VL 4 Flashcards
Two principal approaches to measuring plant resistance to herbivores
Antibiosis
(how suitable the plant is for the herbivore)
- herbivore fitness
- intrinsic plant traits underlying herbivore fitness
Antixenosis
(how much damage or how many herbivores a plant attracts)
- herbivore presence
- herbivore damage
Two types of direct resistance
Physical resistance
- cuticle wax, lignification
- trichomes, spines, thorns
- secretory defenses
- calcium oxalate crystals
Chemical resistance
- Reduced digestibility
- Toxins
Cuticle, wax and lignification
Border of a plant and first line of defense
primarily useful for maintaining osmotic balance with environment
has some defensive capabilities against insects
structurally diverse among species but exhibits the organization of a composite material consisting in cutin and suberin, a polyester that is partly covered and interspersed with waxes
How does wax reduce insect adhesion?
Roughness hypothesis
wax-dissolving hypothesis
contamination hypothesis
fluid-absorbtion hypothesis
Lingnin
Lignin is the second most abundant polymer, after cellulose, found in nature
Lignin provides terrestrail plants with physical structure
synthesized by the phenylpropanoid pathway (as are tannins, flavonoids and other polyphenols)
can be induced locally to prevent or hinder feeding
Trichomes, spines and thornes
Limit herbivory by blocking access to plant cells
Glandular thrichomes can:
- trap insects and/or secrete chemical deterrents
- encapsulate volatile chemical deterrents
spines/thorns:
originally thought to only be a deterrent against larger, vertebrate herbivores
recent study shows that spines restrict movement of caterpillars
Secretory defenses - three types
Resin
Gum
Latex
Resin
Lipid-soluble mixture of volatile and non-volatile terpenoid and/or phenolic compounds
this mixture of secondary metabolites is not involved in growth metabolism. It is not “phloem sap”
(Epithelial cell lining secretes resin)
Resin evolved 320 million years ago
Amber is fossilized plant resin
Effectivity since its evolution is proven by enclosures in amber
Gum
Neither terpenoid, nor phenolic in origin (but can contain both)
True gums are complex chains of hydrophilic polysaccharides
Broken plant cell walls result in cavities in the tissue which are filled with sugars
Latex
Composed of a complex mixture of hydrophobic molecules (lipid soluble) including tannins, proteins, alkaloids, oils and gum
secreted by laticiferous tissues
Calcium oxalate crystals
Raphides: prismatic monoclinic crystals of Ca oxalate
produced in cells called idioblasts
a large “styloid” spans the entire leaf
Raphids faciliate poisoning of the gut in two steps:
- mechanical prcking
- injection of a harmful protease
Chemical resistance - reduced digestibility
chemical resistance - toxins
gut disruption / destruction as plant defense
- can kill or slow growth of herbivourous insects
- generally, prevents the insect from reaching maturity
4 Types of toxins
Cyclopeptides
Lectins
Alkaloids
Glucosinlates
Cyclopeptides
Large class of small plant proteins that are post-translationally processed into a continuous loop structure
result in swollen gut due to epithelial cell damage
Lectins
Plant lectins are proteins that bind specifically to carbohydrates on the cell surface
highly selective
several lectins are known to cause antibiotic effects in Coleoptera, Hemiptera or Lepidoptera
Alkaloids
“True alkaloids” contain nitrogen in a heterocyclic molecule and originate from amino acids
* E.g. Nicotine, Cocaine and Morphine
“Pseudoalkaloids” – alkaloid-like compounds that do not originate from amino acids.
* E.g Caffeine
Other alkaloid groups exist
Example of alkaloid
Nicotin:
unique toxin of the Solanaceae family
binds to acetylcholine receptors and causes paralysis in insects, at high concentrations even death
Neonicotinoids:
Very high specifity to insect acetylcholine receptors
most effective insecticides but kills beneficial insects as effectively
Glucosinolates
Mustard oils -> two component system called “mustard oil bomb”
unique to brassicaceae
commonly induced by herbivory
initially evolve to deter herbivory
Have resulted in one of the most studied evolutionary arms races in plant-insect interactions
plants evolve new glucosinolates to deter herbivory
Insects evolve new ways to mitigate glucosinolates and repurpose them for other uses
this arms race has resulted in providing humans with the taste that we enjoy in mustard plants
4 contermeasures of insects to direct resistance
specialized behaviour
detoxification
sequestration of toxic compounds
highjack host’s metabolism
Example of chemical resistance in insects: Nicotine resistance
Manduca sexta (tobacco hornworm) is specialized for life around Solanaceae
The adult hawkmoth feeds (and pollinates) Solanaceae
Manduca caterpillars move unmodified nicotine through ther gut and excrete via their Malpighian tubules
Cotesia parasitoid larvae exhibit increased mortality when feeding on Nicotiana-fed caterpillars
Sequestration and modification of nicotine
Unmodified nicotine repels spiders
Manduca sexta larvae do not metabolize nicotine
Nicotine oxidation reduces Spodoptera exigua’s headspace-nicotine and renders it more susceptible to predation by spiders than M. sexta, which exhales unmetabolized nicotine
Glucosinolate sequestration and modification
p. 52 - 54
Example of specialized behavior
Cutting the midrib eliminates the distal outflow of latex
58 - 72
Effect of Daktulosphaira vitifoliae Phylloxeridae on wine production in Europe
p. 58 - 70
