Insects that feed on toxic plants (adaptation)

Prepared by: Hael Saeed Ahmed Raweh (436107622)

Ph.D. 2nd SemesterCollege of Food and Agriculture SciencesKing Saud University, Riyadh, KSA

Mechanisms of Plant Defense Against Insect Herbivores

Phenolics

phenolic

glycosides

bound

phenolics

condensed

tannins

lignin

Terpenes

Monoterpenes

diterpene

acids

Secondary Metabolites

Defense-related

proteins

perxoidases

polyphenol oxidase

hydrolysable

tannins

N-containing

Alkaloids

Mustard oils

Physical

Defenses

Spines, thorns

cutins

waxes

suberins

• Plants evolve defense chemicals in response to attacks by

insects

• While insects counter-evolve detoxification systems.

Resistance Mechanisms In Insects

Survival insects employ a variety of resistance mechanisms, including:

• Detoxification, target site modifications, and nerve insensitivity.

• Detoxification occurs when toxins are modified by reduction, oxidation, and conjugation reactions,

resulting in the excretion of modified toxin molecules.

• The metabolic defense system in insects generally involves three main groups of enzymes acting in

three phases against a number of insecticides and phytochemicals.

Resistance Mechanisms In Insects

• Phase first involves reduction of toxicity of substrates by cytochrome P450 monooxygenases (CYPs).

• In second phase hydrophobic toxic compounds are converted to hydrophilic products by action of the

glutathione S-transferases (GSTs), uridinediphosphate (UDP)−glucuronosyltransferases (UGTs), and

carboxylesterases (COEs) facilitating their excretion.

• Finally, phase third includes ATP binding cassette (ABC) and major membrane transporters that actively

pump conjugated xenobiotics out of the cell (Dawkar et al., 2013).

Insect adaptation strategies

• Behavioral adaptation

• Detoxification of plant chemical defense compounds

• Sequestration

Behavioral adaptation

Bursera squirts toxic

compounds (terpenes)

when the vein is cut

Blephara beetles cut the

veins before they start

feeding

Detoxification of Plant Chemical Defense Compounds

• General detoxification enzymes: P450's,

• Specific detoxification enzymes: example (Nitrile Specifier Protein) NSP.

What happens when a Pieris rapae larvae feeds on a brassicales plant?

By NSP to convert it to a non-toxic form.

Senecio jacobaea

contain

Pyrrolizidine

Alkaloids (PA's)

PA's sequestered

by larvae of

tigermoths

Tyria jacobaea

(Arctiidae)

• Sequestration

PA's transferred

to adults

- Males produce aphrodisiacs from PA's

- Males transfer PA's to females by mating

(nuptial gift)

- Females are better protected against predators

- Females transfer PA's to eggs for protection

Examples of adaptive:2

• Scientists at the Max Planck Institute for Chemical Ecology in Jena, Germany, now found the opposite

mechanism: a defensive compound of the wild tobacco species Nicotiana attenuata which is toxic with sugar

molecules bound to it and a glycosidase in the gut of the tobacco hornworm Manduca sexta which removes

one sugar from this toxin to convert it to a non-toxic form.

• This is the first time that the role of deglycosylation in detoxification as an insect counter-adaptation.

• Scientists showed that removing a sugar molecule from a plant’s defensive compound can also result in

detoxification. (Poreddy et al., 2015).

Lyc4 RGHGL

Lyc4

Another way to avoid contact with the toxin is by suppressing plant defences.

• The induction of plant defences by chewing insects is well documented, and usually involves salivary elicitors.

• Oral secretions of several caterpillar species have recently been shown to suppress or reduce host-plant

defences.

• The principal component of the tobacco earworm Helicoverpa zea (Lepidoptera) saliva is a glucose oxidase

that decreases the level of nicotine induced in leaves from tobacco Nicotiana tabacum (Musser, R.O. et al.,

2002).

Helicoverpa zea that decreases the level of nicotine induced in leaves from

tobacco Nicotiana tabacumglucose oxidase

• The only well documented example is from two unrelated lineages, the specialist monarch butterfly and

two leaf beetle (Chrysochus sp) species, which are all resistant to ouabain, a toxic cardenolide found in

their milkweed host plants.

