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Induced responses in clover to an herbaceous mite.

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Ridsdill-Smith et al.
Archives of Insect Biochemistry and Physiology 51:170–181 (2002)
Induced Responses in Clover to an Herbaceous Mite
James Ridsdill-Smith,* Emilio Ghisalberti, and Yong Jiang
Halotydeus destructor feeding on subterranean clover cotyledons can cause severe damage. The mites live on the soil surface
and move up onto plants to feed. Foraging behaviour consists of palpating, probing, and feeding with frequent transitions
between them. Sustained feeding is made up of a series of short (1–2 min) feeds separated by periods of palpating. The
mites tend to feed in aggregations, and are attracted to cotyledons damaged by other mites feeding or by mechanical
damage. Mites can distinguish between resistant and susceptible cotyledons within 30 min and resistance is antixenotic due to
deterrence. Study of the mechanisms shows this to be induced plant resistance. Several green leaf volatiles are involved in the
plant/mite interaction. After feeding commences, 2-E-hexenal is released that at low concentrations is attractive to mites,
perhaps causing the feeding aggregations. The wound-induced C8 compound, 1-octen-3-one, plays a significant role in the
deterrence of cotyledons of resistant subterranean clover varieties to H. destructor. Damaged cotyledons of resistant varieties
produce more 1-octen-3-one that those of susceptible varieties. Screening for resistance has identified varieties from Italy
showing resistance. H. destructor does not occur in Europe. Production of damage-induced volatiles by the resistant plants
may have resulted from invasion by herbivores or pathogens, but not from coevolution with these mites. The responses of H.
destructor are probably an adaptation to these general plant defensive compounds. Arch. Insect Biochem. Physiol. 51:170–
181, 2002. © 2002 Wiley-Liss, Inc.
KEYWORDS: induced plant resistance; Halotydeus destructor; subterranean clover; 1-octen-3-one
INTRODUCTION
Redlegged earth mite, Halotydeus destructor
(Tucker) (Acari: Penthaleidae), was accidentally introduced to Australia in 1917 from South Africa
and spread through southern Australia by 1934
(Swan, 1934; Ridsdill-Smith, 1997; Qin, 1997).
Mostly it is found on the soil surface, but moves
up onto plants to feed (Gaull and Ridsdill-Smith,
1996). Ridsdill-Smith (1997) reviewed the biology
and control of this species. It is a truly polyphagous species with a very wide host plant range, but
is considered a particular pest of annual clover
plants in pastures, such as subterranean clover, Trifolium subterraneum (L.). Redlegged earth mite is
active for about 6 months of the year during the
cool moist winter months, spending the summer
as a diapausing egg (Ridsdill-Smith and Annells,
1997). Populations of 12,000 mites/m2 are common in pastures of southern Australia and cause
severe losses of productivity to the pastures. Mites
feed on all stages of clover in pastures, but the
greatest damage occurs in autumn when oversummering mites hatch at much the same times
as the annual clover germinates. At these times,
high mite populations can destroy clover seedlings.
Thus, a high priority for the clover- breeding program is to develop resistance in subterraneum clover cotyledons to protect the seedlings.
Screening methods to test for resistance in clover seedlings against H. destructor were developed
by Gillespie (1993). The screening is carried out
in the glasshouse with rows of different varieties
in wooden (or plastic) containers at a rate of about
seven varieties per container, and the mites are thus
given a choice. Feeding damage is assessed after 1
Centre for Legumes in Mediterranean Agriculture, University of Western Australia, Australia
Contract grant sponsor: Australian Wool Innovation.
*Correspondence to: Dr. James Ridsdill-Smith, CSIRO Entomology, Private Bag No 5, Wembley, WA 6913, Australia. E-mail: James.Ridsdill-Smith@csiro.au
Received 6 May 2002; Accepted 22 August 2002
© 2002 Wiley-Liss, Inc.
