Herbivory Herbivory noun herbivory plural noun herbivories The

Herbivory

Herbivory • noun: herbivory; • plural noun: herbivories • The eating of plants, especially ones that are still living. • "in response to herbivory, plants defend themselves with arrays of structural and chemical weapons"

Plant defenses against herbivores • Plants can’t run away from herbivores • Plants can’t hide – leaves must be exposed too catch light for photosynthesis. • Plants are usually too abundant to be cryptically colored! • Therefore, plant defenses operate in situ, either directly or indirectly Used with permission from the National Science Foundation Multimedia Gallery

Types of defense Structural Defenses 1) spines, prickles, and other painful attachment structures 2) organization of photosynthetic tissues to minimize herbivore access to photosynthate The sensitive plant (Mimosa pudica) mimics the appearance of a dead or wilting plant by folding and drooping its leaves when touched. From the wikimedia free licensed media file repository

Types of defense Structural Defenses 3) protecting delicate or critical structures beneath toughened or hardened layers Temporal Defenses Making critical parts unpredictable in time (and/or space) can minimize the damage done by herbivores. Plants can reduce herbivory by producing calcium oxalate crystals in the form of (a) long, needle-like raphides (b) shorter and stouter styloids (c) spherical druses Used with permission © 2012 Nature Education

Plants may reduce herbivory by growing in areas inaccessible to herbivores as represented in the figure above. Spatial refuges The dashed red lines represent the area where a particular herbivore can reach a plant and feed. Refuge types shown here include (a) a geological refuge on top of a cliff or plateau. (b)a biotic refuge beneath plants capable of excluding or repelling the herbivore. (c) spatial refuges occurring below or above the browse line for the herbivore. Used with permission © 2012 Nature Education

Mechanical defenses Chemical Defenses 1) toxic substances, 2) substances which reduce the digestibility or effective quality of consumed food 3) pharmaco-active substances. Most chemical defenses (other than tannin production) are offshoots of normal metabolic pathways in the leaves. An outline of 'normal' and secondary product metabolism reveals that there is a close metabolic relationship among many toxic, pharmacoactive and inhibitory defenses.

Plant Response to Herbivores Plants damaged by insects can release volatile chemicals to warn other plants of the same species These volatile molecules can also function as an “early warning system” for nearby plants of the same species. Methyl jasmonic acid can activate the expression of genes involved in plant defenses From the wikimedia free licensed media file repository

Jasmonic Acid Levels of jasmonic acid rise in response to damage This hormone can trigger many types of plant such as alkaloids The action of jasmonic acid induces the transcription of many genes involved in plant defense Jasmonic acid turns on genes for proteinase inhibitor.

Plant defense traits Plants use a variety of mechanical (toughness, spines), chemical (alkaloids, phenolics, terpenoids, latex – the realm of chemical ecology), developmental, and phenological defenses Defenses may also be classified with reference to their production: Constitutive – produced by & present in the plant irrespective of attack Induced – produced by & present in the plant in response to attack

Plant defense traits Resistance traits Those traits that “reduce herbivory” Avoidance (antixenosis) traits Those traits that “affect herbivore behavior; ” i. e. , deter or repel herbivores Antibiosis traits Those that “reduce herbivore performance” Tolerance traits Those traits that “reduce the impact of herbivory on fitness”

Resistant vs Tolerant Resistant Tolerant Susceptible

Resistant vs Tolerant Benefits of defense are obvious in the presence of herbivores Resistant Tolerant Susceptible

Resistant vs Tolerant Costs of defense are obvious in the absence of herbivores Resistant Tolerant Slide courtesy of Alyssa Stocks Hakes; modified from the original Susceptible

Direct Defense Morphological Characters Cutin, Waxes, and Suberins

Cutin, Waxes, Suberins All plant parts exposed to the atmosphere are coated with layers of lipid material that reduce water loss and help block the entry of pathogen fungi and bacteria. They are made of hydrophobic compounds which have water-repelling properties These compounds are non-polar Fatty acids are one type of hydrophobic compound

Cutin It is found most above ground It is a macromolecule, a polymer composed of long fatty acid chains that are attached to each other by ester linkage, creating a rigid three dimensional network It was a major component of plant cuticle, a multilayered secreted structure that coats the outer cell wall of epidermis on the areal parts Plants’ cuticles is composed of a top coating of wax, often vary with the climate in which they live. From the wikimedia free licensed media file repository

