13

Those related to CELL EXPANSION.

Question 14

If the shoot tip is separated by gelatin, what happens when light hits it?

Answer 14

It will curve, as gelatin is permeable.

Question 15

If the tip is separated by mica, what happens when light hits it?

Answer 15

It will not curve, as mica is impermeable.

Question 16

What is auxin polar transport, in the shoot apex?

Answer 16

This is a downward movement of auxin.

An auxin transporter (one of the PIN proteins) is inserted in the plasma membrane at the lateral face of cells of the shoot (lower end of cells).
Auxin is pumped OUT of these efflux transporters and accumulates in the cells on the shady side.

This stimulates elongation of the cells on the shady side, causing the shoot to bend toward the light.

Question 17

What are auxins’ acid growth capabilities?

Answer 17

A drop in pH of the cell wall space, via H+-ATPase, leads to microfibril loosening and longitudinal expansion of the cell.

Question 18

How do auxins control branching pattern?

Answer 18

Via APICAL DOMINANCE. They inhibit lower-down buds from dividing, thus allowing the main shoot to be dominant.

Question 19

What other processes do auxins promote?

Answer 19

• Root formation: Indole butyric acid is added to rooting powder.
• Phototropic and gravitropic responses

Question 20

What processes do cytokinins function within?

Answer 20

Cytokinins are the major regulators of cell division in plants.

Question 21

What is the discovery of cytokinins linked to?

Answer 21

Plant tissue culture: Growing plants or plant tissues in completely defined, sterile conditions.

Question 22

What molecule is cytokinin structurally similar to?

Answer 22

Purine, the nucleotide base as in DNA.

Question 23

What other plant hormones do cytokinins work with?

Answer 23

They work in combination with auxin, promoting cell division.

Question 24

What role does cytokinin play in apical dominance?

Answer 24

Presence of cytokinin in an area causes preferential movement of nutrients towards it.

When applied to lateral buds, they help in their growth, despite the presence of an apical bud. They stimulate the division of cells in these axillary buds.

In other words, they act antagonistically to auxin, which promotes apical dominance.

Question 25

Where is cytokinin generated, and where does it function?

Answer 25

It is generated in the root meristem, spreading thereon to the shoots and upper parts of the plant.
They help maintain the shoot apical meristem, but reduce the size of the root meristem.

Question 26

What other phenomenons do cytokinins play a role in?

Answer 26

Conteracting senescence, the dropping of leaves in the Fall.

They are the cause of green islands, a response to wounding.

Question 27

How can we manipulate the tissue culture of plants?

Answer 27

By playing around with the balance and ratios of auxin:cytokinin.

High cytokinin : Low auxin = Gives shoots
Low cytokinin : High auxin = Gives roots

Low levels of cytokinin and auxin = No growth
High levels of cytokinin and auxin = Growth of undifferentiated cells (“callus”)

Question 28

How does cytokinin interact with auxin when the apical meristem is removed?

Answer 28

Removal of the apical meristem cuts off the supply of auxin. IPT genes are activated, cytokinin genes are turned off.
Cytokinin moves into the adjacent bud.
Cytokinin activates that other bud’s growth. The branch meristem starts producing auxin, and the axillary bud grows out.

Question 29

What is Agrobacterium tumefaciens?

Answer 29

A plant pathogenic soil bacterial that causes crown gall disease.

Question 30

What is crown gall disease?

Answer 30

A mass of undifferenciated cells, similar to a tumor, forming at the ‘crown’ above the soil surface.

Question 31

How does Agrobacterium work?

Answer 31

It induces the plant cells to divide unnaturally, overriding any plant signals.

Agrobacterium contains a plasmid with genes for auxins and cytokinin biosynthesis. It does not synthesize these genes itself, but rather has evolved a very sophisticated mechanism to integrate these genes into the plant DNA.

The cells are now genetically transformed, making excess auxin and cytokinin, which together cause the division of cells = Crown gall.

Question 32

What ability do some plant pathogens have?

Answer 32

The ability to make plant hormones as part of their disease strategy. This alters plant growth, making better conditions for the pathogen.

Question 33

How have plant biologists used this Agrobacterium trick?

Answer 33

We can now move any “gene of interest” into Agrobacterium, which then transfers these into the plant for us.

Question 34

What is the catch with the Agrobacterium trick?

Answer 34

It only works if you can regenerate an entire plant from a single cell = totipotency. This is a limiting factor for many plant species!

You need to be able to select those cells that have received the new DNA, and use plant hormones to stimulate them to divide and make a new plantlet, which is slow and painstaking.

Question 35

What is the primary function of gibberellins?

Answer 35

They are a type of important regulators of elongation.