DOI: 10.1002/arch.10063
Published online in Wiley InterScience (www.interscience.wiley.com)
Archives of Insect Biochemistry and Physiology
Induced Resistance to Herbaceous Mite
and 2 weeks. Levels of resistance to cotyledons are
assessed using a visual scale of 1–10 where 1 is no
feeding damage, through to 5 where half the upper cotyledon surface has feeding damage (silvering patch) to 10 where the whole cotyledon is dead
or dying. Of about 7,500 subterranean clover introductions to Australia, 675 have been screened
with mites and 18 show seedling resistance (Ridsdill-Smith and Nichols, 1998). Thirteen of the resistant introductions were collected in Italy, of
which 10 were from Sicily. The redlegged earth mite
does not occur in Europe at all, and so this resistance is not the result of coevolution. It is likely
that this seedling resistance in subterranean clover
is due to general plant defense mechanisms that
protect the plants against other biotic stresses, and
that the mites have adapted to respond to them.
Over a number of years, different aspects of the
plant-insect interactions involved in T. subterraneum
cotyledon resistance to H. destructor have been investigated. A picture has developed of an induced
resistance mechanism involving the production of
plant volatiles that deter mites from feeding, but
that are not specific responses to mite feeding. The
term “induced resistance” is used in the present
context to refer to a rapid response by the plant to
biotic or abiotic challenges. In this study, we review the current state of knowledge of the mechanisms involved in the resistance, and consider how
this reflects on our understanding of plant defense
systems.
MITE FEEDING BEHAVIOUR
In pastures, most H. destructor are on or near
the soil surface with only 10% at any one time on
the upper canopy (Gaull and Ridsdill-Smith,
1996). The mites move up onto the canopy of the
plants where they feed on the upper adaxial surfaces of the leaves. In the annual pastures of
southwestern Australia, the mites show a preference for feeding on subterranean clover leaves.
While the proportion of mites is distributed approximately in proportion to the relative abundance of the subterranean clover in the pasture
(50%), the proportion feeding was higher on the
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171
clover (74%) (Gaull and Ridsdill-Smith, 1996).
This indicates that the mites are selecting their
hosts for feeding after searching on all the plants
in the pasture. A characteristic of the feeding mites
is that they are in aggregations of 5–6 mites. In
the field, an average of 88% of aggregated individuals are feeding, whereas only 23% of solitary
individuals are feeding (Gaull and Ridsdill-Smith,
1996). The mites feeding in aggregations leave
patches of feeding damage, and this provides an
easy way to visually assess the amount of feeding
from the size of the patch. The aggregations provide a benefit to the feeding mites, in terms of increased mean weight gain over a 2-h period (Gaull
and Ridsdill-Smith, 1997). Feeding mites require
applying mechanical physical force to penetrate the
upper epidermis of the plants (Jiang and RidsdillSmith, 1996b), and it is possible that the aggregations may assist mites in penetrating the epidermis.
When feeding, the mite braces its legs on the
epidermis by clamping the tarsal claws into the surface, leaving triangular dents (Ridsdill-Smith et al.,
1997). One hole 3 mm in diameter is made in each
cell with the movable digit of the chelicera, and
the mites suck out the cell contents with a pharyngeal pump. The side-walls of the cells collapse,
and the upper surface changes from convex to flat.
Air in the cells probably causes the silvery appearance. Feeding is restricted to the epidermal and subepidermal cells, which is where resistance factors
are likely to be found.
The repertoire of foraging behaviour has three
elements. Palpating is where the mites run on the
leaf surface continually tapping it with the first pair
of legs and palps. Movement is continual and directional. Probing is where there are regular stops,
continual changing of position, and momentarily
the mouthparts are applied to the plant surface.
Feeding is where the mite is stationary, with legs
braced and mouthparts applied to the leaf surface.
Also, occasionally the mite is stationary and immobile with the first pair of legs held out straight
in front of the mite. Host acceptance behaviour
has been observed using starved marked mites in
a group. During a 70-min observation period, mites
spend 66% of their time in aggregations, and 83%
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Ridsdill-Smith et al.
of that is feeding, while solo/pairs of mites only
spend 65% of time feeding (Gaull and RidsdillSmith, 1997).
