Plant Defenses Flashcards
What are some examples of mutualistic relationships between plants and other species?
» Mycorrhizal fungi
» Nitrogen-fixing bacteria
» Pollinating animals
Why do plants even need protection or defense?
- 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
What are the different types of plant defenses?
» Physical
» Biological
» Chemical
» Behavioural responses
What are physical barriers in plants?
- Physical barriers are the first line of defense seen at both macroscopic level and Cellular level
What are the physical barriers at macroscopic level?
» 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)
What are the physical barriers at cellular level?
» 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)
How can physical barriers be breached?
» Damage from grazing herbivore
» Microbes can also gain entry at open stomata
How does the plant get rid of things like microbes and harmful bacteria
- 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
What does PAMP stand for?
PAMP = pathogen associated molecular patterns
What is PAMP?
- 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
Why do plants need the Effector-triggered immunity?
Some bacteria have evolved chemicals (“effectors”) to suppress the PAMP response
How does the Effector-Triggered Immunity work?
- 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
What is another form of defence besides the PAMP and Effector methods?
- 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)
What are the types of “plant poisons” or chemicals that plants make as a defense?
» 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
What is clover disease?
- 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
How do plants “call for help”?
- 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
What is another method of defense besides the PAMP and Effector methods and besides the chemical releases?
- 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
What is mimicry in plants?
- 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
What is an example of complex behavioral response in plants?
- 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
What is another example of complex behavioral responses in plants?
» 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
