Chemical Defence Flashcards
What do we mean by Plant Defence?
How about - Any morphological, biochemical or structural characteristic that reduces the likelihood of herbivory or its subsequent impact on plant fitness?
includes chemical, physical, latex/sap, stinging trichomes and biological (plants invite ants to defend the plant)
Pre-eminence of Chemical Defence?
Most of the plant defence literature deals with chemical defences
They are relatively easy to quantify and link to other aspects of plant ecophysiology and biochemistry
Plant Metabolites
Primary Metabolites (Carbohydrates, Proteins, Lipids) contribute to photosynthesis, respiration, assimilation, cell division etc.
Plants have loads of organic compounds (Secondary Metabolites) with no obvious function in growth or development
Vast array of Secondary Metabolites but with a more restricted taxonomic distribution
Plant Secondary Metabolites
What are they for?
PSMs have been associated with:
UV protection
Growth regulation
Pollinator attraction
Pathogen Defence
Nutrient cycling
Allelopathy (plants produce biochemicals that influence the growth, survival, development, and reproduction of other organisms.)
Thermal protection
Secondary Metabolites or Primary Defenses?
So PSM may have evolved for a number of reasons unconnected with anti-herbivore defence
But they currently help protect the plant against herbivores (a ‘neutral defence’ sensu Edwards 1989?)
If an increase in the amount of a PSM is correlated with a decrease in herbivore attack/performance, then the PSM probably fulfils a defensive function
Defence or Not?
PSMs are not linked to growth or development and are taxonomically restricted
They are involved in many varied interactions/processes
May have evolved for one or more other functions and later co-opted into defence
A brief history of Chemical Defence
As ‘secondary metabolites’ most defensive compounds were ignored as functionless waste products
Fraenkel (1959) was one of the first to explore the evolutionary role of PSMs as anti-herbivore defences
Since then we have identified a number of defensive compounds that can be categorised into 3 chemically distinct groups
The big three
(shikimic acid pathway) nitrogen-containing secondary products
*left over carbon of shikimic acid pathway) phenolic compounds
(acetyl coa pathway) terpenes-carbon based compounds
Terpenes (or Terpenoids)
Largest class of PSM
Synthesised from Acetyl-CoA (a Carbon-carrier in the Krebs Cycle)
Characterised by 5-Carbon Isoprene Units
Sesquiterpenes (15-C terpenes) have 3 Isoprene Units
Diterpenes (20-C terpenes) have 4 Isoprene Units
isoprene unit is main structure which cause. monoterpene (2 units) diterpenes (4 units) changes molecular weight, can be frequently emitted from the plant, signal to herbivores that plants taste bad (only light molecular weight.
Some common examples of terpenes
Pyrethroids (Chrysanthemum) highly insecticidal - used in commercial insecticides (used to kill bedbugs)
Tetrahydrocannabinol (THC) – medical use
In conifers monoterpenes accumulate in resin ducts – toxic to insects (pine smell)
Many terpenoids are volatile (smelly)
Volatile Terpenes occur in Eucalypt species, the Citrus and Mint families.
Pine Terpenes through the ages
Iason et al (2011) looked at the role of different Terpenes in Scots Pine
Trees are attacked by different herbivores throughout their life-history
Stage & herbivore herbivory
Seedlings - (slugs) -alpha pinene effect against slug
Old saplings - (deer)- delta^3-carene and beta-ocimene deters larger herbivores like deer
Mature Trees - capercaillie (bird) -alpha-pinene, delta^3carene and beta-ocimene (uses all 3 to deter bird)
Describe foaming grasshoppers and milkweed
milkweed, break a bit you get sticky latex full of alkaloid, very toxic the only thing that will eat it is that grasshopper (its can carpmentalise alkaloids, if you annoy grasshoppers it produces out of thorax spit like substance, smells looks and kills predators so colouration shows to NOT eat advertise through colours
Phenolics
Nearly 10,000 different compounds
Characterised by an hydroxyl group (OH) on an ‘aromatic ring’
In higher plants synthesised mainly via the Shikimic acid pathway (creates amino acids)
Some common Phenolics
Lignin – provides mechanical support but also reduces digestibility (its why plants are cruddy) (dense form of phenolic. It reduces digestibility. good for structure but also deters herbivores (cruddiness)
Tannins – are both feeding deterrents and have toxic impacts on growth and survival.
