Plant Resistance to Insect Grazing

 

1. Non-preference (Antixenosis)                       Insects choose not to feed or oviposit

 

2. Antibiosis                                                     Results in smaller adults or larval death

 

3. Tolerance

 

Antixenosis

 

1. Absence of an attractant

2. Presence of a repellant

3. Unfavorable balance between the two.

4. Tissue toughness

5. Nutrient deficiency

 

Canby is an aphid (Amphorophora agathanica) resistant raspberry. Phloem sap is deficient in sugar and nitrogenous compounds which are suitable for the aphid.

If only presented with the plant, mortality occurs in the first two intars.

Underfield conditions – aphids, probed and fed for no more than 24h before becoming “restless” and abandoning the plant.

 

6. Waxes

 

            Major components of waxes are alkanes.

            In rice, application of epicuticular wax from resistant variety IR64 to susceptible

IR22 detered probing by brown planthopper.

 

6. Structural deterrents

 

HAIR – PUBESCENCE  

 

But some insects prefer “hairy” plants e.g. Hessian fly (Magetiola destructa)

Cotton bollworm (Helicoverpa zea) female hangs on to hairs when ovipositing

 

TRICHOMES act by

 

1) Simple impedance

2) Trap insects using – hooks or exudates from glandular Trichomes

 

Trichomes in Solanum, Lycoperscium, Nicotinia, oin contact with atmospheric oxygen produce a insoluble black substance.

 

            Polyphenoloxidase and peroxidase activities – leading to oxidative cross-linking.

 

 

Plant Resistance to Insect Grazing : Secondary Metabolism

 

1o Metabolites                          Nucleotides, lipids, organic acids – produced by all organisms.

 

2o Metabolites                          Specific chemicals “tailored” to the need of specific organism.

 

2o Metabolites

 

1. Made during certain developmental phases of the plant.

2.Restricited to specific storage sites.

Intracytoplasmic  -vacuole (eg. cyanogenic glucosides) and plastids (in chloroplasts ; phenylpropanes and flavanoids)

Extracytoplasmic       cell walls, sub-cuticular spaces. Terpenes, generally restricted to resin ducts and glandular hairs.

 

FUNCTION OF 2O METABOLITES         

 

Plant Growth Hormones           auxins, cytokinins, ABA, gibberelic acid

Insect Attractors                       colour favanoids and carotenoids (see Insect Host selection)

Insect Repellents                       e.g terpenoids

 

 

TERPENOIDS

These compounds are not induced following wounding but following insects damage.

 

Terpenoid diversity is a major source of the distinctive “signature” – associated with each insect/plant interaction.

 

The signature is produced either

 

(i) insect encoded enzymes

e.g. spider mites (Tetranychus urticae) produce (3S)-(E)-nerolidol synthase (1) to release monoterpene 4,8-dimethyl – 1-3(E) 7 nanatriene (2).

 

This attracts the predatory mite Phytoseiulus persimilis.

 

(i)                              Insects producing elicitory compounds.

 

e.g. Manduca sexta 

induced the synthesis of mono- and sesquiterpenes as well as a jasmonate burst.

 

Examination of M. sexta  regurgitant identified fatty-acid – amino acid conjugates which could elicit the “feeding signature” from Nicotinia attenuata.

 

Some monoterpenes are directly toxic to insects –

 

Very varied group derived form acetyl CoA or glycolytic intermediates. –

 

Acetate-mevalonate pathway.

 

Form by the fusion of 5 carbon “isoprene units” thus the isoprenoid pathway → trepenoid pathway.

 

 

 

 

 

 

 

 

 


Derived from the condensation of three molecules of Acetyl CoA.

 

A series of complex polymers are based on this isoprene unit.

 

MONOTERPENOIDS. C10 terpenoids (2 x C5)

 

Volatile → giving plants their characteristic odor.

The “essential oils” – menthol, camphor and a-piene are all monoterpenes

 

LIMONENE and GERMINOL are constituents of flower scents – attracting wind pollinators.

 

PYRETHROIDS – found on leaves and flowers of Chrysanthemum species. These are neurotoxins which cause hyperexcitation, unco-oridinated movement and insect paralysis.

Synthetic analogues are now important commercial insectides

 

CITONELLOL  is an oviposition-deterrent to leaf hoppers (Amrasca devastans)

 

SESQUITERPENOIDS   C15 terpenoids (3 x C15) –

 

Can be either aliphatic or cylic.

 

1. DRIMANE SKELETAL TYPE -  most potent insect feeding deterrents

 

e.g.       POLGONIOL from Polygonum

            WARBURGANOL from Warburgia.