RESPONSES TO SEEDLINGS OF SUSCEPTIBLE AND
RESISTANT VARIETIES
the two cotyledons divided by the total number of
mites on both cotyledons. The number of mites
on cotyledons is positively correlated with feeding
damage over 3 h (67% of variability), and damage
scores of cotyledons from 1–2 week screening
choice experiments are correlated with deterrence
on the same varieties in 3-h choice experiments
(93% of variability). The choice made by mites in
the 3-h experiments is relative. Mites show 74%
deterrence to resistant variety 1 compared to a susceptible control, and 45% to resistant variety 2
against the same control, but they also show 83%
deterrence to resistant variety 2 compared with resistant variety 1 (Fig. 1).
While the total time spent feeding by a starved
marked mite is significantly greater on the cotyledon of a susceptible than a resistant variety, the
mean duration of each separate feeding event is
similar on both at around 1.5–2 min (Gaull and
Ridsdill-Smith, 1997). The mites are continually
moving between probing and feeding. The transitions from probing to feeding and back on the resistant variety are 62% of those on the susceptible
variety. The mites have only rudimentary eyes and
chemical cues are likely to be important in host
plant acceptance.
When H. destructor are confined over 7 or 14
days to varieties of subterranean clover that are resistant and susceptible with a single variety per pot
in a non-choice situation, mites on resistant varieties produce 45% of the progeny of mites on susceptible varieties, and feeding damage on resistant
varieties is 45% of feeding damage on susceptible
varieties (Ridsdill-Smith, 1995). When mites are
given a choice of seedlings of a resistant and a susceptible variety in the same pot over 7 days, feeding damage on the resistant varieties is only 23%
of that on the susceptible variety. Mites also readily
discriminate between detached cotyledons of resistant and susceptible varieties in a 3-h choice tests
on soil in a Petri dish (Jiang and Ridsdill-Smith,
1996a). When mites are given a choice of two detached cotyledons of the same variety, similar numbers occur on each, but given a choice between a
resistant and a susceptible variety, the choice is
strong and occurs within 30 min. Thus, the mites
are distinguishing the resistant cotyledons by
antixenosis or deterrence. This deterrence is measured by the difference in numbers of mites on
Cotyledons damaged by H. destructor feeding are
more attractive to mites in choice trials than un-
Fig. 1. Mite preference in choice between subterranean
clover cotyledons over 3 h. Mites showed a clear choice
between resistant and susceptible varieties, but were also
able to distinguish between the two resistant varieties. Data
from Jiang and Ridsdill-Smith (1996a) and Wang et al.
(2001).
RESPONSES TO DAMAGED COTYLEDONS
Archives of Insect Biochemistry and Physiology
Induced Resistance to Herbaceous Mite
damaged cotyledons (Jiang and Ridsdill-Smith,
1996a). Individual marked mites spend twice as
much time feeding and twice as long in total activities on cotyledons previously damaged by mite
feeding in a choice situation than on a previously
undamaged cotyledon (Gaull and Ridsdill-Smith,
1997). When mechanical cuts (about 15) are made
with a razor blade in the upper surface of the cotyledons, the exposure of the cell contents also increased the attractiveness of the cotyledon relative
to an undamaged control (Jiang and Ridsdill-Smith,
1996a) (Fig. 2). However, while mechanical damage increases the attractiveness of cotyledons both
of a susceptible and a resistant variety, there are
still more mites on the damaged susceptible than
the damaged resistant cotyledon (Jiang and Ridsdill-Smith, 1996a).
Damaged cotyledons remain attractive to mites
for some time after the damage occurs when the
damage is caused by mite feeding or mechanically
(Jiang et al., 1997). For the mite feeding damage
treatment, 200 mites are released into pots containing 15 seedlings and allowed to feed for 18 h,
before being removed with a compressed air sucker.
For the mechanical damage treatment, a spiral
metal screw is pressed into the leaf in 10 places.