Question 36

What is the function of gibberellins in stems?

Answer 36

They mediate stem elongation.

Question 37

What effect do gibberellins have in fruit development?

Answer 37

They enhance fruit development.
When fruits have seeds, the gibberellins are made by those seeds and promote growth of fruit.
In the case of seedless fruits, spraying with gibberellins leads the same effect.

Question 38

How do gibberellins promote seed germination?

Answer 38

Gibberellins are released by the embryo within the seed, which activates metabolism and triggers seed cells to synthesize starch-degrading enzymes.

Question 39

Why don’t we need to add any plant hormones to our tissue culture plantlets once they are growing?

Answer 39

This relies on the plant cells’ ability to regenerate a whole plant.

Question 40

If you have a plant with too much gibberellin, what would you expect it to look like?

Answer 40

Hyper-elongated stems due to the excess gibberellins! They will be tall and spindly.

Question 41

What does a mutant with a defective gene for the gibberellin receptor look like?

Answer 41

It cannot detect gibberellin and is therefore considered to be a ‘dwarf mutant’, with stunted stems (lack of elongation)

Question 42

Agrobacterium tumefaciens causes crown gall disease by modifying plant hormone levels in affected plant cells. Agrobacterium rhizogenes causes formation of roots, not tumor-like galls. Which hormone do you think has most likely been affected?

Answer 42

Auxin, which stimulates more root growth phenotypes.

Question 43

Why don’t we need to add any plant hormones to our tissue culture platelets once they are growing? Is it because of argobacterium?

Answer 43

Once it’s growing, with roots and leaves, it doesn’t need hormones! It only needs hormones in order to start the process. After that, they become self-sustainable. Once formed, the plant can make its own hormones and regulate itself.

Question 44

There is a tomato mutant “wilty” which is deficient in a plant hormone and which wilts more easily under drought than wild type plants. 1. What is most likely to be hormone affected in this mutant? 2. What would be an easy test of your hypothesis? 3. What other problems, abnormalities, or phenotype might this mutant have?

Answer 44

1. Abscisic acid (ABA)
2. This depends on whether the plant can’t generate ABA, or whether it does not have the receptor itself.
   - We can spray the plant with ABA, and see whether or not it responds (do the stomata close?)
   - Complement the ABA defect by adding ABA back in.
3. Seeds would germinate too soon, unable to follow the proper cues for germination.
   Seeds are made dormant so they can await the right conditions for germination. This is thanks to ABA.

Question 45

What maintains the germination and the growth process?

Answer 45

The ratio of ABA:gibberellin

Question 46

4. a) What can fungal pathogens do, allowing them to infect and enter plant cells?
   b) Name some mechanisms by which a plant cell tries to stop them.

Answer 46

a. Appressorium, through which they force their way through the cell wall.

b. The cell wall (papilla) can be reinforced;
Toxins, enzymes that attack the fungi;
Hypersensitive cell death refers to the programmed cell death. It is hypersensitive because you will see the brown spots on the plant leaves, where the cells have died. Programmed response by the plant.

Question 47

5. A) If you have a mutant tomato fruit that cannot make ethylene, what would be its phenotype? B) How might you identify this mutant? C) What would happen to this tomato fruit if you sprayed it with ethylene?

Answer 47

a. They would be green and hard, as they cannot ripen!
Ethylene starts making sugar, softens, destructs cell walls.
b. Try to obtain a triple response. You could also just look for those that never ripen. They can’t get around physical obstacles during growth.
c. It would ripen, since it cannot make ethylene itself! However, if it doesn’t have an ethylene receptor, then it can’t detect the ethylene = no response

Question 48

6. You spray one tomato plant with jasmonic acid, and one with water. Three days later you put a caterpillar on each plant. One week after this, you inspect the caterpillars. Describe some differences between these two caterpillars that you might expect to see, and why? (I.e. What would jasmonic acid do to these plants?)

Answer 48

The ones with jasmonic acid: The JA sprayed plant would reduce the growth of the caterpillars. JA is the defense chemicals. The caterpillar wouldn’t grow as fast, or they would be dead (or a bit of both).
JA contains protease inhibitors, thus the insect can’t digest the nutrients. However, the caterpillars will try to fight back by creating new types of digestive enzymes.

Question 49

7. Two species of caterpillar feeding on a plant can cause similar amounts of leaf loss and tissue damage, but the plant responds slightly differently in its defense. Suggest an explanation, and how to test it.

Answer 49

The caterpillars may have different chemical compositions within their saliva, one of which triggers a stronger defense mechanism from the plant.
As well, the timing of the feeding (temporal variation), whether the damage is quick, or occurs in bits throughout the course of the day, triggers different responses from the plant.