Plants have evolved various constitutive and induced defense strategies against herbivores. Constitutive defenses include physical defenses like thorns and trichomes as well as chemical defenses like alkaloids, terpenes, and glucosinolates. Induced defenses are activated in response to herbivore attack and can be direct like proteinase inhibitors or indirect like releasing volatile signals to attract natural enemies of herbivores. Key phytohormones like jasmonic acid and salicylic acid mediate induced defense responses by activating genes related to toxins, repellents, and signal production. Understanding these plant defense mechanisms could help develop new ecofriendly pest management approaches for agriculture.

1. SURENDRA SINGH SHEKHAWAT
GI 9048
2015PPM 04

2. Introduction
 As autotrophs, plants require effective
defense mechanisms to survive in the world
of heterotrophs.
 A range of adaptations evolved
by plants which improve their survival and
reproduction by reducing the impact
of herbivores.
 Plans face challenges from various
herbivores and microbes at all stage and in
all organs.

3. DEFENCE STRATEGIES OF PLANTS AGAINST INSECTS
• Tolerance
• Avoidance
• Defence (Physical or
chemical)
• Association with other
spp.
Plants have 4
basic strategies
by which they
reduce the
impact of
herbivory-

4.  Constitutive or Direct
• Physical characters
• Biochemicals
 Induced
• Induced direct
• Induced indirect
Defence

6. Constitutive defence
 Always present in the plant.
 Multitude of structures and chemicals incorporated into their tissues.
 They can deter, repel and interfere with the development and reproduction
of herbivores.
 It may depend on the texture and composition of the plant surface.
 Presence of anatomical structures such as :-
Veins, thorns, silica, trichomes and resin ducts etc.
 Absence of nutrients.
 Presence of hormones that affect the development.

7. Cont...
 Secondary metabolites.
 Alkaloids to terpenes, phenolics, steroidal compounds, cyanogenic glucosinolates and
mustard oil.

8. Plants physiological characters in defence

9. Cont…

10. Induced defence
 Involves the production of chemicals or physiological structures, or removal of
nutrients essential to herbivore, in response to attack.
 May take several forms :-
(a) Nutrient removal
(b) Cell lignification
(c) Controlled chemical biosynthesis
(d) Uncontrolled chemical biosynthesis

11. Biochemistry of induced resistance
 Environmental conditions can increase, or decrease, the level of a plant`s intrinsic
chemical defence.
 Insect feeding on plant tissues could elicit defensive metabolism in the plant (Akazawa
et al., 1960).
1. Highly volatiles, which elicit negative insect orientation with regard to source, and may
negatively affect the insect before physical contact.
2. Less volatiles, which prevent or reduce feeding if the insect contacts the plant.
3. Chemicals which are non volatile variously affects the herbivore`s nutritional and
developmental physiology after feeding.

12. INDUCED DIRECT DEFENCE
 Affects the attackers directly through toxins, repellents, digestibility reducers,
spines and thorns(Koiwa et al.,1997)
 Defence induced by mechanical wounding in a plant results in induction of
proteinase inhibitors in tomato and potato, leading to a reduction of the
digestibility of plant tissues(Green and Ryan,1971).
 Wounding by Manduca sexta, tobacco hornworm caterpillar induce the
production of nicotine(Green and Ryan, 1971).
 Caterpillars, Helicoverpa zea and leafminer flies, Liriomyza trifoli induce
different combinations of polyphenol oxidase, peroxidase, lipoxygenase and
proteinase inhibitors in tomato plants(Stout, et al., 1994).

13. Proteinase inhibitors
 Inhibit proteinase activities of herbivores.
 PIs bind to the digestive enzymes in the insect gut and inhibit their activity.
 Thereby reduce protein digestion resulting in the shortage of amino acids.
 Slow development and starvation of the insects.
PI in plants species Against Insect species
Sorghum bicolor Schizaphis graminum
Tomato Manduca sexta
Helicoverpa armigera
Solanum nigrum Spodoptera littoralis
Spodoptera exigua
Transgenic Arabidopsis/ tobacco Spodoptera littoralis

14. Herbivore-induced plant volatiles
 Attracts the natural enemies of the herbivores
 Act as feeding and/or oviposition deterrent.
 HIPVs are the lipophilic compounds with higher vapor pressure which are released
from the leaves, flowers, and fruits into the atmosphere, and into the soil from the roots
by plants in response herbivore attack (Arimura G, Matsui K, Takabayashi J,2009).
 Plant volatiles such as methyl salicylates and the C16- Homoterpene 4, 8, 12-
Trimethyl-1, 3(E), 7(E), 11- Tridecatetraene [(E, E)-TMTT] have been found to attract
the predatory mites (de Boer JG et al., 2004).