Damaged and undamaged plants are grown in
separate pots, and detached when ready to test at
1.5 h (feeding damage), 3 h (mechanical damage),
1, 3, and 7 days after damage had occurred. Mites
are given a choice of a damaged or an undamaged
173
cotyledon, and numbers of mites on each cotyledon counted over 3 h. Preference is expressed from
the average number per cotyledon. Mites prefer
damaged cotyledons for 4 days on the susceptible
variety, and 7 days on the resistant variety (Fig. 3).
Preference is greater with mite damaged than mechanically damaged cotyledons on most occasions,
especially for the resistant variety. The plants are
growing in soil and presumably continue to produce the factor that attracts mites from damaged
cotyledons.
EFFECTS OF VOLATILES FROM THE PLANTS
Since we were unable to establish that secondary metabolites were involved in the resistance
mechanism in subterranean clover cotyledons, we
tested the possible role of volatile compounds. The
method used successfully by other authors on mites
was a glass tube olfactometer (Dabrowski and
Rodriguez, 1971). However, for H. destructor the
response to volatile compounds depends on the
method of their presentation to the mites (Jiang
et al., 1996b). In a glass tube assay, two flasks (8
cm diameter) joined by a U-shaped glass tube (2
cm diameter) is used with screen cloths across the
tube at the joints with the flasks to limit mite
movement. Cotyledons from a susceptible subterranean clover variety are frozen in liquid nitrogen
and ground up in a mortar. Different weights of
the powder are placed in one of the flasks, and
Fig. 2. Mite preference in
choice between an undamaged control and a mechanically damaged subterranean
clover cotyledon over 3 h.
Mites show a preference
for mechanically damaged
cotyledons both with a susceptible and a resistant subterranean clover variety. Data
from Jiang and Ridsdill-Smith
(1996a).
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174
Ridsdill-Smith et al.
Fig. 3. Mite preference in choice between subterranean
clover cotyledons with mite feeding damage or mechanical damage over time for 7 days. Each bioassay lasted for
3 h with mites given a choice between freshly detached
cotyledons. Feeding damage was the result of feeding by
200 mites in pots containing 15 seedlings of resistant or
susceptible varieties. Mechanical damage was the result
of pressing the tip of a screw into the upper surface of a
cotyledon in 10 places. Plants were then left attached to
the growing seedling until testing. Mites detected damaged cotyledons for up to 7 days. Data from Jiang et al.
(1997).
the other flask contains no powder as a control.
Forty mites are released in the middle and counted
after 5 and 10 min. There is no consistent difference in mite numbers in relation to the amounts
of tissue, and the mites do not seem to be able to
detect the directions from which the volatiles are
coming (Fig. 4). In the second method, volatiles
are presented in a Parafilm membrane sachet with
1% glucose added as a phagostimulant. The relative concentrations of volatiles are presented at 4,
40, 100, and 400% of the in vivo concentration
found in a susceptible variety (labelled 1 to 4 in
Fig. 4). Mites respond strongly to concentrations
when presented in this system, being attracted at
concentrations below 100% in vivo and deterred
at 400% in vivo (Fig. 4). The membrane sachet
test provides the better test to detect concentration
effects of volatiles. It is clear that the method of
presentation is important for detecting effects of
volatile compounds on H. destructor behaviour
through olfactory and gustatory responses.
Volatile compounds produced from damaged
subterranean clover cotyledons are trapped onto
charcoal and identified using GC/MS. The major
compounds collected are similar from susceptible
and resistant varieties but differ in their relative
amounts. The major compounds were 1-octen-3one, 1-octen-3-ol, and 2-E-hexenal, making up
77% of the volatile fraction (Jiang et al., 1997;
Wang et al., 2001). Levels of these volatile compounds are determined in the headspace of artificially damaged cotyledons of six resistant and four
susceptible subterranean clover varieties and expressed relative to n-hexanol (Jiang et al., 1996a).
