Content uploaded by Jack C Schultz
Author content
All content in this area was uploaded by Jack C Schultz on Jan 08, 2014
Content may be subject to copyright.
1
Plants release volatiles after herbivore attack in a highly regulated
fashion. These compounds attract natural enemies and function
as indirect defenses. Whether neighboring plants ‘eavesdrop’
on these volatile signals and tailor their defenses accordingly
remains controversial. Recent laboratory studies have identified
transcriptional changes that occur in plants in response to certain
volatiles. These changes occur under conditions that enhance the
probability of signal perception and response. Field studies have
demonstrated repeatable increases in the herbivore resistance
of plants growing downwind of damaged plants.
Addresses
Max Planck Institute for Chemical Ecology, Department of Molecular
Ecology, Winzerlaer Strasse 10, Jena 07745, Germany
*e-mail: baldwin@ice.mpg.de
Current Opinion in Plant Biology 2002, 5:
1369-5266/02/$ — see front matter
© 2002 Elsevier Science Ltd. All rights reserved.
DOI 10.1016/S-1369-5266(02)00263-7
Abbreviations
FAC fatty-acid–amino-acid conjugate
HI herbivore-induced
JA jasmonic acid
MeJA jasmonic acid methyl ester
PAL phenylalanine ammonia lyase
VOC volatile organic compound
Introduction
Plants are masters of gas exchange, not only literally
building forests from gases taken from the air but also
releasing complex bouquets of volatile organic compounds
(VOCs) back into the air. This remarkable ability fuels the
expectation that plants communicate through volatile
signals. Although ‘communication’ is a loaded term that
means different things to different researchers, most would
accept a definition with the minimal requirement that
information be exchanged, regardless of ‘intent’ or fitness
consequence for either party. Two decades ago, researchers
reported that wounding or herbivore attack resulted in
changes in the herbivore resistance, or to the secondary
metabolites that mediate this resistance, not only of the
attacked plants but also of plants growing nearby. In some
experiments, aerial transfer of information was the most
parsimonious way in which the results could be interpreted
[1], causing the phenomena to be dubbed ‘talking trees’ by
the popular press. Given that neighboring plants compete
for resources and that selection is unlikely to favor plants
that provide information to competitors, the phenomena
should be more aptly called ‘eavesdropping elms’.
Experiments published in the past two years have been
highlighted in reviews [2–4] and have rekindled interest in
these phenomena. Last year, evidence for inter-plant
communication was compiled in a special issue of
Biochemical Systematics and Ecology [5
••
]. Here, we review
the evidence concerning how emissions are controlled, the
signals involved, and the responses of downwind plants.
We also summarize the challenges for future research.
Regulation of the composition, and the
temporal and spatial patterns, of VOC release
After herbivore attack, plants release complex bouquets of
volatiles into the air from their vegetative tissues. The
release of some constituents is likely a passive consequence
of damage to the compartments (e.g. vacuoles or trichomes)
in which VOCs (or their precursors) are stored. The release
of other constituents has been demonstrated to result from
de novo synthesis and is tightly controlled. Even metabo-
lites that occur in substantial pools in undamaged leaves
may be actively discharged. For example, mechanical
damage to Artemesia tridentata leaves causes the release of
large amounts of jasmonic acid methyl ester (MeJA) into
the air under field conditions. When compared to the pools
of MeJA found in the leaves, the epimeric composition of
the released MeJA is highly enriched in the thermodynam-
ically unstable and biologically active enantiomer (i.e.
3R,7S MeJA) [6
•
], suggesting that the released material is
newly synthesized or somehow epimerized during release.
Flowers and other reproductive organs are known to
discharge complex blends of VOCs with distinct temporal
patterns, and evidence is emerging that herbivore-induced
(HI) releases from vegetative tissue are similarly regulated.