Waxes Complex mixtures of long-chain lipids that are extremely hydrophobic. The most common components of waxes are straight chain alkanes and alcohol of 25 to 35 carbon atoms. They are synthesized by epidermal cells. They exuded through pores in the epidermal cell wall by an unknown mechanism. From the wikimedia free licensed media file repository

Suberin It was formed from fatty acids but has a different structure from cutin. It was often within roots. It can protect against pathogens and other damage. It can form transport barriers between the soil and the roots Older parts of roots more suberized A cell wall constituent Endodermis has suberin side walls Pollard, M. et al. , (2008) Building lipid barriers: biosynthesis of cutin and suberin. Trends in Plant Science, 13 (5), 236 -246.

Secondary metabolites End points of metabolism with no strictly defined function Organic compounds that appear to have no direct function in photosynthesis, growth, or respiration, but They protect primary metabolism by deterring herbivores, reduce tissue loss and avoid infection by microbial pathogen They also attract pollinators and seed-dispersing animals, They act as an agent of plant-plant competition They are formed from the byproducts or intermediates of primary metabolism

Three Principal Groups of Secondary Metabolites Terpenes Lipid toxins synthesized from acetyl Co. A or from basic intermediates of glycolysis Phenolic compounds Aromatic substances formed via the shikimic acid pathway or the malonic acid pathway Nitrogen containing secondary products (alkaloids) Alkaloids which are synthesized primary from amino acids

Terpenes They can be produced in response to herbivore feeding, and to attract predatory insects and parasites of the feeding herbivore. They are constituents of essential oils Building block- 5 C isoprene unit They are classified by the number of isoprene units: monoterpenes-1, diterpenes-4 From the wikimedia free licensed media file repository

Phenolic Compounds Secondary metabolites which contain a hydroxyl functional group on an aromatic ring They are heterogenous group: Some are water soluble only in organic solvents Some are water soluble carboxylic acids and glycosides Some are insoluble polymer Many serves as defense compounds against herbivores and pathogens Other function in attracting pollinators and fruit dispensers

N-containing secondary compounds Those are encountered less commonly in plants than the phenolics and terpenoids Those are important in view of their bioactivity as drugs and toxins They are synthesized from aliphatic and aromatic amino acids via shikimic acid pathway Different classes 1. Alkaloids, 2. Cyanogenic Gycosides, 3. Glucosinolates, 4. Nonprotein amino acids

1. Alkaloids Most important nitrogen containing secondary products Often alkaloids are used as medicines for humans cocaine, nicotine, and caffeine used as stimulants and sedatives. Wild tobacco can “sense” which herbivore is feeding on it. It normally produces nicotine (an alkaloid) in response to herbivore feeding. But if nicotine-tolerant caterpillars are feeding, the tobacco produces terpenes instead. These terpenes can attract the predators of the herbivore. From the wikimedia free licensed media file repository

2. Cyanogenic Gycosides Release the toxic gas hydrogen cyanide. plants must have enzymes to break down the compounds and release a sugar molecule yielding a compound that can decompose to form HCN. Glycosides and enzymes which break them down are usually spatially separated (in different cellular compartments or different tissues)

3. Glucosinolates These compounds release volatile defensive substances, “mustard oils”, (often herbivore repellents) Plants like cabbage, broccoli, and radishes (Brassicaceae family) have these. From the wikimedia free licensed media file repository

4. Non-protein amino acids These amino acids are not incorporated into proteins but instead act as protective substances Can “mistakenly” be incorporated into protein and therefore resulting in a nonfunctional protein.

Functions of Secondary Metabolites in Plants The secondary metabolites have no function in the physiology of the plants They are formed as a result of an overspill from the primary metabolism They make a valuable contribution to the relationship between plant and their environment Plant utilized secondary metabolites as antibiotics or signaling agent during the interaction with herbivore (or pathogen) They play an important role in strategies: Structural level, phenyl propanoids are the major component of wall polymers lignin and suberin Inducible defence antibiotics originated from phenolics, and terpenoids (phytoalexins)

