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 + CO 2
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
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