The mechanisms that control floral emissions are only just
being examined [7
•
,8], and the molecular and physiological
controls over HI releases are not understood. Most
HI-VOCs can also be found in the floral headspace of some
species and are derived from phenolic, terpenoid and
fatty-acid metabolic pathways that utilize both stored
reserves and recently fixed carbon. After herbivore attack,
HI-VOCs are released both locally from damaged tissues
and systemically from undamaged tissues in discrete
temporal patterns. Some constituents are emitted at
maximum levels during daylight hours and become
undetectable at night [9,10
•
], others have nocturnal
maxima [11]. With the recent development of instrumenta-
tion that allows the real-time analysis of emission patterns
[12
••
], the temporal and spatial complexity of these patterns
will be more readily characterized.
Wounding plays an important role in eliciting the VOC
release. In some plant species, mechanical damage can pro-
voke releases of the same VOCs as are elicited by herbivory.
In many plant species, however, the HI-VOC release differs
from that elicited by mechanical wounding. Exogenous
Volatile signaling in plant–plant–herbivore interactions:
what is real?
Ian T Baldwin*, André Kessler, Rayko Halitschke
jasmonic acid (JA) treatments can trigger a VOC release [13],
but the exact ratios of constituents in such VOCs can
sometimes differ from those in VOCs released after
herbivore attack [9,10
•
,14]. When herbivores attack, they
not only cause damage but also introduce saliva-derived
compounds to the wound sites. Fatty-acid–amino-acid
conjugates (FACs) in herbivore saliva have been shown to
elicit both an endogenous JA burst as well as a HI-VOC
release in native tobacco [15]. If these FACs are removed
from the oral secretions, eliciting activity is lost, but is
regained when synthetic FACs are added back [15]. These
results demonstrate that FACs probably activate the
endogenous jasmonate cascade in a manner that differs
from that effected by wounding. A FAC containing a
17-hydroxy functionality (i.e. volicitin), which causes a
HI-VOC release in corn but not in lima bean [16], was
found to be just as effective or less effective in eliciting a
HI-VOC release than exogenous applications of JA [10
•
].
Whether or not volicitin elicits a JA burst that is associated
with the HI-VOC release in corn remains to be determined.
The study of HI-VOC releases has focused primarily on
species other than Arabidopsis, largely because the small
stature and low emission rate of Arabidopsis challenge the
analytical sensitivity of volatile detection systems. A recent
study solved this problem by using short-day conditions to
prolong vegetative growth, thereby producing abnormally
large Arabidopsis plants [17
•
]. This strategy allowed Van
Poecke et al. to demonstrate that plants infested with Pieris
rapae larvae attracted Cotesia rubecula parasitoid females,
and that this attraction was correlated with greater
emissions of methyl salicylate, myrcene and two nitriles.
The attractiveness of the nitriles, which are likely derived
from larval feces, is noteworthy as the recently cloned
TASTY locus, which confers susceptibility to feeding by
Trichoplusia ni larvae, encodes genes that are responsible
for nitrile biosynthesis [18]. These results suggest that
nitrile production may attract both herbivores and their
predators. Arabidopsis’ small stature makes it unlikely
that the attraction of parasitoids or predators by volatile
emissions is under current selection as an indirect defense
in this species. Nevertheless, the powerful genomic tools
available in Arabidopsis make it a valuable system for the
identification of signal cascades and downstream genes
that are involved in VOC release.
Are plants the receiver for VOC emissions?
Predators and parasitoids of insect herbivores are clearly
attracted to HI-VOC releases [19], which have been shown
to function as a powerful indirect defense for plants [14].
Whether or not other plants respond to these emissions
remains controversial, largely because the experimental
conditions under which ‘communication’ has been demon-
strated are seldom found in nature. Receptors are known for
the volatile intra-plant signal ethylene, but not for the larger
molecular weight volatiles of the HI-VOC release. Without
a receptor and signal transduction system to amplify the signal,
information transfer from plant to plant would probably
depend on the diffusion and convection of the volatile
signal between sender and receiver. It would therefore
require a high release concentration or accumulation after
exposure. Moreover, if large quantities of the signal must
be released, plants must face the challenge of being
insensitive at their release sites while remaining sufficiently
sensitive to perceive the signal after the many-fold dilution
that inevitably occurs during air transport.