15. Cont…
 Most frequent component of the HIPVs is methyl salicylate (MeSA) reported in the
headspace of many insect-infested plants including lima bean, and Arabidopsis (Chen F. et
al., 2003).
 MeSA is a component of many leaf and floral blends and MeSA baited sticky cards attract
many insect predators including the big- eyed bug, Geocoris pallens Stal., ladybird
beetle, Stethorus punctum picipes (Casey), green lacewing Chrysopa nigricornis and other
natural enemies (James DG, 2003)

16. Cont…
 Inhibition of oviposition of cabbage moths Mamestra brassicae by MeSA released during
infestation (Ulland et al., 1960).
 MeBA (methylate benzoic acid) has also been detected from insect-infested plants.
 S. frugiperda infestation in rice induces emission of about 30 volatiles, including MeSA and
MeBA, which are highly attractant to the natural enemies such as, Cotesia marginiventris.
 HIPVs has the potential of attracting crop pests. For example, Colorado potato
beetles, Leptinotarsa decemlineata (Say) is attracted to a blend of volatiles consisting of cis-
3-hexenyl acetate, linalool, and MeSA

17. Cont…
 In black mustard, trichomes density and glucosinolates levels were elevated after feeding
by Pieris rapae (Traw MB, 2002).
 Trichome density increased in plant, Alnus incana as a result of damage by beetles (Baur
R; Binder S; Benz G., 1991).
 Increase in trichome density in response to herbivory between 25 to 100%.
 Positive correlation has been observed between natural enemies abundance and trichome
density.
 Trichome exudates also serve as extra floral nectar (EFN) for scelonid egg parasitoid, of
squash bugs, Gryon pennsylvanicum (Olson DL; Nechols JR., 1995).

18. Seven-spotted Lady Beetle feeding at
an extrafloral nectary on Wild Senna
(Senna hebecarpa)

20. Induced indirect defence
 It promotes the effectiveness of natural enemies of herbivores through the provision of
shelter, food and the production of signals that enable carnivorous arthropods to locate
the herbivores (Kessler and Baldwin, 1997).
 Emission of volatiles after damage that attract carnivorous enemies of herbivores.
 Eg. Parasitoid Aphidius ervi, its aphid host Acyrthosiphon pisum and
 Its host plant Vicia faba- shown that feeding by aphid alters the composition of
volatiles released by the plant (Wadhams et al., 1999).

21. 1. A volatile signal
is released as
the caterpillar
eats a leaf.
2. Female wasp is attracted
by the volatile signal,
finds caterpillar, and lays
eggs.
3. Wasp larvae
feed on the
caterpillar and
then emerge.
Volatile signal
Larvae
4.Larvae continue to feed on
the caterpillar after it dies,
but not the plant. The larvae
then spin cocoons to pupate.

22. Applications
 Soyabean var. “Davis” attractive to Mexican bean beetle, Epilachna varivestis, but
larvae emerged from eggs can`t survive further or died.
 Feeding on foliage of this var. “Davis” by attracted adult female caused a premature
termination in their egg laying.
 Volatiles of G. max breeding introduction PI 227687 strongly repellent to adult of this
beetle (Burden and Norris, 1994).
 Microplitis demolitor attracted by the volatile 3-octanone in G. max, Soybean
looper, Pseudoplusia includens and more arrested by quaiacol, found in its host`s frass.
 Quaiacol was only detected in Pseudoplusia includens frass when this herbivore fed on
this plant and was never detected in volatiles from intact host plants (Ramachandran et
al., 1991)

23. Role of phytohormones in induced resistance
 Many signal transduction pathways mediated by a network of phytohormones.
 Regulating plant growth, development, and defense mechanisms.
 A number of plant hormones have been implicated in intra– and inter- plant
communication in plants, damaged by herbivores.
 Most of the plant defense responses against insects are activated by signal-transduction
pathways mediated by JA, SA, and ethylene.
 Specific sets of defense related genes are activated by these pathways upon wounding
or by insect feeding.
 These hormones may act individually, synergistically or antagonistically, depending
upon the attacker.

24. Jasmonic acid
 Naturally occurring, non-toxic compound.
 Function of JAs in defence proposed by Farmer & Ryan,1992.
 Wounding caused release of linolenic acid(LA), precursor of JAs from membrane lipids.
 Important role in plant’s defence in response to wounding.
 JA plays a role in insect and disease resistance: many genes during plant defence are
induced by JA, and ethylene may act together in defence responses.

26. Applications
 JA seed treatment stimulates the natural anti-pest defenses of the plants that germinate
from the treated seeds, without harming plant growth.
 Exogenous application of JA on rice plants elicits the production of proteinase inhibitors,
phytoalexins, PRs, and salt-induced proteins (Tamogamia et al., 1997; Rakwal and
Komatsu, 2000; Rakwal et al., 2001; Kim et al., 2003) and it may increase the emission
of volatiles.
 JA application to rice plants on the host-searching behavior of the rice brown
planthopper Nilaparvata lugens and its mymarid egg parasitoid Anagrus nilaparvatae.

27. Cont..
 Exogenous application of MeJA increases the release of volatile organic
compounds (Halitschke et al., 2000), which enhances the mortality rates of
the herbivores by attracting the natural enemies of herbivores (Kessler and
Baldwin, 2001).
 Appl. of jasmonates affects trichome formation, glucosinolate biosynthesis
and terpenoid emission.
 JA-treatment reduces larval mass of S. exigua and induces the attraction
of Cotesia rubecula parasitoids.

28.  Complexity of volatile blend and large numbers of different herbivore enemies
suggests that many of these defenses remain to be characterized.
 Application of these defense strategies in an agricultural setting might offer new
ecofriendly approaches to increase insect resistance in crops.
CONCLUSION