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Defense Mechanism in Plants Against Insects

The document discusses various defense mechanisms that plants have developed to combat insect herbivores, highlighting their ability to recognize insect attacks and deploy appropriate responses. Key defense strategies include physical barriers, chemical deterrents, and signaling pathways involving jasmonic acid that trigger gene activation for defense compounds. Additionally, the document details specific plant bioactive compounds and physiological responses initiated in reaction to herbivore presence, showcasing the intricate co-evolution between plants and insects over millions of years.

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Assignment On Defense Mechanism in Plants Against Insects Advanced Insect Physiology ENT 602 JAYANT YADAV (2017A39D)
OUTLINE •INTRODUCTION • RECOGNITION OF INSECT HERBIVORE ATTACK • EARLY EVENTS IN THE PLANT DEFENSE • REGULATION OF DEFENSE RESPONSES • TYPES OF DEFENSE RESPONSES
INTRODUCTION • Plants and insects living together for more than 350 million years • Evolutionary between plants and insects resulted in the development of defence system in plants that has the ability to recognize signals from damaged cells • Activates the plant immune response against the insects • Plants have the ability to distinguish between herbivory and mechanical damage, such as hail and wind, as well as to recognize oviposition. • This feature is needed to avoid wasting expensive defence resources, since production and release of defence responses only benefits herbivore challenged plants.
Video No. 1: Plant Defence Mechanism (By TED)
Recognition of Insect Herbivore Attack • Conjugation of plant- and herbivore-derived precursors result in the formation of fatty acid-amino acid conjugates (FACs). • N-17-hydroxylinolenoyl-L-glutamine (volicitin ): First identified in Spodoptera exigua (beet armyworm) oral secretions • Inceptins: Disulfide-bonded peptides formed by proteolytic fragments of chloroplastic ATP synthase γ-subunit, produced through the digestion of plant proteins in the gut of Spodoptera frugiperda (fall armyworm) Elicitors (Present in saliva of insects)
• Caeliferins: Disulfoxy fatty acids, were identified in the oral secretions of Shistocerca americana (American bird grasshopper) and it release of volatile terpenoids • Bruchins: Long-chain α,ω-diols, esterified at one or both oxygen atoms with 3-hydroxypropanoic acid found in Callosobruchus maculatus (cowpea weevil) • β-glucosidase: In the oral secretion of the larvae of Pieris brassicae elicits the release of volatile organic compounds that attracts the parasitic wasp Cotesia glomerata Doss et al 2000
Bonaventure et al 2011 Herbivore Associated Elicitors
• Insect oviposition fluids can give rise to defense responses in the plant • Many female adult herbivorous insects lay eggs directly into plants, and some species are known to perceive insects’ oviposition activities and deploy defenses responses OVIPOSITION FLUIDS
Oviposition induced plant responses acting against the herbivores
Hilker andMeiners, 2006 Contd….
Early Events in the Plant-Insect Interaction • Changes in the transmembrane potential (Vm) appear immediately upon herbivory damage • Changes in the intracellular Ca2+ concentration and generation of H2O2 • Kinases and phytohormone jasmonic acid (JA) are detectable within minutes • After 1 h, gene activation followed by metabolic changes (Mithöfer and Boland, 2008)
Membrane potential changes • The plant plasma membrane is in direct contact with the environment, and is therefore able to recognize outer changes and initiate cascade events leading to apossible defense response. • Herbivore feeding will lead to an immediate change in the cell membrane potential (Vm), or modulate the ion flux at the plasma membrane level. • The Vm changes induced by herbivory are followed by afast electric signal (action potential), which travels through the entire plant from the point where the signal was induced.
• Calcium ions function as a second messenger in several plant signaling pathways. • In healthy cells, the cytosolic Ca2+ concentration is lowerthan in the apoplastic fluid andcellularorganelles. • This creates adriving force for the influx of Ca2+ into the cytosol, via channel proteins where it acts as a messenger to induce defense related signals. Ca2+ Homeostatis
Systemic Signaling • In plants attacked by insect herbivores, the expression of several defense genes is induced in undamaged leaves • Several components have been identified that are involved in the systemic induction of defense responses • Systemin peptides • Oligogalacturonides (OGAs) • Jasmonates
Regulation of Defence Responses  Levels of jasmonic acid rise in response to herbivor damage.  This hormone can trigger many types of plant defenses including bioactive compounds.  The action of jasmonic acid induces the transcription of many genes involved in plant defense.  Jasmonic acid turns on genes for proteinase inhibitor.
Fig: A simplified scheme for co-evolution between plants and their herbivores.
(War et al 2012)
Types of Defense Responses
• Plants produce physical barriers against insect herbivores. • Produce compounds that exert repellent, anti-nutritive or toxic effects on the herbivores.