Most studies of plant–plant communication through
volatiles have exposed plants to unrealistically high
concentrations of putative volatile signals and have
enclosed them in air-tight chambers without replenishing
CO
2
for long periods of time. When plants that are below
their CO
2
compensation points are fumigated, they are
likely to receive abnormally high exposures to HI-VOCs.
Moreover, many researchers have used excised leaves or
shoots rather than intact plants in their assays. The use of
excised leaves increases the probability of a response for
two reasons. First, as has been demonstrated in corn [10
•
]
and lima bean [20], excised leaves release more HI-VOCs
than do intact plants. Second, excised leaves are more
sensitive to HI-VOCs than are intact plants. Working with
the lima bean system, Arimura and coworkers [20]
demonstrated that excised leaves responded to HI-VOCs
by increasing transcripts encoding the pathogenesis-related
proteins β-1,3 glucanase (Pr-2) and chitinase (Pr-3), pheny-
lalanine ammonia lyase (PAL), lipoxygenase (LOX) and
farnesyl pyrophosphate synthetase (FPS). When intact
plants were exposed to the same volatile treatments,
however, transcription of the LOX and FPS genes did not
increase [20]. This excellent study also highlights another
trend that increases the chances of seeing a plant–plant
response: the use of transcriptional changes as a response
variable. Transcriptional changes are likely rapid indicators
of a response that do not necessarily represent a full
commitment to a change in defense phenotype. Nevertheless,
they demonstrate that the plant has perceived the signal and
provide valuable insight into the environmental signals
that induce a response.
Although most studies of plant–plant communication
sacrifice ecological realism to increase the likelihood of
seeing a response, the results provide tantalizing hints as to
how signals could be examined under more realistic condi-
tions. The most sustained effort, from work by Takabayashi
and colleagues with lima bean plants [21
••
], has demonstrated
that intact plants that are exposed to HI-VOCs in sealed
chambers increase the transcription of Pr-2, Pr-3 and PAL,
as well as of genes involved in ethylene biosynthesis (i.e.
those encoding S-adenosylmethionine [SAM] synthase,
1-aminocyclopropane-1-carboxylic acid oxidase [ACO] and
SAM decarboxylase [SAMDC]). When exposed to HI-VOCs
in the cuvette of an open flow photoacoustic spectropho-
tometer, so as to permit the measurement of ethylene release
in real time, plants emitted more ethylene [21
••
]. This is
consistent with the transcriptional control of ethylene release
in responses to HI-VOCs. Interestingly, the research by
2 Biotic interactions
Takabayashi and colleagues also demonstrated an important
role for green leaf volatiles, that is C6-alcohols and alde-
hydes, in eliciting the PR-protein transcriptional responses
in uninfested leaves [20]. These results are consistent
with earlier findings that suggest an intra-plant signaling
function for green leaf volatiles [22,23]. Clearly, more
attention should be given to the potential signaling roles of
compounds with α-β-unsaturated carbonyl groups [4].
Results from field studies raise fewer concerns about
ecological realism, but the replication of results and
identification of responsible mechanisms can be onerous
in this research. Work on inter-plant signaling between
native tobacco and sagebrush by Karban and colleagues
[24–26] represents the best-replicated study to date. In
five consecutive field seasons, the mechanical clipping of
sagebrush increased the herbivore resistance of native
tobacco plants that were transplanted to within 10–15 cm
of the clipped sage relative to the resistance of tobacco
plants transplanted to be adjacent to unclipped sage.
Experiments from one field season strongly implicated
aerial (as opposed to below-ground) transfer of information
[24]. Clipping of sagebrush dramatically increased the
release of the biologically active enantiomer (3R,7S) of
MeJA [24], but ongoing research [6
•
] has not confirmed
that this constituent is the biologically active signal.
