Volatiles (practical too)

Question 1

Constitutive defences​

Answer 1

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​

Question 2

Plants are plastic​

Answer 2

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?​

Question 3

Induced defence definition​

Answer 3

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​

Question 4

Inducing defences​

Answer 4

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​

Question 5

Inducing nicotine ​

Answer 5

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​

Question 6

Jasmonates (JAs)​

Answer 6

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 ​

Question 7

How do plants trigger JA release?​

Answer 7

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​

Question 8

Wounding and Ca2+ transmission ​

Answer 8

All of these processes are stimulates by the herbivores saliva as damage to the plant by scissors did not induce this response

Question 9

McConn Trick?​

Answer 9

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​

Question 10

Benefits of Induction​

Answer 10

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

Question 11

Induced defences summary

Answer 11

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 ​

Question 12

Volatiles​

Answer 12

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)​

Question 13

Some common volatiles​

Answer 13

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)

Question 14

Volatiles - how to detect them​

Answer 14

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​

Question 15

A GC-MS trace​

Answer 15

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 ​

Question 16

How do we know insects track VOCs?​

Answer 16

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​

Question 17

Do VOCs have a defensive role?​

Answer 17

VOCs are continually released by leaves (constitutive?) ​
But when damaged, the types and amount of VOCs released changes (induced defence?)​

Question 18

Herbivore deterrence and VOCs​

Answer 18

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

Question 19

Plantago VOCs​

Answer 19

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 ​

Question 20

Not just something in the air​

Answer 20

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​

Question 21

Talking Trees?​

Answer 21

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​

Question 22

Spit

Answer 22

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​

Question 23

VOCs - the downsides?​

Answer 23

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​

Question 24

Host-location & VOCs​

Answer 24

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​

Question 25

Volatiles​ summary

Answer 25

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 ​