Physical factors • Laticifers and Oleoresins • Waxes and Crystals • Trichomes • Spines • Thicker Leaves
Crop Morphological structure Effect on insect soybean Hairy trichomes Prevent insect eggs from reaching the epidermis and the larvae starve after hatching Potato Tomato Glandular trichomes secrete oils that repel aphids Cotton Glandular trichomes Attract pink boll warm Crucifer Wax bloom Deter feeding by DBM Onion Round leave Resistant to thrips Okra Long trichomes Resistant to Amrasca biguttula Sorghum Leaf glossiness Shoot fly Effect of physical factors on insects
Laticifers • Plants contain networks of channels in vascular tissues called laticifers ducts • More than 50 latex producing plant families, Asclepias (milkweeds) is the one most studied • Latex of Cryptostegia grandiflora (rubber wine) may be transported 70 cm upwards to the wounding site, where it, upon exposure to air, will coagulate and there by trap small insect larvae.
Oleoresins • Oleoresins (often termed resin or pitch) mixture of terpenoids and phenolics, and are stored in high pressurized intercellular spaces . • Herbivore damage results in upward movement of resin . • When resin is exposed to air, the highly volatile monoterpenes and sesquiterpenes evaporate, leaving insects trapped in the solidifying resin acids.
•Epicuticular waxes form films and crystals that cover the cuticle of most vascular plants •Oviposition of Pieris brassicae on Arabidopsis thaliana induce changes in the wax composition, increasing the amount of fatty acid tetratriacontanoic acid , while decreasing the amount of tetracosanoic acid . These changes lead to attraction of the egg parasitoid (Blenn, et al 2012) Waxes
Trichomes • Trichomes : specialized hairy structure found on aerial plant parts • Trichome density negatively effect – Ovipositional behavior – Feeding – Larval nutrition of insect pests – Interfere with the movement of insect – Type - Straight, spiral, hooked, branched, or unbranched and can be glandular or non glandular
Types of Trichome
The glandular trichomes of the plant Roridula gorgonias release an extremely adhesive, visco- elastic, resinous secretion that traps a variety of insects and show mutualistic effect of mirid bug Pameridea roridulae on its prey (Voigt and Gorb , 2008)
Hooked trichomes and resistance of phaseolus vulgaris to Empoasca fabae • Two cultivars of the bean, Phaseolus vulgaris L., differing in densities of hooked trichomes were examined for resistance to the potato leafhopper, Empoasca fabae • Leafhoppers were impaled by these epidermal appendages, leading to wounding and death • Nymphal survival on the cultivar, “California Light Red Kidney” (hooked trichome density: 2000/cm2) was significantly less than that on “Brasil 343” (hooked trichome density: 400/cm2) (Pillemer and Tingey, 2011)
Direct Chemical Defence Responses • Bioactive Specialized Compounds • Hypersensitive Response • Digestibility Reduction • Reallocation of Resources
A. Bioactive Specialized Compounds • Alkaloids • Benzoxazinoides • Cyanogenic Glucosides • Terpenoids • Phenolics • Proteinase Inhibitor • Non protein Amino Acid • Polyphenol Oxidases • Primary metabolites used for growth, development and reproduction • Secondary metabolites known as bioactive specialized compounds, are used to protect the plant against insects (herbivores) (Engelberth et al 2006)
Alkaloids •Alkaloids (>15,000 different alkaloids found in 20% of all vascular plants), are prevalently found in the Leguminosae spp. (legumes), Liliaceae spp. (lilies), Solanaceae spp. •Alkaline contain nitrogen in a heterocyclic ring e.g., nicotine and atropine •Pseudoalkaloids, such as caffeine and solanidine, are alkaline but not derived from amino acids •Solanum demissum (nightshade potato) containing the alkaloid demissine is resistant to Leptinotarsa decemlineata (Colorado beetle) and Empoasca fabae (potato leafhopper)
Benzoxazinoides •Grammineae Family (maize, rye and wheat) produces the defense- related bioactive specialized compounds 2,4-dihydroxy-1,4-benzoxazin- 3-one-glucoside (DIBOA-Glc) and dihydroxy-7-methoxy-1,4- benzoxazin-3-one-glucoside (DIMBOA-Glc, from indole-3-glycerol phosphate • Conversion is catalyzed by BX1-BX9, of which BX1 cleaves off the glycerol phosphate, BX2-BX5 (Cytochrome P450s CYP79C1-4) catalyze the reactions forming DIBOA, BX8/BX9 add the stabilizing glucosyl group, and BX6-BX7 assists in the conversion from DIBOA- Glc to DIMBOA-Glc • DIMBOA has been shown to confer resistance to Ostrinia nubilalis (first-brood European corn borer) (Glauser et al 2011)
Cyanogenic Glucosides • Cyanogenic glucosides (CNglcs ) are amino acid derived glucosides, originating from aromatic or branched-chain amino acids • Tyrosine (Dhurrin in Sorghum bicolor) •Valine and isoleucine in Lotus japonicus •CNglcs are stored in the vacuole plant tissues. •When the plant tissue is fragmented, for instance due to feeding, the CNglcs are exposed to β-glucosidases located in plastids or which leads to hydrolysis and the formation of a sugar and a cyanohydrin that spontaneously decomposes into toxic hydrogen cyanide (HCN) and a ketone or aldehyde.