Conclusions and future prospects
The increasing sensitivity of analytical instrumentation
has recently allowed plant VOC emissions to be character-
ized in real time [12
••
,27]. Unless receptors for these
putative volatile signals are discovered, however, similar
advances will be required to characterize plant ‘immi-
sions’: the signals entering a plant. If the ongoing
transcriptional analyses identify genes that are strongly
regulated by HI-VOCs, these will likely provide a source
of promotors that could be fused to easily characterized
reporters (e.g. β-glucuronidase [GUS] or green fluorescent
protein [GFP]). Plants that have been transformed with
such reporter genes could provide much needed informa-
tion about how plants respond to volatile signals under
natural conditions. While some researchers continue to
look for evidence of aerial communication, others are
examining communication below ground [28]. Below-
ground signaling, in contrast to VOC signaling, limits the
dialog to immediate neighbors and competitors, and is not
affected by wind direction, but will require additional
sophisticated approaches to disentangle it.
References and recommended reading
Papers of particular interest, published within the annual period of review,
have been highlighted as:
• of special interest
••of outstanding interest
1. Baldwin IT, Schultz JC: Rapid changes in tree leaf chemistry
induced by damage: evidence for communication between plants.
Science 1983, 221:277-279.
2. Agrawal AA: Communication between plants: this time it’s real.
Trends Ecol Evol 2000, 15:446.
3. Pickett JA, Poppy GM: Switching on plant genes by external
chemical signals. Trends Plant Sci 2001, 6:137-139.
4. Farmer EE: Surface-to-air signals. Nature 2001, 411:854-856.
5. Dicke M, Bruin J: Chemical information transfer between plants:
•• back to the future. Biochem Syst Ecol 2001, 29:981-994.
This special issue consolidates the best current evidence for plant–plant
signaling.
6. Preston CA, Laue G, Baldwin IT: Methyl jasmonate is blowing in
• the wind, but can it act as a plant–plant airborne signal? Biochem
Syst Ecol 2001, 29:1007-1023.
The authors describe an elegant experimental approach to manipulating a
putative active component in sagebrush emissions (i.e. 3R,7S MeJA) that
elicits resistance in native tobacco.
7. Kolosova N, Gorenstein N, Kish CM, Dudareva N: Regulation of
• circadian methyl benzoate emission in diurnally and nocturnally
emitting plants. Plant Cell 2001, 13:2333-2347.
The study clearly shows that the total amount of substrate (i.e. benzoic acid)
in the cell regulates the rhythmic, environmentally independent emission of
methyl benzoate. The authors suggest that similar mechanisms are involved
in plants that emit methyl benzoate either diurnally (e.g. snapdragon) or
nocturnally (e.g. tobacco and petunia).
8. Vainstein A, Lewinshon E, Pichersky E, Weiss D: Floral fragrance.
New inroads to an old commodity. Plant Physiol 2001,
127:1383-1389.
9. Halitschke R, Kessler A, Kahl J, Lorenz A, Baldwin IT:
Ecophysiological comparison of direct and indirect defenses in
Nicotiana attenuata. Oecologia 2000, 124:408-417.
10. Schmelz EA, Alborn HT, Tumlinson JH: The influence of intact-plant
• and excised-leaf bioassay designs on volicitin- and jasmonic
acid-induced sesquiterpene volatile release in Zea mays. Planta
2001, 214:171-179.
This study demonstrates that experimental protocols (i.e. the use of explants,
time of elicitation, etc.) can dramatically influence VOC emission.
11. De Moraes CM, Mescher MC, Tumlinson JH: Caterpillar-induced
nocturnal plant volatiles repel nonspecific females. Nature 2001,
410:577-580.
12. Kunert M, Koch T, Boland W: Ultrafast sampling and gas
•• chromatographic analysis of plant volatiles. J Sep Sci 2002,
in press.
Analytical sensitivity has limited the temporal resolution of analyses of VOC
releases. This study uses a new method (zNose™) that solves this problem.