Feeding damage scores for cotyledons on seedlings
of these varieties are measured in separate screening
trials over 2 weeks in choice situations (Gillespie,
personal communication). Quadratic regressions of
feeding damage as a function of concentration (log
scale) are used to determine which compounds
best predict damage. The r2 is greater for 1-octen3-one (0.834) than for 1-octen-3-ol (0.307) or 2E-hexenal (0.321), suggesting a key role for 1octen-3-one in the cotyledon resistance.
Archives of Insect Biochemistry and Physiology
Induced Resistance to Herbaceous Mite
175
ROLE OF 1-OCTEN-3-ONE
Fig. 4. Mite preference for volatiles from a susceptible
subterranean clover variety and presented in two different
bioassays. In the glass tube bioassay, mites had a choice
between different quantities of ground-up cotyledon tissue (0.04, 0.10, 0.50, or 5 g) and air. The distribution of
mites was measured after 5 and 10 min. In the Parafilm
sachet, mites had a choice of volatiles collected from the
headspace of crushed cotyledons, and the control was
without the volatiles. The volatiles were dissolved in Tween
80 (a detergent) to which 1% glucose was added, and
made up to 4, 40, 100, or 400% of in vivo concentration.
Mite numbers on each membrane were counted over 3 h.
For each method, concentrations are expressed as 1 to 4,
although the concentrations the mites were exposed to
may not have been completely comparable. Mites responded better to the Parafilm sachet bioassay. Data from
Jiang et al. (1996b).
Fig. 5. Mite preference in 3-h bioassays for cotyledons
of a susceptible and a resistant subterranean clover variety grown in the glasshouse with and without shade. Shade
provided by placing a cardboard box over the seedlings
with four openings allowing dim light. Cotyledons were
December 2002
In a separate experiment, cotyledons of a susceptible variety and a resistant variety, grown under shading, are completely avoided by mites in
choice feeding tests (Jiang et al., 1996a) (Fig. 5).
Analysis of the volatile compounds show that the
levels of the C8 volatile compounds increase by
more than 50%, whereas 2-E-hexenal decreases by
9% in both shaded varieties. These results emphasize the significance of 1-octen-3-one (1) in resistance (Jiang et al., 1996a).
The influence of the volatile compounds 1octen-3-one and 2-E-hexenal on H. destructor was
investigated by placing droplets on the surface of
a cotyledon of a susceptible subterranean clover
variety (Jiang et al., 1997). The preference or deterrence of mites was measured in choice tests
using cotyledons with droplets containing the compound, a solubilizing agent and 1% glucose, and
the control with no compound. The treatments
consist of a range of concentrations, applied either as a single 5-ml droplet on the basal part of
the cotyledon (where mites do not usually feed),
or as five 0.5-ml droplets spread over the cotyledon surface. The volatile fraction from cotyledons
was tested at levels from 4 to 400% of in vivo,
used 12–13 days after sowing. Concentrations of octenone
were measured from shaded and unshaded seedlings.
Mites were adversely affected by the octenone in the
shaded cotyledons. Data from Jiang et al. (1996a).
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Ridsdill-Smith et al.
and the individual metabolites at levels equal to
or above the in vivo level. At low concentrations,
mites show a significant preference for both compounds, and at high concentrations a significant
deterrence (Fig. 6). Mites are attracted to cotyledons with five scattered droplets of 2-E-hexenal
at concentrations 50–1,000 ppm, while with 1octen-3-one the cotyledon with droplets is attractive at 1 ppm and deterrent at 10,000 ppm. In
contrast, with a single larger droplet there is a
much stronger concentration-dependent effect
(Fig. 6). 2-E-hexenal is attractive at 10–100 ppm
and deterrent at 10,000 ppm, while 1-octen-3-one
is not attractive, but deters mites at 100 ppm and
all mites are killed within 3 h at 10,000 ppm.
From these results, we conclude that 2-E-hexenal
is probably the factor that attracts mites to damaged cotyledons, including those on which mites
are feeding, and 1-octen-3-one is the volatile compound that is a primary cause of the antixenotic
deterrence of mites from cotyledons of resistant
subterranean clover varieties. A contribution of the
other minor volatile compounds to resistance cannot be ruled out.
