Plant Defenses

Question 1

What are some examples of mutualistic relationships between plants and other species?

Answer 1

» Mycorrhizal fungi
» Nitrogen-fixing bacteria
» Pollinating animals

Question 2

Why do plants even need protection or defense?

Answer 2

- As primary producers, plants are at the base of most foodwebs, and are subject to attack by a wide range of organisms
» Herbivorous animals
» Parasitic fungi
» Bacteria
» Viruses

Question 3

What are the different types of plant defenses?

Answer 3

» Physical
» Biological
» Chemical
» Behavioural responses

Question 4

What are physical barriers in plants?

Answer 4

• Physical barriers are the first line of defense seen at both macroscopic level and Cellular level

Question 5

What are the physical barriers at macroscopic level?

Answer 5

» Waxy cuticle
» Epidermis
» Periderm
» Large spines or thorns ≠ large herbivores
» Fine hair or spines (“trichomes”) ≠ herbivorous insects
• Some trichomes produce toxic chemical cocktails (eg azadirachtin, pyrethrum)

Question 6

What are the physical barriers at cellular level?

Answer 6

» Tough cells (plant cell wall, sclerenchyma cells) → difficult to eat
» Silica used to create “plant stones” (“plytoliths”) → really (!) difficult to eat (reduces growth rate of herbivorous insects, abrades teeth)

Question 7

How can physical barriers be breached?

Answer 7

» Damage from grazing herbivore

» Microbes can also gain entry at open stomata

Question 8

How does the plant get rid of things like microbes and harmful bacteria

Answer 8

• Once the enemy is inside, it is important to be able to recognize it – this is where the plant immune system kicks in
• Two types of immune responses
  » PAMP-triggered immunity
  » Effector-triggered immunity

Question 9

What does PAMP stand for?

Answer 9

PAMP = pathogen associated molecular patterns

Question 10

What is PAMP?

Answer 10

• PAMP-triggered immunity:
  » Based on detection of non plant molecules
  » When plant detects an alien molecule (eg, flagellin protein specific to bacteria), it produces broad-spectrum antimicrobial chemicals with fungicidal and bactericidal properties

Question 11

Why do plants need the Effector-triggered immunity?

Answer 11

Some bacteria have evolved chemicals (“effectors”) to suppress the PAMP response

Question 12

How does the Effector-Triggered Immunity work?

Answer 12

• Effector-triggered immunity: if a plant detects these effectors, it activates the effector-triggered immunity, which triggers:
  » Hypersensitive response: basically kills the plant tissue at the site of infection
  » Systemic acquired resistance (“call to arms”): infection site produces alarm chemical (methyl salicylate → salicylic acid), which induces plantwide expression of defense genes

Question 13

What is another form of defence besides the PAMP and Effector methods?

Answer 13

• The immune system releases toxic (antimicrobial) chemicals
• Plants also produce other toxic chemicals (plant poisons)
• Very large variety of chemistry, especially within flowering plants (angiosperms – one of the reason for their success)

Question 14

What are the types of “plant poisons” or chemicals that plants make as a defense?

Answer 14

» Nicotine (very toxic to insects)
» Digoxin – used for heart medication
» Highly toxic alkaloids like morphine, strychnine, coniine (hemlock)
» Chemicals that mimic moulting hormones (“phytoecdysteroids”), thereby
interfering with insect moulting

Question 15

What is clover disease?

Answer 15

• Chemicals in clover mimic the potent animal female hormone estradiol (“phytoestrogen”)
  » Genistein, daidzein, coumestrol
• “Clover disease”:
  » Infertility in sheep fed on fields of clover
  » Big problem in WA and NSW in 1960s

Question 16

How do plants “call for help”?

Answer 16

• When injured, plants release volatile alarm chemicals
  » Alert nearby plants of the danger
  » Attract carnivorous insects (“recruit bodyguards”)
  » Attract insectivorous birds
• Some plants attract ants (providing them with food), to serve as a standing army (eg, Acacia and Pseudomyrmex ants) against herbivores
  » Some ant colonies even attack nearby plants

Question 17

What is another method of defense besides the PAMP and Effector methods and besides the chemical releases?

Answer 17

• Biological warfare
• Plants can silence RNA of pathogens, targeting genes for growth, development and survival of the attacker
  » Using small interfering RNA molecules
  » Holds great promise for crop protection

Question 18

What is mimicry in plants?

Answer 18

- Some plants produce leaves to mimic …
» Look un-inviting
» Being partially eaten
» The presence of eggs (dissuading egg laying insects from doing so)
» Being a carnivorous insect

Question 19

What is an example of complex behavioral response in plants?

Answer 19

• Wild tobacco plant response to herbivory
  » Releases nicotine (neurotoxicant) when fed upon
  » Some caterpillars immune to nicotine, but … plant detects saliva from those caterpillars, in which case it releases a volatile chemical messenger, which attract “insect mercenaries” that eat caterpillars

Question 20

What is another example of complex behavioral responses in plants?

Answer 20

» The “evil lollipop” – plants provide a nice sugary treat for caterpillars, which make them smell and attracts land predators (eg, lizards)
» Ultimately, if too many caterpillars (from its normal, night-time pollinator moth), it can even change pollinators bloom during day instead of at night → switches from moth to hummingbird