Chemical Defence​

Question 1

What do we mean by Plant Defence?​

Answer 1

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)

Question 2

Pre-eminence of Chemical Defence?​

Answer 2

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​

Question 3

Plant Metabolites​

Answer 3

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​

Question 4

Plant Secondary Metabolites
​What are they for?​

Answer 4

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​

Question 5

Secondary Metabolites or Primary Defenses?​

Answer 5

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 ​

Question 6

Defence or Not?​

Answer 6

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​

Question 7

A brief history of Chemical Defence​

Answer 7

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​

Question 8

The big three

Answer 8

(shikimic acid pathway) nitrogen-containing secondary products
*left over carbon of shikimic acid pathway) phenolic compounds
(acetyl coa pathway) terpenes-carbon based compounds

Question 9

Terpenes (or Terpenoids)​

Answer 9

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.

Question 10

Some common examples​ of terpenes

Answer 10

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.

Question 11

Pine Terpenes through the ages​

Answer 11

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)

Question 12

Describe foaming grasshoppers and milkweed

Answer 12

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

Question 13

Phenolics​

Answer 13

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)​

Question 14

Some common Phenolics​

Answer 14

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​

Question 15

Nitrogen-compounds​

Answer 15

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)

Question 15

Phenolics down under​

Answer 15

A general pattern emerges for Proteaceae seedlings attacked by invertebrates and vertebrates

Increased phenolics means reduced herbivory ​

Question 16

Alkaloids in Action​

Answer 16

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.

Question 17

Cyanogenic glycosides​

Answer 17

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​

Question 18

flowers

Answer 18

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

Question 19

Glucosinilates​ (makes brussels taste bad)

Answer 19

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​

Question 20

The White response to GLSs​

Answer 20

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

Question 21

An Evolutionary Arms Race?​

Answer 21

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​

Question 22

Chemical Defence​ summary

Answer 22

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?​