• Phenolics, derived from phenyl alanine (such as caffeic and ferulic acid), phenylpropanoid lactones (known as coumarins) and benzoic acid derivatives (such as vanillin and salicylic acid ). • The cotton phenolic pigment gossypol has repellent effects against numerous insects and is toxic to Heliothis virescens (tobacco bollworm), Heliothis zea (bollworm) and several other insects Phenolics
• Tannins - naturally occurring plant polyphenols. Their main characteristic is that they bind and precipitate proteins and stored in vacuoles of plants. Effect on insects  Bind to salivary proteins and digestive enzymes including trypsin and chymotrypsin resulting in protein inactivation • Lignin heterogeneous polymer composed of phenolic compounds that gives the cell rigidity. Lignin is the primary component of wood, and cell walls that become “lignified” are highly impermeable to pathogens and difficult for small insects to chew Effect on insects  Insoluble, rigid, and virtually indigestible, lignin provides an excellent physical barrier
Glucosinolates •Glucosinolates (GSL) are sulphur- and nitrogen-containing compounds found extensively in Brassicaceae •The GSL are divided into Three groups based on the amino acid precursor of the side chain: • Aliphatic GSL (50%) derived from methionine •Indole GSL (10%) synthesized from tryptophan •Aromatic GSL (10%) derived from phenylalanine or tyrosine •Flee beetle Phyllotreta cruciferae feeds preferably on older cotyledons of Sinapis alba (white mustard), due to the lower levels of the GSL ( Bodnaryk , 1991)
•Terpenoids are biosynthesized from acetyl-CoA or glycolytic intermediates. •They are classified by the number of isoprene units or five-carbon elements (CH3–CH2–CH–(H3C)2) 10-carbon terpenes are called monoterpenes 15-carbon terpenes are sesquiterpenes 20-carbon terpenes are diterpenes 25-carbon terpenes are sesterterpenes 30-carbon terpenes are triterpenes 40-carbon terpenes are tetraterpenes terpenes with even more isoprene units polyterpenes •Many of them play a role in plant defense, both as components in resin or as volatiles, acting as antifeedants, repellents, toxins or as modifiers of insect development (Aharoni et al 2005) Terpenoids
• Phytoecdysones are plant steroids that have the same basic structure as insect molting hormones and thus interfere with molting. These compounds sometimes cause death of the insect herbivore. • Phytoecdysteroids are classed as triterpenoids PHYTOECDYSONES
(a) Nicotine, a true alkaloid derived from aspartate and ornithine (b) DIMBOA, a benzoxazinoide derived from indole-3-glycerol phosphate (c) Dhurrin, a cyanogenic glucoside derived from tyrosine (d) Sinalbin, a glucosinolate derived from tyrosine (e) Canavanine, a nonprotein amino acid derived from L-homoserine (f) Salicylic acid, a benzoic acid derived phenol (g) limonene, a terpenoid derived from geranyl pyrophosphate (Caspi et al 2008) Structures of plant bioactives
Ibanezet al 2012 Examples of Plant Bioactive Specialized Compounds With Insecticidal Activity
Contd…..
B. Hypersensitive response • Plant’s response to herbivore results in the formation of necrotic plant tissue and neoplamal growth that isolates the invader from plant • Plants can cast eggs off their leaves • When an insect deposits its eggs onto a plant, the plant may respond with growth of neoplastic tissue and formation of necrotic tissues that results in detachment of eggs.
(a) Eggof smallcabbagewhite butterfly, inducinghypersensitive response–like necrosisin Brassica nigra. (b) Eggof Heliothissubflexainducingneoplasticgrowth in Physalis angulata leaf. Hilker andFatouros, 2014
C. Digestibility Reduction •Plants produce a number of defense proteins that reduce insect herbivores ability to digest the plant •Anti-digestive proteins limit the rate of enzymatic conversion of ingested food •Anti-nutritive proteins limit the utilization of food by altering physical availability or chemical identity •Five major classes of defense proteins are •Protein inhibitors • α-amylase inhibitors • Lectins •Chitinases • Polyphenol oxidases (Falco et al 2001)
•Endopeptidases and Exopeptidases found in midgut region , used by insect herbivores to cleave peptide bonds Serine proteases (trypsin and chymotrypsin ) Cysteine Aspartic acid proteases Metalloproteinases •Plants have inhibitors for all four classes of proteinases Delay larval development without directly causing mortality  Proteolytic activity of midgut enzymes and decrease the availability of amino acid In sugarcane, trypsin inhibitors detected in leaves, lateral buds and seed tissue, which effect sugarcane borer larval devlopment (Falco et al 2001) Protease Inhibitors
•The lectin-like α-amylase inhibitors (α-AI) are found in cereal seeds, such as Triticum spp.(wheat) and Hordeum vulgare (barley), and in monocots, such as S. bicolor and Z. Mays •The activities of these inhibitors are directed against α-amylases from insects , used for starch breakdown •Transgenic Wheat α-AIs can inhibit Tenebrio obscurus (mealworm), Tribolium spp. (flour beetles), Sitophilus spp. (wheat weevils) and Oryzaephilus spp. (grain beetles) (Falco et al 2001) α- Amylase Inhibitors
•Polyphenol oxidase (PPO) enzymes cause the typical browning of plant extracts, mainly fruits, and damaged tissues •PPOs appear frequently upon wounding, and are therefore suggested to play a defensive role •Polyphenol oxidase (PPO) enzymes, over expression genes decreased the growth rate 2.5-fold in S. litura, and increased the mortality up to 3.3-fold •PPOs can also be combined with specific phenolic substrates in glandular trichomes to produce a kind of “super glue” to trap smaller insects Polyphenol Oxidases
• Lectins are sugar-binding proteins produced by plants as a defense response • When lectins come into contact with the glycoproteins lining the intestinal area of insect herbivores, they are assumed to inhibit the absorption of nutrients Lectins
QuantificationofNICTABAaccumulationintobaccoleavesafterfeeding bydifferentherbivores Vandenborreet al 2011
Spottedknapweedknapweed moth Newingham et al 2007 Fig: Allocating nitrogen away from aherbivore D. Reallocation of Resources To protect valuable resources, they might be reallocated by the plant upon attack
Indirect Defence Response The term “Indirect Defence ” is used when plants attract, nourish or house natural enemy.