• In this case, a single amino-acid substitution in the target site of ouabain (the Na+, K+-ATPase) is

responsible for resistance (Labeyrie & Dobler., 2004).

Cardenolides Nervous system (depressing activity);

Na+, K+-ATPase (specific inhibitor)Target-site mutation

• Danaus plexippus (Lepidoptera) Late-instar larvae deactivate the defence of milkweeds by cutting veins

before feeding.

Cardenolides Nervous system (depressing activity);

Na+, K+-ATPase (specific inhibitor)Canal trenching behaviour

• In many insects, Schistocerca americana (Orthoptera); Hypera brunneipennis

(Coleoptera) Zygaena sp. (Lepidoptera); Clossiana euphrosyne (Lepidoptera)

Heliconius sara (Lepidoptera) a large proportion of the accumulated toxic plant

compounds can be excreted, or lost with exuvia during the moult (Zagrobelny,

M. et al., 2004).

Cyanogenic glycosides Electron transport (inhibition

of mitochondrial cytochrome

oxidase)

Ingestion avoidance;sequestration anddetoxification

• In the aphid Myzus persicae, the overproduction of the glutathione S-transferases (GSTs)

is probably responsible for the adaptation of the insect to glucosinolates and

isothiocyanates contained in its Brassicaceae host plants (Francis, F. et al., 2005).

Glucosinolates Respiration (inhibition) Detoxification by GSTs

• Another interesting example is the response of the diamondback moth Plutella xylostella (Lepidoptera) to the

defence system of its crucifer host plants, the ‘mustard oil bomb’.

• This system relies on the co-secretion of glucosinolates and a specific enzyme (myrosinase), each stored in

separate cell compartments.

• Following tissue damage, glucosinolates are hydrolysed by myrosinase into highly toxic products, such as

isothiocyanate.

• The diamondback moth has developed an original adaptive strategy based on the modification of ingested plant

glucosinolates by a sulfatase gut enzyme, preventing their hydrolysis by plant myrosinase, thus disarming the

‘mustard oil bomb’ (Ratzka, A. et al.,2002).

Glucosinolates Respiration (inhibition) Detoxification by a glucosinolate sulfatase

Formation of nitriles

instead of isothiocyanate

• The cabbage white butterfly Pieris rapae (Lepidoptera) has developed a different adaptive mechanism by

redirecting toxic isothiocyanate formation toward non-toxic nitrile formation with a specific gut protein

(Wittstock, U. et al., 2004).

• Cyanide in P. rapae is detoxified by β-cyanoalanine synthase and rhodanese activity into nontoxic β-

cyanoalanine and rhodanide (Stauber et al., 2012).

• *Another example is the adaptation of cactophilic Drosophila species from the

Sonoran desert to the specific allelochemicals contained in their host plants, where

several unrelated P450-encoding genes are induced by different isoquinoline

alkaloids, suggesting the exploitation of evolutionarily distant P450 enzymes in the

adaptation of the various Drosophila species to their different host cacti (Fogleman,

J.C. 2000).

Detoxification by P450s

• When the polyphagous arctiid moth Estigmene acrea and Tyria jacobaeae (Lepidoptera) feeds on

species of Asteraceae, it not only sequesters pyrrolizidine alkaloids (PAs) contained in those plants,

but also detoxifies them by N-oxidation catalysed by a specific flavin-dependent monooxygenase

(Hartmann, T. et al., 2005).

Detoxification by N-oxidation

and sequestration

• Larvae of the cowpea bruchid Callosobruchus maculatus (Coleoptera) fed on a diet containing the soybean

cysteine protease inhibitor soyacystatin N (scN) activate an array of counterdefence genes to adapt to this

toxin.

• Using microarray technology, Monn et al. 2004, suggested that the overexpression of cathepsin-like cysteine

proteases has a key role in the adaptation of this insect to ingested scN by saturating the protease inhibitor

molecules.

Overexpression of insensitive proteaseProtease inhibitors Digestive system (inhibition of protease)

• The glucose oxidase enzyme in the saliva of the beet armyworm Spodoptera exigua decreases the

transcript level of key regulatory genes involved in the early steps of the defence pathways of the

Medicago truncatula plant (Bede, J.C. et al., 2006).