Tannins are what makes tea, apples and red wine taste astringent
Nitrogen-compounds
Synthesised from amino acids
Chemically diverse and found in about 20% of vascular plants
Alkaloids are characterised by the heterocyclic ring (N and C atoms)
A variety of human use (e.g. Cocaine, Codeine, Strychnine, Nicotine)
most plants don’t have excess nitrogen (legumes fabaceae are more likely to use these as defence)
Phenolics down under
A general pattern emerges for Proteaceae seedlings attacked by invertebrates and vertebrates
Increased phenolics means reduced herbivory
Alkaloids in Action
Hanley et al (1995 J. Ecol)
Senecio was the only species to survive attack by slugs during the seedling stage
ragwort full of alkaloid (kills horses) deters slugs too normally outcompeted by dandelion if there aren’t any snails at the start of growth. Ragworts put much more effort into defences and not into growth so there is a cost of defense.
Cyanogenic glycosides
CGs release the gas Hydrogen Cyanide (HCN) common in Fabaceae and Rosaceae (almonds!)
Release is a two-step process
1. Sugar cleaved from CGs by hydrolytic enzyme to form cyanohydrin
2. HCN released by decomposing cyanohydrin (accelerated by lytic enzymes (spit form herbivores)) locally releases cyanide HCN decomposes quickly into cyanide
cyanide can kill themselves with cyanide through respiration so by keeping this separate in tissues
CG and enzymes (hydrolytic & lysic) are stored separately - mechanical disruption allows mixing and HCN release
flowers
Many Hakeas are bird pollinated with conspicuous inflorescences
But pollinator attractiveness and accessibility leaves you vulnerable to flower eaters
Hakeas solve this by producing floral HCN (Hanley et al 2009)
big red obvious flowers great for pollination however bad as herbivores can see them easily. so how do they defend themselves? These flowers are laced with cyanide,
birds don’t disrupt… they just suck up pollen and nectar
Glucosinilates (makes brussels taste bad)
GLSs break down to form isothiocyanates and nitriles in the Brassicaceae (Sprouts!)
Release is a two-step process
1. Glucose cleaved from GLS by hydrolytic enzyme to form aglycone
2. Aglycone releases sulphate and re-arranges to form isothiocyanates and nitriles (catalysed by saliva of herbivores)
GLS and enzymes are stored separately until mechanical disruption allows mixing
The White response to GLSs
GLSs are an effective defence against many herbivores ….but not the cabbage white butterfly
Gravid females use the highly volatile isothiocyanate smell to locate plants (Renwick et al 1992)
Larvae redirect the GLS hydrolysis reaction to produce less toxic nitriles (Wittstock et al 2004)
very attracted to female butterflies, detect isothiocyanate, looks for chemical, as larvae can detoxify chemical and advantage as caterpillars then have all the plant for themselves, caterpillars take in isothiocyanates too so won’t be eaten
An Evolutionary Arms Race?
Despite a diverse array of chemical weapons, all plants get eaten
Erhlich & Raven (1964) proposed the reciprocal (co)evolution of plant defence and herbivore tolerance
Chemical Defence summary
Defence or Not?
PSMs are not linked to growth or development and are taxonomically restricted
They are involved in many other interactions/processes
May have evolved for one or more other functions and later co-opted into defence
The Chemical Arsenal
Carbon-based defences
-Terpenes – Largest group and highly volatile
-Phenolics – Common, cruddy and astringent
Nitrogen-based defences
-Alkaloids – Diverse and useful to us
-Cyangenic glycosides – mixed with enzyme yield HCN
-Glucosinilates - mixed with enzyme yield isothiocyanate
Evolutionary Arms Race?