Two mechanisms are involved in Indirect Defence Response 1. Herbivore-induced Plant Volatiles (HIPV) 2. Extra-floral Nectar
Video No. 2: Plant Defence Mediated By HIPV Production
HIPV: HIPV can mediate indirect defences by attracting foraging carnivores predators and parasitoids
1. Volatiles •More than 1000 volatile organic compounds (VOCs), primarily consisting of 6-carbon aldehydes, alcohols, esters and various terpenoids are released from plant flowers, vegetative parts or roots • VOCs are used to attract pollinators and predators or repel herbivores •Green-leaf volatiles (GLVs) are isomers of hexanol, hexenal or hexenyl acetate •Roots VOCs: Z. mays roots attacked by Western corn rootworm Insect larvae release the sesquiterpene (E)-β-caryophyllene as well as small amounts of α-humulene and caryophyllene oxide which attract its natural enemy (Rasmann et al 2005)
•Insect oviposition fluids give rise to defence responses in the plant as well, making the plant attract egg-eating predators or strengthen its defense in case of a potential future insect herbivore attack (Hilker and Meiners, 2006) •Oviposition of P. brassicae on leaves of Brassica oleracea (Brussels sprouts) changes the leaf surface chemicals leading to attraction of the egg parasitoid Trichogramma brassicae (Fatouros et al 2005) 2. Ovipositional Fluids
3. Extrafloral Nectar • Extrafloral nectar (EFN) appear in more than 70 plant species spanning angiosperms, gymnosperms and ferns, indicating that it is evolutionary more ancient than floral nectar • In contrast to floral nectar, used to attract pollinators, EFN is secreted on leaves and shoots to attract predators and parasitoids
Extrafloral nectar as an herbivore-induced defense trait Family Species Herbivore Trait enhanced Bignoniaceae Catalpabignonioides (Indian bean) Ceratomiacatalpae (CatalpaSphinx) Sugarcontent inEFN Euphorbiaceae Ricinuscommunis (Castor) Spodopteralittoralis EFNvolume Euphorbiaceae Triadicasebifera (Chinese tallow tree) Gadirthainexacta, and Grammodesgeometrica Secretionof total solids Malvaceae Gossypiumherbaceum (Cotton) Spodopteralittoralis EFNvolume Martin, 2015
Ant visitation to extrafloral nectaries decreases herbivory and increases fruit set in Chamaecrista debilis AlvesandClaro, 2010
CONCLUSION  Insects and plants co-evolved and not only developed morphological defense mechanisms active against insect herbivory, but also genetic transformations allowing the production of volatile chemicals.  Attacked plants use these volatile chemicals as arsenals and signals against attacking pests. These chemicals not only repell herbivores but also control their population, by signalling invitations sent to their predators or parasites.  The interaction between herbivores and biotic environment is thus largely based on plant mediated mechanisms, including constitutive traits like modifications in plants anatomy and physiology, or herbivore induced changes in host biochemistry.  Plants also evolved direct strategies to repel herbivores, through induced and constitutive defence mechanism. The trichomes constitute a defense feature against a variety of insects.  Due to co-evolution of synthesis of herbivore repelling volatiles in plants and their modification by herbivores, cyclic population changes, concerning both the herbivores and primary producers, can be also affected.
Video No. 3: Plant Defence Mechanism Briefing
FUTURE OUTLOOK  Although induced resistance has attained a considerable momentum recently, and has attracted the attention of scientists in evolutionary ecology, entomology, plant physiology and biotechnology, much of the underlying mechanism have still remained unanswered.  There is a need to understand the herbivore- specific signal molecules, their identification, mode of action and further signal transduction.  An understanding of induced resistance in plants can be utilized for interpreting the ecological interactions between plants and herbivores and for exploiting in pest management in crops.  The future challenge is to exploit the elicitors of induced defense in plants for pest management, and identify the genes encoding proteins that are up and/or down regulated during plant response to the herbivore attack, which can be deployed for conferring resistance to the herbivores through genetic transformation.
Defense Mechanism in Plants Against Insects

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Defense Mechanism in Plants Against Insects

  • 1.
    Assignment On Defense Mechanism inPlants Against Insects Advanced Insect Physiology ENT 602 JAYANT YADAV (2017A39D)
  • 2.
