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Plant Volatiles – Chemical Defense Mechanisms
Symbiotic,
antibiotic,
and
defense
relationships
Acacias
–
sugar composition
adjusted 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.
“Tri-trophic” Interactions
Plant
predator’s
Plant
predator
-
predator
Herbivore
parasitic
Insect
“Tri-trophic” Interactions
forced regurgitating
feeding damage
maize, cotton, etc.
e.g. Spodoptera littoralis
parasitic wasps
Schnee et al. (2006) The products of a single maize
sesquiterpene synthase form a volatile defense
signal that attracts natural enemies of maize herbivores.
PNAS 103, 1129
JA biosynthesis – abbreviated
From plant signaling
to insect response via
VOC – volatile
organic compounds
Jasmonates
Terpenes
Farmer & Ryan (early 90s) –
volatile signals from plant to plant
Turlings TCJ, Loughrin JH, McCall PJ, Rose USR,
Plants respond to caterpillar feeding
Lewis WJ, Tumlinson JH (1992) How caterpillardamaged plants protect themselves by attracting
parasitic wasps. PNAS 92, 4169.
Healthy, undamaged corn seedlings
1
C6
6 hours after start
of caterpillar feeding
5
C10
Some peak IDs (LC-MS):
1,2,3 – 3-hexenal; 2-hexenal;
3-hexenol
5- linalool; 9 – β-farnesene;
10 - nerolidol
C15
IS1,2 – internal standards
9
10
C15
Feeding on cotton
Change in composition over time of attack.
Signaling compounds (or degradation products)
at low levels.
jasmone
pinene
1st day
linalool
indole
farnesene
3rd day
Emitted 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
Leaves scratched, then added
caterpillar regurgitate
LOX-products from corn
Induced complex
compounds
Recognition – timing, composition and nature of compounds
Signals in
caterpillar “spit”
induce
plant
biodefense
WMD
by recruiting
allied forces
Based on
Isoprene &
Isoprenoid metabolism
acetoacetyl-CoA + acetyl-CoA > HMG-CoA > mevalonate >>>> isopentenyl-PP
C4 + C2
>
C6
>
C5 + CO2
Isoprene
Isopentenyl-PP
Dimethylallyl-PP
C5
C5
Geranyl-PP
C15 – farnesyl-PP
Cyclic sesq.
(cadinene)
C20 - Geranyl-geranyl-PP
Sesquiterpene type –
phytol (retinol, retinal)
6β-acetoxy-24-methyl12, 24-dioxoscalaran-25-al
(pacific sponge)
C25 – Sesterterpines > abundant, non-volatile
C30 - Triterpenes > steroid source structure, abundant, non-volatile
C40 - Carotenes > carotenoid source structure, abundant, non-volatile
Induction of sesquiterpene synthases
maize
Wasps fly straight to damaged leaf from downwind, not to a wounded leaf,
but to wounded leaves treated with regurgitated midgut sap of insect.
Gene to Product
maize
What happens when the gene is expressed in Arabidopsis ?
A single transgene/ protein generates the entire spectrum!
… but will the wasps know?
Wt and transformed Arabidopsis – wasps in central compartment
• naïve wasps
wt
• trained on Arabidopsis
tr
• trained on maize
Side result –
wasps must learn by
trial & error, i.e.,
there are other cues;
signals that connect
wasp & caterpillar
P < 0.01
One could use the contraption for other experiments
Western
Corn rootworm
Diobrotica
v. virgifera
Metabolomics
to the
Rescue!
A major problem in US agriculture –
is there a natural biodefense strategy (i.e., no chemicals)?
One could use the contraption for other experiments
Maize
Western
Corn rootworm
Nematode
Rasmann et al. (2005)
Nature 434, 731.
Trimorphic interaction involving a entomopathogenic nematode
Experiments similar to the wasp
predation experiment
• Identification of attractant
• Why is US maize not protected
• Does it work in the field
• Isoprenoids in the soil?
Sesquiterpene hydrocarbons in maize
A – leaf inducible, B – ubiquitous; C – root specific
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