Volatiles (practical too) Flashcards
Constitutive defences
A Constitutive defence is always expressed
i.e. the plant continually allocates resources to chemical or structural defence on the ‘expectation’ that herbivory will happen
The defence functions independently of damage
Plants are plastic
Many traits vary according to environment
Plastic phenotypes are not genetically fixed (although plasticity is under genetic control)
Herbivores are a key part of the environment so can plant respond to them also be plastic?
Induced defence definition
Changes in plant traits decreasing the negative fitness consequences of herbivore attack
*Increased plant resistance
Mobilisation of resources towards increased chemical or structural defence
*Increased plant tolerance
Mobilisation of resources towards regrowth following herbivore attack
Inducing defences
Achieved via variation in gene transcription rates
Following attack a plant will down-regulate genes coding for one given process and up-regulate genes coding for defence The result is increased anti-herbivore defence following herbivore attack
Inducing nicotine
Following damage by tobacco hornworm (Manduca sexta)
tobacco (Nicotiana tobacum) plants down-regulated genes for photosynthesis and up-regulated genes involved in defence, so tobacco plants produce more nicotine after being attacked
Jasmonates (JAs)
JA biosynthesis is rapidly activated following wounding
JAs induce the up-regulation of genes involved in secondary metabolite synthesis
JAs also stimulate synthesis of protease inhibitors that interfere with insect digestive proteases to stop nitrogen assimilation from leaf protein
JAs travel through the plant and stimulate defence in ungrazed leaves on the same plant
How do plants trigger JA release?
Tissue damage triggers electrical activity that propagates rapidly (cm min-1) through the plant
These “slow wave potentials” (SWPs) are characterized by rapid (<2 s) and massive (>50 mV) membrane depolarizations
Glutamate receptor-like (GLR) ion channels mediate Ca2+ membrane depolarization and JA synthesis in damaged and undamaged plant tissues
JAs activate transcription factors in the cell nucleus, leading to changes in gene expression
Wounding and Ca2+ transmission
All of these processes are stimulates by the herbivores saliva as damage to the plant by scissors did not induce this response
McConn Trick?
Some mutant lines of Arabidopsis produce only low level of JAs
These mutants had low resistance against herbivore attack - fungal gnats (Bradysia impatiens)
Application of exogenous methyl jasmonate reduced mortality from 80% to ≈12%
Methyl JA induced transcription of wound-responsive genes to increase chemical defence
Benefits of Induction
1: Maintaining constitutive defences is costly
-ve relationship between competitive ability (C) & constitutive defence suggests a cost
+ve relationship between competitive ability (D) & induced defence implies benefit
2: Keep the herbivores guessing
Variation in defence via induction reduces opportunity for herbivore adaption
Induced defences summary
Induced Defences
Plant traits are plastic, so option to increase defence in response to herbivore attack .
Induction achieved by electrical (Ca2+) signalling triggered by GLR proteins – JAs are released across the plant
Protease inhibitors reduce N-assimilation; genes involved in secondary metabolite synthesis are upregulated
Induction has less impact on plant competitive ability and may reduce herbivore adaptation
Volatiles
In-fact many plant chemicals are volatile – i.e. molecules readily escape by evaporation into the air
Traditionally associated with flowers
Now linked to many different plant structures (leaves, roots, etc)
Some common volatiles
Terpenes
C10 monoterpenes and C15 sesiquiterpenes (eucalyptus)
(Pinaceae, Myrtaceae, Geraniaceae + many others)
Green Leaf Volatiles (GLVs)
Lipid-based C6 aldehydes, alcohols, esters
Less obvious (to us) but taxonomically widespread (grassy smell)
Volatiles - how to detect them
Gas Chromatography-Mass Spectrometry (GC-MS)
After injection, compounds separate as they travel along the column
Compounds retained by the column and then elute (come off) from the column at different times (called the ‘Retention Time’)
Mass spectrometer captures and then ionizes (breaks down) each compound separately - this aids ID
A GC-MS trace
A good trace should present symmetrical, non-overlapping peaks
Peaks on the trace compared with known standards using a chemical library to allow identification of each compound
How do we know insects track VOCs?
GC-EAG allows simultaneous passage of VOCs through a GC and across an insect antenna (implanted with an electrode
This yields a ‘coupled trace’ showing insect response to individual compounds emitted by plants
Do VOCs have a defensive role?
VOCs are continually released by leaves (constitutive?)
But when damaged, the types and amount of VOCs released changes (induced defence?)
Herbivore deterrence and VOCs
Snails given a choice between Plantago lanceolata pellets made from plants of different ages or lettuce ‘standard’ in olfactometer trials.
Material from older plants was progressively avoided
Plantago VOCs
We compared volatile profiles from 1- and 8-week old Plantago lanceolata pellets
Big increase in GLVs in 8-week old material
Suggests snail olfaction influenced by GLVs
Not just something in the air
Wiesemeier et al (2007) showed volatile release in the inter-tidal alga Dictyota dichotoma
When these compounds were added to agar, the amphipod Amphithoe longimana showed a strong preference for controls
Talking Trees?
Evidence that VOCs released by a plant under herbivore attack could induce defence in undamaged neighbours remained elusive
Arimura et al (2000) first to show that undamaged leaves could activate defence genes when exposed to VOCs from leaves damaged by herbivores
Interestingly there was no effect (in lima beans - Phaseolus lunatus) when undamaged leaves were exposed to VOCs from artificially wounded leaves
Spit
VOC induction (and non-volatile secondary metabolites) may depend on contact with specific oral secretions (‘elicitors’)
Pierre et al (2011) showed how above-ground VOC emission by turnips following root damage (by cabbage rootfly) differed from the blend released when attacked by cabbage white caterpillars.
Various combinations of lytic enzymes (β-glucosidase) likely to elicit production of new VOCs – specific to a particular herbivore elicitor
VOCs - the downsides?
VOC synthesis involves an energetic/ nutrient cost (diverts resources from growth and reproduction
Used by parasitic plants to find host-plants
Herbivores can use VOCs to locate food-plants
Host-location & VOCs
The plant has little control over who detects the VOCs and how they use the information
Some herbivores use VOC release as a host-plant locator
This is particularly well studied in Lepidoptera
Cabbage White Butterflies use the highly volatile isothiocyanate to locate plants upon which to lay eggs. So VOC release can increase plant ‘apparency’ to herbivores
Volatiles summary
Plants emit various VOCs, dominated by GLVs & terpenes
Techniques - GC-MS, GC-EAG
A clear defensive role - signalling constitutive defence and a deterrent in their own right
VOCs signal attack to neighbours ‘Talking Trees’
But there are costs – reduced growth, greater Apparency