Terpenoids

Nervous system (inhibition of acetylcholine

esterase); feeding (deterrent owing to physical

barrier and bitterness); growth and development

inhibitor (pheromone analog)

Repression of genes involved

in biosynthetic pathways

Ostrinia furnacalis maize contain cyclic hydroxamic

acids (cHx) Feeding

UDP-glucosyltransferases (UGT)

activity in the midgut and excretion of

non-toxic glucosides

Enzyme

• A number of gramineous plants such as maize contain cyclic hydroxamic acids (cHx) that are toxic to many

herbivores such as Ostrinia furnacalis (Kojiima et al., 2010).

.

• Burnet moth larvae are able to sequester cyanogenic glucosides from their food plant as well as to carry

out de novo biosynthesis.

• Here we show that three genes (CYP405A2, CYP332A3 and UGT33A1) encode the entire biosynthetic

pathway of cyanogenic glucosides in the Burnet moth Zygaena filipendulae (Zagrobelny &Moller,. 2011).

Zygaena filipendulae Detoxification

Caterpillar defence droplets: poisoning the predators

by (β-cyanoalanine synthase) Sequestration

• The generalists cabbage moth Mamestra brassicae, cotton bollworm Helicoverpa armigera (both Lepidoptera,Noctuidae), fall armyworm, Egyptian armyworm and the cabbage looper, which feed on glucosinolate-containing plants, conjugate toxic isothiocyanates with glutathione by GST activity in their gut and subsequentlyexcrete non-toxic products with the frass (Schramm et al., 2012).

Mamestra brassicae and Helicoverpa armigera (GST) Glutathione S-transferase activity, probably in the gut and excrete non-toxic products with the frass

• Insect herbivores that are adapted to low to medium levels of defence compounds can regularly switch to other host

plants to avoid ingestion of lethal doses by dietary mixing a feeding behaviour mainly exhibited by generalists.

• Hostplant switching may dilute excessive detrimental effects of any single plant defence compound.

• For example, larvae of the lepidopteran generalists yellow woolly bear Spilosoma virginica and tiger moth Grammia

incorrupta (both Lepidoptera, Arctiidae) can feed on high iridoid-glucoside-containing plants like Plantago

lanceolata (Plantaginaceae) (Pankoke et al., 2012).

Spilosoma virginica, Grammia incorrupta Inducible reduction of endogenous insect BGD

(β-glucosidase;) activity in the midgut.Host-plant switching

Protection against predators & natural enemies• Insects to avoid plant poisons is by sequestering and deploying the poisons for their own pheromone

system and defense.

• Lepidoptera sequesters plant secondary metabolites such some terpenes, phenols and many nitrogen-

containing compounds and uses them as toxic or unpalatable to predators.

• Insects frequently detoxify or sequester plant defense compounds and, sometimes, they result in

protection against parasitoids and predators being used as toxic or unpalatable at defense.

• Sequestering specialists have developed the ability to incorporate these compounds with relative

impunity, ingesting, transporting and depositing the substances to be sequestered in parti- cular sites of

the larvae, adult body and even in the eggs.

An example of this adaptation is illustrated by the tobacco hornworm.

• This insect accumulates the nicotine synthesized by tobacco plants in its

own body which is toxic to most insects and uses it as a deterrent to

parasitoids.

• Monarch butterfly caterpillars have evolved the ability to store toxins known as cardenolides, obtained

from their milkweed diet, specifically to make themselves poisonous to birds, as has at least one other

species of milkweed-munching caterpillar.

• Sequestration of cardenolides in the body (as a defence against predators) is associated with the three

levels of Na+/K+-ATPase resistance.

Conclusions

• The co-evolution of plants and insects is very interesting.

• Plants have developed efficient mechanisms to protect them against

herbivory while insects have found diverse ways of avoiding negative effects

of their host plants defense mechanisms.

• Even though many workers have attempted to study plant-insect interaction,

our knowledge is still limited.

• The better understanding of this process will allow us to achieve more

effective methods for the biological control of insect pests with natural

products by the development of new plant varieties with enhanced chemical

defenses .

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