    OUTLINE •INTRODUCTION • RECOGNITION OFINSECT HERBIVORE ATTACK • EARLY EVENTS IN THE PLANT DEFENSE • REGULATION OF DEFENSE RESPONSES • TYPES OF DEFENSE RESPONSES
  • 3.
    INTRODUCTION • Plants andinsects living together for more than 350 million years • Evolutionary between plants and insects resulted in the development of defence system in plants that has the ability to recognize signals from damaged cells • Activates the plant immune response against the insects • Plants have the ability to distinguish between herbivory and mechanical damage, such as hail and wind, as well as to recognize oviposition. • This feature is needed to avoid wasting expensive defence resources, since production and release of defence responses only benefits herbivore challenged plants.
  • 4.
    Video No. 1:Plant Defence Mechanism (By TED)
  • 6.
    Recognition of InsectHerbivore Attack • Conjugation of plant- and herbivore-derived precursors result in the formation of fatty acid-amino acid conjugates (FACs). • N-17-hydroxylinolenoyl-L-glutamine (volicitin ): First identified in Spodoptera exigua (beet armyworm) oral secretions • Inceptins: Disulfide-bonded peptides formed by proteolytic fragments of chloroplastic ATP synthase γ-subunit, produced through the digestion of plant proteins in the gut of Spodoptera frugiperda (fall armyworm) Elicitors (Present in saliva of insects)
  • 7.
    • Caeliferins: Disulfoxyfatty acids, were identified in the oral secretions of Shistocerca americana (American bird grasshopper) and it release of volatile terpenoids • Bruchins: Long-chain α,ω-diols, esterified at one or both oxygen atoms with 3-hydroxypropanoic acid found in Callosobruchus maculatus (cowpea weevil) • β-glucosidase: In the oral secretion of the larvae of Pieris brassicae elicits the release of volatile organic compounds that attracts the parasitic wasp Cotesia glomerata Doss et al 2000
  • 8.
    Bonaventure et al2011 Herbivore Associated Elicitors
  • 10.
    • Insect ovipositionfluids can give rise to defense responses in the plant • Many female adult herbivorous insects lay eggs directly into plants, and some species are known to perceive insects’ oviposition activities and deploy defenses responses OVIPOSITION FLUIDS
  • 11.
    Oviposition induced plantresponses acting against the herbivores
  • 12.
  • 13.
    Early Events inthe Plant-Insect Interaction • Changes in the transmembrane potential (Vm) appear immediately upon herbivory damage • Changes in the intracellular Ca2+ concentration and generation of H2O2 • Kinases and phytohormone jasmonic acid (JA) are detectable within minutes • After 1 h, gene activation followed by metabolic changes (Mithöfer and Boland, 2008)
  • 14.
    Membrane potential changes •The plant plasma membrane is in direct contact with the environment, and is therefore able to recognize outer changes and initiate cascade events leading to apossible defense response. • Herbivore feeding will lead to an immediate change in the cell membrane potential (Vm), or modulate the ion flux at the plasma membrane level. • The Vm changes induced by herbivory are followed by afast electric signal (action potential), which travels through the entire plant from the point where the signal was induced.
  • 15.
    • Calcium ionsfunction as a second messenger in several plant signaling pathways. • In healthy cells, the cytosolic Ca2+ concentration is lowerthan in the apoplastic fluid andcellularorganelles. • This creates adriving force for the influx of Ca2+ into the cytosol, via channel proteins where it acts as a messenger to induce defense related signals. Ca2+ Homeostatis
  • 16.
    Systemic Signaling • Inplants attacked by insect herbivores, the expression of several defense genes is induced in undamaged leaves • Several components have been identified that are involved in the systemic induction of defense responses • Systemin peptides • Oligogalacturonides (OGAs) • Jasmonates
  • 17.
    Regulation of DefenceResponses  Levels of jasmonic acid rise in response to herbivor damage.  This hormone can trigger many types of plant defenses including bioactive compounds.  The action of jasmonic acid induces the transcription of many genes involved in plant defense.  Jasmonic acid turns on genes for proteinase inhibitor.
  • 19.
    Fig: A simplifiedscheme for co-evolution between plants and their herbivores.
  • 20.
  • 21.
  • 22.
    • Plants producephysical barriers against insect herbivores. • Produce compounds that exert repellent, anti-nutritive or toxic effects on the herbivores.
  • 23.
    Physical factors • Laticifersand Oleoresins • Waxes and Crystals • Trichomes • Spines • Thicker Leaves
  • 24.
    Crop Morphological structureEffect on insect soybean Hairy trichomes Prevent insect eggs from reaching the epidermis and the larvae starve after hatching Potato Tomato Glandular trichomes secrete oils that repel aphids Cotton Glandular trichomes Attract pink boll warm Crucifer Wax bloom Deter feeding by DBM Onion Round leave Resistant to thrips Okra Long trichomes Resistant to Amrasca biguttula Sorghum Leaf glossiness Shoot fly Effect of physical factors on insects
  • 25.