Fig. 6. Mite preferences in 3-h bioassays for cotyledons
of susceptible subterranean clover variety droplets containing different concentrations of 2-E- hexenal or 1-octen-3one. There was one 5-ml droplet per cotyledon or five
0.05-ml droplets, with controls of cotyledons with droplets without the compounds. Preference decreased with
INDUCED RESISTANCE: DISCUSSION AND CONCLUSIONS
H. destructor spend 90% of their time on or near
the soil. They move up to feed in the upper canopy
of pastures, on the dorsal (adaxial) surface of
leaves. Most feeding occurs in aggregations, and
mites in such groups benefit from greater weight
gain. The mites are polyphagous with a very high
range of higher plants on which they can feed
(Ridsdill-Smith, 1997). These range from species
on which populations increase rapidly, to plants
where mites cause damage but survival is low and
few progeny are produced. In addition, they are
able to maintain populations feeding on lower
plants at the soil surface (McDonald et al., 1995).
Mite foraging behaviour is a simple sequence from
palpating to probing to feeding, but sustained feeding is made up of a series of short (1–2 min) bouts
of feeding followed by a return to probing. In this
way, the mite is continually sampling its environment, and is able to detect rapidly between plants
for feeding. This ability to choose also provides
for us a useful tool for studying the mechanisms
of resistance, but also means that mite choices are
increasing concentration, mites being attracted at low concentrations and deterred at high concentrations. Octenone
was more deterrent than hexenal, and concentration-dependent responses were greater with the larger droplet.
Data from Jiang et al. (1997).
Archives of Insect Biochemistry and Physiology
Induced Resistance to Herbaceous Mite
made using relative differences. For example, the
mites do select between two resistant subterranean
clover varieties.
In subterranean clover cotyledons, the green leaf
compounds identified as affecting H. destructor, act
as attractants at low concentrations and deterrents
at high concentrations. The mites respond in a
dose-dependent way when the volatiles are presented in a Parafilm membrane with glucose as a
feeding stimulant. This mimics the way that
volatiles would be affecting mites on a plant. The
response is initiated on plants after the initial probing and feeding, and thus is induced. Two volatile
compounds, 2-E-hexenal and 1-octen-3-one, are
mainly involved. It is the quantity produced that
affects mite behaviour. Thus 2-E-hexenal in general seems to be acting to attract mites, which may
help explain how feeding aggregations are formed.
However, 1-octen-3-one seems to be the cause of
deterrence. Mites are more attracted to damaged
than undamaged cotyledons of a resistant variety.
Presumably, the concentration of volatiles decreases
with distance from the point of feeding puncture,
and may thus attract mites from a distance (on
the cotyledon), but when they arrive at the feeding site concentrations of 1-octen-3-one may be
high enough to cause deterrence. The induced responses occur whether the damage is from feeding mites or artificial/mechanical and so are not
specific to the mites. Indeed, natural resistance is
found in subterranean clover varieties collected in
Italy where these mites do not occur, and so is not
coevolved.
It has been shown that fatty acid oxidation
products from the lipoxygenase pathway can play
an important part in plant defenses against attack
of herbivorous insects or animals, fungal or bacterial pathogens. Recent studies indicate that the response of plants to different types of wounding
involve common responses as well as specific ones
determined by the cause of the damage (Pickett
and Poppy, 2001). Considerable interest has been
shown in the observations that the production of
volatile compounds at the wound site increases
significantly. The volatiles produced in general
wound responses arise from the catalytic activity
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177
of lypoxygenases (LOX), a group of enzymes believed to be present in all green plants (Gardner,
1991). These enzymes operate, principally, on the
two important fatty acids linoleic and linolenic
acid and produce hydroperoxides that are degraded by hydroperoxy lyases (HPL) to a suite of
C6-aldehyde and alcohol compounds (Hatanaka,
1993; Mau et al., 1994) (Fig. 7). These compounds
appear to possess signalling properties. They are
released in sufficient quantities to be detected by
animals, can trigger formation of phytoalexins,
and reduce insect feeding rates. Importantly, they
have been shown to induce a subset of defenserelated genes such as those involved in the phenylpropanoid pathway and the LOX pathway (Bate
and Rothstein, 1998).
