1.3 Phytohormones

Question 1

Main points on phytohormones

Answer 1

• provide important information to reach fundamental decisions (development, defense)
• can operate independently, but also antagonistic or synergistic
• mutant plants are useful tools
• SA, Et and JA are important hormones in defense responses
• one defense pathway may go at the expense of other pathways

Question 2

Defence decision example

Answer 2

In order to activate the SA hormone (effective against bacteria)

• Plant must recognize attacker as being ‘bacterium’
• Recognition must lead to production of SA, but not JA
• SA-mediated defense active against bacterium or fungus
• SA suppresses JA-mediated defense (insects, nematodes)
• Plant becomes more vulnerable to insects or nematodes

Question 3

Antagonistic hormone pairs

Answer 3

Auxin Cytokinin

Auxin Strigolactone

Gibberellin Abscisic acid

Salicylic acid Jasmonic acid

Question 4

Hormone-mediated resistance to attackers

Answer 4

• Systemic Acquired Resistance = SA/MeSA signalling
• Induced Systemic Resistance = Et and JA signalling
• Wound responses = MeJA signalling

Question 5

Three plant hormones important in defense

Answer 5

• Salicylic acid (against bacteria and biotrophic fungi)
• Ethylene (against necrotrophic fungi)
• Jasmonic acid (against insects and necrotrophic fungi)

Question 6

Attackers can manipulate hormone-mediated defenses

Answer 6

• Some bacteria produce the JA analogue coronatine

- This suppresses SA activity making the plant less resistant to bacterial attack

Question 7

Synergistic hormones

Answer 7

• Synergistic hormones have the same effect in a target cell and therefore amplify the results
• e.g. Ethylene and jasmonic acid

Question 8

Defence decisions

Answer 8

• Plants have many types of attackers and many possible responses
• Therefore they must a defence decision base on all available evidence
• This decision determines the hormone pathways that are activated

Question 9

What is a hormone

Answer 9

A signal molecule produced in one tissue/cell

type and exerting an effect in a different tissue/cell type

Question 10

Auxin

Answer 10

Aux (root formation, apical dominance)

Question 11

Cytokinin

Answer 11

CK (shoot formation, cell division)

Question 12

Gibberellin

Answer 12

GA (seed germination)

Question 13

Abscisic acid

Answer 13

ABA (seed dormancy, water homeostasis)

Question 14

Strigolactone

Answer 14

SL (inhibits shoot branching)

Question 15

Ethylene

Answer 15

Et (ripening, abscission, senescence, defense)

Question 16

Salicylic acid

Answer 16

SA (defense against microbes)

Question 17

Jasmonic acid

Answer 17

JA (wound response, defense against insects)

Question 18

Hormone transport

Answer 18

Via vascular sytem OR cell to cell transport

Ethylene gas can travel via the air outside the plant

Question 19

Which hormones can be methylated?

Answer 19

SA and JA can be methylated to Me-SA and Me-JA (volatiles)

Question 20

Pleiotropic

Answer 20

Producing or having multiple effects from a single gene

Question 21

Studying hormones

Answer 21

• Application of hormones or inhibitors
• Mutants defective in hormone production
• Mutants defective in hormone perception and response .
• Mutants overproducing hormone or “overreacting”
• Mutants often have pleiotropic, developmental phenotypes