    Laticifers • Plants containnetworks of channels in vascular tissues called laticifers ducts • More than 50 latex producing plant families, Asclepias (milkweeds) is the one most studied • Latex of Cryptostegia grandiflora (rubber wine) may be transported 70 cm upwards to the wounding site, where it, upon exposure to air, will coagulate and there by trap small insect larvae.
  • 26.
    Oleoresins • Oleoresins (oftentermed resin or pitch) mixture of terpenoids and phenolics, and are stored in high pressurized intercellular spaces . • Herbivore damage results in upward movement of resin . • When resin is exposed to air, the highly volatile monoterpenes and sesquiterpenes evaporate, leaving insects trapped in the solidifying resin acids.
  • 27.
    •Epicuticular waxes formfilms and crystals that cover the cuticle of most vascular plants •Oviposition of Pieris brassicae on Arabidopsis thaliana induce changes in the wax composition, increasing the amount of fatty acid tetratriacontanoic acid , while decreasing the amount of tetracosanoic acid . These changes lead to attraction of the egg parasitoid (Blenn, et al 2012) Waxes
  • 28.
    Trichomes • Trichomes :specialized hairy structure found on aerial plant parts • Trichome density negatively effect – Ovipositional behavior – Feeding – Larval nutrition of insect pests – Interfere with the movement of insect – Type - Straight, spiral, hooked, branched, or unbranched and can be glandular or non glandular
  • 29.
  • 30.
    The glandular trichomesof the plant Roridula gorgonias release an extremely adhesive, visco- elastic, resinous secretion that traps a variety of insects and show mutualistic effect of mirid bug Pameridea roridulae on its prey (Voigt and Gorb , 2008)
  • 31.
    Hooked trichomes andresistance of phaseolus vulgaris to Empoasca fabae • Two cultivars of the bean, Phaseolus vulgaris L., differing in densities of hooked trichomes were examined for resistance to the potato leafhopper, Empoasca fabae • Leafhoppers were impaled by these epidermal appendages, leading to wounding and death • Nymphal survival on the cultivar, “California Light Red Kidney” (hooked trichome density: 2000/cm2) was significantly less than that on “Brasil 343” (hooked trichome density: 400/cm2) (Pillemer and Tingey, 2011)
  • 32.
    Direct Chemical DefenceResponses • Bioactive Specialized Compounds • Hypersensitive Response • Digestibility Reduction • Reallocation of Resources
  • 33.
    A. Bioactive SpecializedCompounds • Alkaloids • Benzoxazinoides • Cyanogenic Glucosides • Terpenoids • Phenolics • Proteinase Inhibitor • Non protein Amino Acid • Polyphenol Oxidases • Primary metabolites used for growth, development and reproduction • Secondary metabolites known as bioactive specialized compounds, are used to protect the plant against insects (herbivores) (Engelberth et al 2006)
  • 34.
    Alkaloids •Alkaloids (>15,000 differentalkaloids found in 20% of all vascular plants), are prevalently found in the Leguminosae spp. (legumes), Liliaceae spp. (lilies), Solanaceae spp. •Alkaline contain nitrogen in a heterocyclic ring e.g., nicotine and atropine •Pseudoalkaloids, such as caffeine and solanidine, are alkaline but not derived from amino acids •Solanum demissum (nightshade potato) containing the alkaloid demissine is resistant to Leptinotarsa decemlineata (Colorado beetle) and Empoasca fabae (potato leafhopper)
  • 35.
    Benzoxazinoides •Grammineae Family (maize,rye and wheat) produces the defense- related bioactive specialized compounds 2,4-dihydroxy-1,4-benzoxazin- 3-one-glucoside (DIBOA-Glc) and dihydroxy-7-methoxy-1,4- benzoxazin-3-one-glucoside (DIMBOA-Glc, from indole-3-glycerol phosphate • Conversion is catalyzed by BX1-BX9, of which BX1 cleaves off the glycerol phosphate, BX2-BX5 (Cytochrome P450s CYP79C1-4) catalyze the reactions forming DIBOA, BX8/BX9 add the stabilizing glucosyl group, and BX6-BX7 assists in the conversion from DIBOA- Glc to DIMBOA-Glc • DIMBOA has been shown to confer resistance to Ostrinia nubilalis (first-brood European corn borer) (Glauser et al 2011)
  • 36.
    Cyanogenic Glucosides • Cyanogenicglucosides (CNglcs ) are amino acid derived glucosides, originating from aromatic or branched-chain amino acids • Tyrosine (Dhurrin in Sorghum bicolor) •Valine and isoleucine in Lotus japonicus •CNglcs are stored in the vacuole plant tissues. •When the plant tissue is fragmented, for instance due to feeding, the CNglcs are exposed to β-glucosidases located in plastids or which leads to hydrolysis and the formation of a sugar and a cyanohydrin that spontaneously decomposes into toxic hydrogen cyanide (HCN) and a ketone or aldehyde.
  • 37.
    • Phenolics, derivedfrom phenyl alanine (such as caffeic and ferulic acid), phenylpropanoid lactones (known as coumarins) and benzoic acid derivatives (such as vanillin and salicylic acid ). • The cotton phenolic pigment gossypol has repellent effects against numerous insects and is toxic to Heliothis virescens (tobacco bollworm), Heliothis zea (bollworm) and several other insects Phenolics
  • 38.