In the response of T. subterraneum to leaf
wounding, the formation of 2-E-hexenal is expected. However, the production of 1-octen-3-one
and 1-octen-3-ol is unusual; only some species of
fungi are known to produce them in significant
quantities (Mau et al., 1994). Moreover, 2-Ehexenal is known to be produced from linolenic
acid but 1-octen-3-one and 1-octen-3-ol arise from
linoleic acid (only the R-enantiomer of 1-octen-3ol is shown in Figure 8, but the formation of the
S-enantiomer is not excluded). The biosynthesis of
1-octen-3-ol from linoleic acid in the mushroom
Agaricus bisporus has been studied (Mau et al., 1994;
Gardner, 1991). It was found that the 10-hydroperoxy derivative was the product of lipoxygenase
action and the precursor of 1-octen-3-ol and 1octen-3-one (Fig. 8). The observation that resistant
varieties of T. subterraneum produced more 1-octen3-one than susceptible varieties, although not
proven, suggests that this difference reflects different amounts of the precursor fatty acids in the cotyledons. In the experiment described here, where
plants grown in shade deter mites, it is the increase
in 1-octen-3-one that causes the deterrence. The
greater production of the C8-volatiles in dark-grown
leaves is a consequence of the higher levels of the
precursor linoleic acid produced under this condition (Harwood, 1980).
The C6-volatiles induce some defense genes,
but at a lower level and a narrower range than
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Ridsdill-Smith et al.
Fig. 7. Basic reactions of the lipoxygenase pathway for the formation of some C6-volatile compounds from linoleic acid (A) and linolenic acid (B).
those induced by jasmonic acid and methyl
jasmonate, which are also LOX-derived metabolites (Bate and Rothstein, 1998). In contrast, the
gene-inducing ability of the C8-compounds produced by T. subterraneum has not been studied.
The only attempt to determine if C8 compounds
induced defense-related genes was a test involving oct-2-en-1-ol, which was found to have no
effect on the induction of LOX mRNA (Bate and
Rothstein, 1998).
Spider mites, Tetranychus species, avoid cotton
seedlings on which co-specifics have fed (Karban
and Carey, 1984; Harrison and Karban, 1986). A
range of compounds is produced when spider
mites feed on lima bean and cucumber leaves and
the blend of compounds varies between the plants
(Dicke et al., 1990). Of these compounds, terpenes and a phenol are attractive to predatory mites,
and Dicke et al. (1990) argue that they are used
by predators as cues to find their herbivorous prey.
These compounds are produced by spider mite
feeding but not by artificial damage, in contrast to
the situation with H. destructor. Terpenoids do not
appear to be present in detectable amounts in damArchives of Insect Biochemistry and Physiology
Induced Resistance to Herbaceous Mite
Fig. 8.
Steps of the lipoxygenase pathway for the formation of C8-volatile compounds from linoleic acid.
aged cotyledons of subterranean clover (Jiang et
al., 1996a).
The resistance mechanisms described here are one
of several that affect H. destructor feeding on different plants. In trifoliate leaves of subterranean clover,
resistance is due to isoflavonoids (Wang et al., 1998),
in Trifolium glanduliferum, the volatile compounds
coumarin and b-ionone (Wang et al., 1999), while
in yellow lupins the resistance mechanism is alkaloids (Wang et al., 2000). However, the mechanism
in subterranean clover cotyledons is the only one that
appears to be induced by mite feeding.
ACKNOWLEDGMENTS
The authors are grateful to Jenny Reidy Crofts
who carried out most of the bioassays, Tanya Pican
and Celia Pavri who helped with many of the experiments, and Dennis Gillespie and Phil Nichols
for advice on screening for resistance and supplying the seed. Dr. Shao Fang Wang provided much
valuable discussion on resistance mechanisms. The
work has been supported by Australian wool growers through the Australian Wool Innovation.
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