Plant Volatiles – Chemical Defense Mechanisms Plant Volatiles – Chemical Defense Mechanisms

Symbiotic, antibiotic,

and defense

relationships

Acacias –

sugar compositionadjusted to

desired ant species

Heil et al. (2005) Postsecretory hydrolysis of nectar sucrose and specialization in ant/plant mutualism. Science 308 (5721)

Plants provide sugars for which ants have no catabolic enzyme.

predator’sPlant predator

predator

PlantPlant

--

HerbivoreHerbivore

--

parasiticparasiticInsectInsect

““Tri-trophic” InteractionsTri-trophic” Interactions

Schnee et al. (2006) The products of a single maizesesquiterpene synthase form a volatile defensesignal that attracts natural enemies of maize herbivores. PNAS 103, 1129

““Tri-trophic” InteractionsTri-trophic” Interactions

maize, cotton, etc.

e.g. Spodoptera littoralis

parasitic wasps

feeding damage

forced regurgitating

JA biosynthesis – abbreviated JA biosynthesis – abbreviated

VOC – volatileorganic compounds

From plant signaling to insect response via

Farmer & Ryan (early 90s) – volatile signals from plant to plant

Jasmonates Terpenes

Plants respond to caterpillar feeding Plants respond to caterpillar feeding Turlings TCJ, Loughrin JH, McCall PJ, Rose USR,

Lewis WJ, Tumlinson JH (1992) How caterpillar-damaged plants protect themselves by attracting parasitic wasps. PNAS 92, 4169.

Healthy, undamaged corn seedlings

6 hours after start of caterpillar feeding

IS1,2 – internal standards

Some peak IDs (LC-MS): 1,2,3 – 3-hexenal; 2-hexenal; 3-hexenol5- linalool; 9 – β-farnesene; 10 - nerolidol

C6

C10

C15

10

9

5

1

C15

jasmone

indole

Feeding on cotton Feeding on cotton

1st day

3rd day

linalool

pinene

farnesene

Change in composition over time of attack.Signaling compounds (or degradation products)at low levels.

Emitted compounds by cottonEmitted compounds by cotton

Start - 2 p.m.5 caterpillars on 6w-old cotton

A – LOX products from cotton

B – constitutive cotton volatiles

C – induced compounds in cotton

Emissions by infected corn over time Emissions by infected corn over time

LOX-products from corn

Induced complexcompounds

Leaves scratched, then addedcaterpillar regurgitate

Recognition – timing, composition and nature of compounds

Signals in Signals in caterpillar “spit”caterpillar “spit”

induce induce plantplant

biodefensebiodefenseWMDWMD

by recruiting by recruiting allied forcesallied forces

Based onBased on

Isoprene &Isoprene &

Isoprenoid metabolismIsoprenoid metabolism

acetoacetyl-CoA + acetyl-CoA > HMG-CoA > mevalonate >>>> isopentenyl-PP

C4 + C2 > C6 > C5 + CO2

Isoprene

Isopentenyl-PP

Dimethylallyl-PP

Geranyl-PP

C5 C5

C20 - Geranyl-geranyl-PPC20 - Geranyl-geranyl-PP

C15 – farnesyl-PPC15 – farnesyl-PP

C25 – Sesterterpines > abundant, non-volatileC25 – Sesterterpines > abundant, non-volatile C30 - Triterpenes > steroid source structure, abundant, non-volatileC30 - Triterpenes > steroid source structure, abundant, non-volatile

C40 - Carotenes > carotenoid source structure, abundant, non-volatileC40 - Carotenes > carotenoid source structure, abundant, non-volatile

6β-acetoxy-24-methyl- 12, 24-dioxoscalaran-25-al

(pacific sponge)

Sesquiterpene type – Sesquiterpene type – phytol (retinol, retinal)phytol (retinol, retinal)

Cyclic sesq.(cadinene)

Induction of sesquiterpene synthasesInduction of sesquiterpene synthases

Wasps fly straight to damaged leaf from downwind, not to a wounded leaf, but to wounded leaves treated with regurgitated midgut sap of insect.

maizemaize

bergamotene

farnesene

sesquiphellandrene

Gene to ProductGene to Product

maizemaize

What happens when the gene is expressed in Arabidopsis ? What happens when the gene is expressed in Arabidopsis ?

A single transgene/ protein generates the entire spectrum!

… … but will the wasps know?but will the wasps know?

Wt and transformed Arabidopsis – wasps in central compartmentWt and transformed Arabidopsis – wasps in central compartment

wt

tr

P < 0.01

• naïve wasps

• trained on Arabidopsis

• trained on maize

Side result – wasps must learn by

trial & error, i.e.,there are other cues;signals that connect

wasp & caterpillar

One could use the contraption for other experimentsOne could use the contraption for other experiments

WesternCorn rootworm

Diobroticav. virgifera

A major problem in US agriculture – is there a natural biodefense strategy (i.e., no chemicals)?

Metabolomicsto the

Rescue!

One could use the contraption for other experimentsOne could use the contraption for other experiments Maize

WesternCorn rootworm

Nematode

Trimorphic interaction involving a entomopathogenic nematode

Rasmann et al. (2005)Nature 434, 731.

Experiments similar to the waspExperiments similar to the wasppredation experimentpredation experiment

• Identification of attractant

• Why is US maize not protected

• Does it work in the field

• Isoprenoids in the soil?

Sesquiterpene hydrocarbons in maizeSesquiterpene hydrocarbons in maize

A – leaf inducible, B – ubiquitous; C – root specific

Terpene synthases in maize Terpene synthases in maize

• Heterologous expression• GC-MS with isotopic tracers• GC-MS of different lines

• Mutational analysis

Sesquiterpene spectrum as affected by mutational analysis of the gene