    • Tannins -naturally occurring plant polyphenols. Their main characteristic is that they bind and precipitate proteins and stored in vacuoles of plants. Effect on insects  Bind to salivary proteins and digestive enzymes including trypsin and chymotrypsin resulting in protein inactivation • Lignin heterogeneous polymer composed of phenolic compounds that gives the cell rigidity. Lignin is the primary component of wood, and cell walls that become “lignified” are highly impermeable to pathogens and difficult for small insects to chew Effect on insects  Insoluble, rigid, and virtually indigestible, lignin provides an excellent physical barrier
  • 39.
    Glucosinolates •Glucosinolates (GSL) aresulphur- and nitrogen-containing compounds found extensively in Brassicaceae •The GSL are divided into Three groups based on the amino acid precursor of the side chain: • Aliphatic GSL (50%) derived from methionine •Indole GSL (10%) synthesized from tryptophan •Aromatic GSL (10%) derived from phenylalanine or tyrosine •Flee beetle Phyllotreta cruciferae feeds preferably on older cotyledons of Sinapis alba (white mustard), due to the lower levels of the GSL ( Bodnaryk , 1991)
  • 40.
    •Terpenoids are biosynthesizedfrom acetyl-CoA or glycolytic intermediates. •They are classified by the number of isoprene units or five-carbon elements (CH3–CH2–CH–(H3C)2) 10-carbon terpenes are called monoterpenes 15-carbon terpenes are sesquiterpenes 20-carbon terpenes are diterpenes 25-carbon terpenes are sesterterpenes 30-carbon terpenes are triterpenes 40-carbon terpenes are tetraterpenes terpenes with even more isoprene units polyterpenes •Many of them play a role in plant defense, both as components in resin or as volatiles, acting as antifeedants, repellents, toxins or as modifiers of insect development (Aharoni et al 2005) Terpenoids
  • 41.
    • Phytoecdysones areplant steroids that have the same basic structure as insect molting hormones and thus interfere with molting. These compounds sometimes cause death of the insect herbivore. • Phytoecdysteroids are classed as triterpenoids PHYTOECDYSONES
  • 42.
    (a) Nicotine, atrue alkaloid derived from aspartate and ornithine (b) DIMBOA, a benzoxazinoide derived from indole-3-glycerol phosphate (c) Dhurrin, a cyanogenic glucoside derived from tyrosine (d) Sinalbin, a glucosinolate derived from tyrosine (e) Canavanine, a nonprotein amino acid derived from L-homoserine (f) Salicylic acid, a benzoic acid derived phenol (g) limonene, a terpenoid derived from geranyl pyrophosphate (Caspi et al 2008) Structures of plant bioactives
  • 43.
    Ibanezet al 2012 Examplesof Plant Bioactive Specialized Compounds With Insecticidal Activity
  • 44.
  • 45.
    B. Hypersensitive response •Plant’s response to herbivore results in the formation of necrotic plant tissue and neoplamal growth that isolates the invader from plant • Plants can cast eggs off their leaves • When an insect deposits its eggs onto a plant, the plant may respond with growth of neoplastic tissue and formation of necrotic tissues that results in detachment of eggs.
  • 46.
    (a) Eggof smallcabbagewhitebutterfly, inducinghypersensitive response–like necrosisin Brassica nigra. (b) Eggof Heliothissubflexainducingneoplasticgrowth in Physalis angulata leaf. Hilker andFatouros, 2014
  • 47.
    C. Digestibility Reduction •Plantsproduce a number of defense proteins that reduce insect herbivores ability to digest the plant •Anti-digestive proteins limit the rate of enzymatic conversion of ingested food •Anti-nutritive proteins limit the utilization of food by altering physical availability or chemical identity •Five major classes of defense proteins are •Protein inhibitors • α-amylase inhibitors • Lectins •Chitinases • Polyphenol oxidases (Falco et al 2001)
  • 48.
    •Endopeptidases and Exopeptidasesfound in midgut region , used by insect herbivores to cleave peptide bonds Serine proteases (trypsin and chymotrypsin ) Cysteine Aspartic acid proteases Metalloproteinases •Plants have inhibitors for all four classes of proteinases Delay larval development without directly causing mortality  Proteolytic activity of midgut enzymes and decrease the availability of amino acid In sugarcane, trypsin inhibitors detected in leaves, lateral buds and seed tissue, which effect sugarcane borer larval devlopment (Falco et al 2001) Protease Inhibitors
  • 49.
    •The lectin-like α-amylaseinhibitors (α-AI) are found in cereal seeds, such as Triticum spp.(wheat) and Hordeum vulgare (barley), and in monocots, such as S. bicolor and Z. Mays •The activities of these inhibitors are directed against α-amylases from insects , used for starch breakdown •Transgenic Wheat α-AIs can inhibit Tenebrio obscurus (mealworm), Tribolium spp. (flour beetles), Sitophilus spp. (wheat weevils) and Oryzaephilus spp. (grain beetles) (Falco et al 2001) α- Amylase Inhibitors
  • 50.