13. Ozawa R, Arimura G, Takabayashi J, Shimoda T, Nishioka T:
Involvement of jasmonate- and salicylate-related signaling
pathways for the production of specific herbivore-induced
volatiles in plants. Plant Cell Physiol 2000, 41:391-398.
14. Kessler A, Baldwin IT: Defensive function of herbivore-induced
plant volatile emissions in nature. Science 2001, 291:2141-2144.
15. Halitschke R, Schittko U, Pohnert G, Boland W, Baldwin IT:
Molecular interactions between the specialist herbivore
Manduca sexta (Lepidoptera, Sphingidae) and its natural host
Nicotiana attenuata. III. Fatty acid–amino acid conjugates in
herbivore oral secretions are necessary and sufficient for
herbivore-specific plant responses. Plant Physiol 2001,
125:711-717.
16. Spiteller D, Pohnert G, Boland W: Absolute configuration of
volicitin, an elicitor of plant volatile biosynthesis from
lepidopteran larvae. Tetrahedron Lett 2001, 42:1483-1485.
17. Van Poecke RMP, Posthumus MA, Dicke M: Herbivore-induced
• volatile production by Arabidopsis thaliana leads to attraction of
the parasitoid Cotesia rubecula: chemical, behavioral, and
gene-expression analysis. J Chem Ecol 2001, 27:1911-1928.
The first study to demonstrate that Arabidopsis releases HI-VOCs that
attract parasitoids to feeding larvae.
18. Lambrix V, Reichelt M, Mitchell-Olds T, Kliebenstein DJ, Gershenson J:
The Arabidopsis epithiospecifier protein promotes the hydrolysis
of glucosinolates to nitriles and influences Trichoplusia ni
herbivory. Plant Cell 2001, 13:2793-2807.
19. Dicke M: Chemical ecology of host-plant selection by herbivorous
arthropods: a multitrophic perspective. Biochem Syst Ecol 2000,
28:601-617.
20. Arimura G, Ozawa R, Horiuchi J, Nishioka T, Takabayashi J:
Plant–plant interactions mediated by volatiles emitted from plants
infested by spider mites. Biochem Syst Ecol 2001, 29:1049-1061.
Volatile signaling in plant–plant–herbivore interactions Baldwin et al. 3
21. Arimura G, Ozawa R, Shimoda T, Nishioka T, Koch T, Kühnemann F,
•• Boland W, Takabayashi J: Herbivory-induced volatiles induce the
emission of ethylene in neighboring lima bean plants. Plant J
2002, 29:87-98.
This work provides the first evidence for the elicitation of transcriptional
changes in intact plants that are exposed to HI-VOCs.
22. Bate NJ, Rothstein SJ: C-6-volatiles derived from the lipoxygenase
pathway induce a subset of defense-related genes. Plant J 1998,
16:561-569.
23. Sivasankar S, Sheldrick B, Rothstein SJ: Expression of allene oxide
synthase determines defense gene activation in tomato. Plant
Physiol 2000, 122:1335-1342.
24. Karban R, Baldwin IT, Baxter KJ, Laue G, Felton GW:
Communication between plants: induced resistance in wild
tobacco plants following clipping of neighboring sagebrush.
Oecologia 2000, 125:66-71.
25. Karban R, Baxter KJ: Induced resistance in wild tobacco with
clipped sagebrush neighbors: the role of herbivore behavior.
J Insect Behav 2001, 14:147-156.
26. Karban R: Communication between sagebrush and wild tobacco
in the field. Biochem Syst Ecol 2001, 29:995-1005.
27. Fall R: Volatile organic compounds emitted after leaf wounding:
on-line analysis by proton-transfer-reaction mass spectrometry.
J Geophys Res 1999, 104:15963-15974.
28. Dicke M, Dijkman H: Within-plant circulation of systemic elicitor of
induced defence and release from roots of elicitor that affects
plants. Biochem Syst Ecol 2001, 29:1075-1087.
4 Biotic interactions