Model of the signaling network for plant defense responses to herbivores Herbivore damage can elicit a Signaling Pathway Induced defenses: Recognition of damage to the local area of plant Transmission of alarm signal to rest of plant; Ca+ ions hydrogen peroxide enzymes. Arimura G-I. , et al. (2011) Recent Advances in Plant Early Signaling in Response to Herbivory. Int. J. Mol. Sci. , 12(6), 3723 -3739

Model of the signaling network for plant defense responses to herbivores Model of the signaling network for plant defense responses to chewing arthropod (caterpillars) and sucking arthropods (aphids and spider mites). Arrows and bars indicate positive and negative interactions, respectively. The overall scenario may differ in certain plant taxa. Arimura G-I. , et al. (2011) Recent Advances in Plant Early Signaling in Response to Herbivory. Int. J. Mol. Sci. , 12(6), 3723 -3739

Model of the signaling network for plant defense responses to herbivores However, in general: Chewing arthropods induce JAdependent defense responses, Piercing-sucking arthropods frequently induce SA-dependent defense responses. Red circles effectors molecules Yellow square elicitors molecules Abbreviations: CDPKs Ca 2+-dependent protein kinases GOX glucose oxidase JAs, jasmonates; MAPK nitrogen-activated protein kinase SA, salicylic acid ROS reactive oxygen species Arimura G-I. , et al. (2011) Recent Advances in Plant Early Signaling in Response to Herbivory. Int. J. Mol. Sci. , 12(6), 3723 -3739

Induced biochemical defenses Hypersensitive reactions (phytoalexins, antimicrobial, s leaf spots, active oxygen radicals disrupt cell membranes, reinforcement of cell walls) Antimicrobials – phytoalexins, phenolics Immunization Local and systemic acquired resistance From the wikimedia free licensed media file repository

Induced biochemical defenses Hypersensitive reactions (phytoalexins, antimicrobials leaf spots, active oxygen radicals disrupt cell membranes, reinforcement of cell walls) Antimicrobials – phytoalexins, phenolics Immunization Local and systemic acquired resistance Zeng, L. et al. , (2016) Regulation of the Rhythmic Emission of Plant Volatiles by the Circadian Clock. Molecules, 21, 124 -134

Phytoalexins (Phyto = “plant” and alexin = “to ward off/”) Low molecular mass antimicrobial metabolites synthesized de novo from primary metabolites in response to infection Structurally diverse group of metabolites with the isoflavonoids. The isoflavonoids phytolaexins are synthesized from the flavonoids branch of the phenylpropanoid pathways Zeng, L. et al. , (2016) Regulation of the Rhythmic Emission of Plant Volatiles by the Circadian Clock. Molecules, 21, 124 -134

Production of Phytoalexins Production of phytoalexins may be stimulated by certain compounds called elicitors. High molecular weight substances found in the cell wall such as glucans, glycoprotein, or other polysaccharides. Gases such as ethylene (C 2 H 4). Plants may prevent the formation of phytoalexins, by the action of suppressors The suppressor also can be a glucan, a glycoprotein, or a toxin produced by the plant itself. Zeng, L. et al. , (2016) Regulation of the Rhythmic Emission of Plant Volatiles by the Circadian Clock. Molecules, 21, 124 -134

Optimal defense theory • Considers costs to plants of antiherbivore defenses • Qualitative defenses cheaper than quantitative defenses? • • Maybe just more appropriate – act quickly against specific herbivores • Lots of debate over the years… Morales, M. et al. , (2016) Oxidative Stress: A Master Regulator of Plant Trade-Offs? Trends in Plant Science, 21 (12), 996 -999.

Optimal defense theory • The cost of defense are one of the constraints on leaf form and function • Photosynthetic capacity, nitrogen, longevity, and susceptibility to herbivores are all related • Quantify these things for a costbenefit analysis Morales, M. et al. , (2016) Oxidative Stress: A Master Regulator of Plant Trade-Offs? Trends in Plant Science, 21 (12), 996 -999.

Optimal defense theory Benefit from a leaf = [rate of carbon gain * carbon gain period] minus [carbon cost of growth and maintenance + losses to herbivory] CO 2 exchange can take care of all except herbivory losses Amount of nitrogen in leaf correlates with photosynthetic capacity Morales, M. et al. , (2016) Oxidative Stress: A Master Regulator of Plant Trade-Offs? Trends in Plant Science, 21 (12), 996 -999.

Effects of herbivores can be dramatic!

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