    •Polyphenol oxidase (PPO)enzymes cause the typical browning of plant extracts, mainly fruits, and damaged tissues •PPOs appear frequently upon wounding, and are therefore suggested to play a defensive role •Polyphenol oxidase (PPO) enzymes, over expression genes decreased the growth rate 2.5-fold in S. litura, and increased the mortality up to 3.3-fold •PPOs can also be combined with specific phenolic substrates in glandular trichomes to produce a kind of “super glue” to trap smaller insects Polyphenol Oxidases
  • 51.
    • Lectins aresugar-binding proteins produced by plants as a defense response • When lectins come into contact with the glycoproteins lining the intestinal area of insect herbivores, they are assumed to inhibit the absorption of nutrients Lectins
  • 52.
  • 54.
    Spottedknapweedknapweed moth Newingham et al2007 Fig: Allocating nitrogen away from aherbivore D. Reallocation of Resources To protect valuable resources, they might be reallocated by the plant upon attack
  • 55.
    Indirect Defence Response Theterm “Indirect Defence ” is used when plants attract, nourish or house natural enemy.
  • 56.
    Two mechanisms areinvolved in Indirect Defence Response 1. Herbivore-induced Plant Volatiles (HIPV) 2. Extra-floral Nectar
  • 57.
    Video No. 2:Plant Defence Mediated By HIPV Production
  • 58.
    HIPV: HIPV can mediateindirect defences by attracting foraging carnivores predators and parasitoids
  • 59.
    1. Volatiles •More than1000 volatile organic compounds (VOCs), primarily consisting of 6-carbon aldehydes, alcohols, esters and various terpenoids are released from plant flowers, vegetative parts or roots • VOCs are used to attract pollinators and predators or repel herbivores •Green-leaf volatiles (GLVs) are isomers of hexanol, hexenal or hexenyl acetate •Roots VOCs: Z. mays roots attacked by Western corn rootworm Insect larvae release the sesquiterpene (E)-β-caryophyllene as well as small amounts of α-humulene and caryophyllene oxide which attract its natural enemy (Rasmann et al 2005)
  • 60.
    •Insect oviposition fluidsgive rise to defence responses in the plant as well, making the plant attract egg-eating predators or strengthen its defense in case of a potential future insect herbivore attack (Hilker and Meiners, 2006) •Oviposition of P. brassicae on leaves of Brassica oleracea (Brussels sprouts) changes the leaf surface chemicals leading to attraction of the egg parasitoid Trichogramma brassicae (Fatouros et al 2005) 2. Ovipositional Fluids
  • 62.
    3. Extrafloral Nectar •Extrafloral nectar (EFN) appear in more than 70 plant species spanning angiosperms, gymnosperms and ferns, indicating that it is evolutionary more ancient than floral nectar • In contrast to floral nectar, used to attract pollinators, EFN is secreted on leaves and shoots to attract predators and parasitoids
  • 63.
    Extrafloral nectar asan herbivore-induced defense trait Family Species Herbivore Trait enhanced Bignoniaceae Catalpabignonioides (Indian bean) Ceratomiacatalpae (CatalpaSphinx) Sugarcontent inEFN Euphorbiaceae Ricinuscommunis (Castor) Spodopteralittoralis EFNvolume Euphorbiaceae Triadicasebifera (Chinese tallow tree) Gadirthainexacta, and Grammodesgeometrica Secretionof total solids Malvaceae Gossypiumherbaceum (Cotton) Spodopteralittoralis EFNvolume Martin, 2015
  • 64.
    Ant visitation toextrafloral nectaries decreases herbivory and increases fruit set in Chamaecrista debilis AlvesandClaro, 2010
  • 65.
    CONCLUSION  Insects andplants co-evolved and not only developed morphological defense mechanisms active against insect herbivory, but also genetic transformations allowing the production of volatile chemicals.  Attacked plants use these volatile chemicals as arsenals and signals against attacking pests. These chemicals not only repell herbivores but also control their population, by signalling invitations sent to their predators or parasites.  The interaction between herbivores and biotic environment is thus largely based on plant mediated mechanisms, including constitutive traits like modifications in plants anatomy and physiology, or herbivore induced changes in host biochemistry.  Plants also evolved direct strategies to repel herbivores, through induced and constitutive defence mechanism. The trichomes constitute a defense feature against a variety of insects.  Due to co-evolution of synthesis of herbivore repelling volatiles in plants and their modification by herbivores, cyclic population changes, concerning both the herbivores and primary producers, can be also affected.
  • 66.
    Video No. 3:Plant Defence Mechanism Briefing
  • 67.
    FUTURE OUTLOOK  Althoughinduced resistance has attained a considerable momentum recently, and has attracted the attention of scientists in evolutionary ecology, entomology, plant physiology and biotechnology, much of the underlying mechanism have still remained unanswered.  There is a need to understand the herbivore- specific signal molecules, their identification, mode of action and further signal transduction.  An understanding of induced resistance in plants can be utilized for interpreting the ecological interactions between plants and herbivores and for exploiting in pest management in crops.  The future challenge is to exploit the elicitors of induced defense in plants for pest management, and identify the genes encoding proteins that are up and/or down regulated during plant response to the herbivore attack, which can be deployed for conferring resistance to the herbivores through genetic transformation.