5.5.2: Controlling plant growth

Question 1

What are auxins?

Answer 1

plant hormones which are responsible for regulating plant growth.

Question 2

What happens if you break the shoot tip (apex) of the plant.

Answer 2

• Plant starts to grow side branches from lateral buds that were previously dormant.
• This is because auxins from the apical bud prevent lateral buds from growing.
• Tip removed→auxin levels in shoot drop→buds grow.

Question 3

How did scientists test the hypothesis that auxins from the apical bud prevent lateral buds from growing?

Answer 3

• Applied a paste containing auxins to the cut end of the shoot.
• Lateral buds did not grow.

Question 4

Which other two hormones are involved in apical dominance?

Answer 4

Abscisic acid- inhibits bud growth.

Cytokines-Promote bud growth

Question 5

Describe in more detail how auxins may explain apical dominance?

Answer 5

• High auxin in shoot keeps abscisic acid levels high in the bud.
• When the tip is removed, abscisic acid levels drop and the bud starts to grow.

Question 6

Describe in more detail how cytokinins may explain apical dominance?

Answer 6

• Directly applying cytokines to buds can override the apical dominance effect.
• High levels of auxin make the shoot apex a sink for cytokinins produced in the roots so most cytokinins go to the shoot apex.
• When the apex is removed, cytokinins spreads out evenly around the plant.

Question 7

What are gibberellins?

Answer 7

Plant hormones responsible for control of cell elongation and seed germination.

Question 8

In Japan, a fungus causes a disease which makes rice grow very tall. What does the fungal compound involved contain?

Answer 8

-Gibberellins including gibberellic acid (GA3)

Question 9

When gibberellic acid is applied to dwarf varieties of plants, they grew taller, what does this suggest?

Answer 9

gibberellic acid is responsible for plant stem growth

Question 10

What dod scientists find when they compared GA1 levels of tall pea plants (homozygous for dominant Le allele) and small pea plants (homozygous for recessive le allele), which were otherwise genetically identical?

Answer 10

They found that the plant with higher levels of GA1 were taller.

Question 11

Scientists needed to show that GA directly causes stem growth, so they needed to know how GA1 is formed. What did they work out?

Answer 11

-The Le gene is responsible for producing the enzyme that converted GA20 to GA1

Question 12

Scientists chose a plant with a mutation that blocks gibberellin production between ent-kaurene and G12-aldehyde.
These plants produced no gibberellins grow to about 1cm tall.
-What did they do to make it grow tall?

Answer 12

• Thy grafted a shoot onto a homozygous le plant (which cannot convert GA20 to GA1), and it grew tall.
• Such a plant with no GA 20 of its own, does not have the enzyme to convert GA 20 to GA1 and it can use the unusual GA 20, from the normal plant.
• Because the shoot grew tall, it confirmed that GA1 causes stem elongation.