 

Blocks the stimulatory effects of glucose, sucrose and inositol on chemosensory receptor located on insect mouthparts.

 

2. SESQUITERPENE LACTONES largest groups of sesquiterpenes  e.g. alanolactone

 

Poisonous to lepidopterans, beetles and grasshoppers – functions by an unknown mechanism.

3. GOSSYPOL – a sesquiterpene dimer.

 

Toxicity arises from complexing with proteins in the insects gastrointestinal tract – inhibiting enzymic activity.

 

4. PHYTOJUVENILE HORMONE

 

Insect juvenile hormone has a farnesol backbone. Depending on substituting groups JH is classified into three groups.

 

Type III JH is closely related to the plant sesquitepene –JAVABIONE

 

The JH –mimicking effects of plant terpenes was first noted in when the cotton bug

(Pyrrhocoris apteru) persistently moulted into the nymph form when cultured with paper pulp from trees.

 

5. ANTIJUVENILE HORMONES

 

Precocene I and II from Ageratum houstoniatum  cause precocious metamorphosis and sterilization on Hemiptera, Homoptera and Orthoptera.

 

Inhibits JH secretion from the corpus allatum  in the insect brain. 

 

DITERPENOIDS  C20 (4 x C5) – “diterpenes” (includes gibberellins).

 

Non-volatile found in the resin of higher plants,

Insect resistance factors which increase insect mortality, reduce growth and inhibit feeding.

 

TRITERPENOIDS C30 (6 x C5)

 

CUCURBITACINS extremely bitter tasting

 

SAPONINS, triterpenoid derived glycoside. Amphipathic – acting as a detergent. Binds to free sterols in the insect gut to reduce uptake into the hemolymph.

 

Reduction in steroids may interfere in insect moulting hormone-.

 

STEROIDS

 

Triterpenoid derived. Non-polar.

 

Phyto-ecdysome has been discoverted which severly inhibits growth, development and reproduction. Not lethal but affects insect fitness.

 

Nitrogenous protective compounds

 

Not in themselves toxic but are readily broken down to give off poisons. Two major examples -

 

CYANOGENIC GLYCOSIDES

 

 

 

 

 

 

 

 

 

 

 


Broken down to give off cyanide (HCN) by a combination of glucosidases (in the vacuole) and hydroxynitrate lyase (cytoplasm).

 

These enzymes only mix when the cell is ruptured e.g following insect grazing.

 

GLUCOSINOLATES

 

In Brassicas, isothiocyanates. are produced from glucosinolates and are systemically emitted

 

 

The distinctive, pungent flavor and odor of mustard, radishes and horseradish is due to isothiocyanates. 

 

Mustard Oil interferes with insect plant selection at two stages

(i)                  Pre-digestive – repelled by pungent odour

(ii)                Post digestive – direct toxic effects.

 

GLUCOSINOLATE HYDROLYSIS

Glucosinolate is degraded by an enzyme - myrosinase

Which depending on the reaction conditions (pH primarily) will give off either isothiocyanates

                                  R      N      C        S

Or nitriles.

                                    R      C      N

Myrosinase is compartmentalized (where) until released following wounding.

 

Isothiocyanate deter grazing EXCEPT –

 

Cabbage white butterfly is attracted by glucosinolates and they are required for egg laying.

 

 

HOW DO INSECTS OVERCOME DEFENCE MECHANISMS?

 

DETOXIFICATION:

 

(I)                 Multifunctional oxidases present in the insect gut.

(II)              During oviposition  - wool wasps (Sirex sp. )  inject a toxin

a.        inhibits polyphenoloxidases. and

b.      spores of a symbiotic fungus into the leaf. Th elatter grown into conductive tissue to- blocks photosynthate translocation from the leaves a

 

BEHAVIOURAL CHANGES . Only feed for 1-2h, not allowing plant defences to build-up.

 

HOST TOXIN SEQUISTRATION

 

1. Western pine beetle (Denroctonus breviconis) is attracted to its host by the terpene –OLEOERSIN -. Females attract male by producing a pheromone – MYRCENE  which is derived from oleoresin.

 

Therefore the population is proportional to the amount of food available.

 

When the population is optimal – females secrete VERBENONE a male repellent substance.

 

2. Larvae of Monarch butterfly (Danaus plexippus) accumulate CARDENOLIDES from milkweed. Larvae can regurgitate gut fluid  with cardenolides – bitter tasting steroid – a potent regurgitant.