Biotic and Abiotic Stress Physiology – Introduction HORT 301 – Plant Physiology November 19,...

Biotic and Abiotic Stress Physiology – IntroductionHORT 301 – Plant Physiology

November 19, [email protected]

Class Notes

Lectures: Biotic – living, abiotic – nonliving/enviromental

Plant defense against insect herbivores and pathogens

Plant responses to herbicides and weeds (Steve Weller)

Drought adaptation and the role of ABA in water stress tolerance

Temperature extremes, thermo-adaptation and cold acclimation

Salinity

Flooding and oxygen deprivation

Toxic heavy metals and hyper-accumulation

Plant Defense against Insect Herbivores and PathogensHORT 301 – Plant Physiology


Taiz and Zeiger, Chapter 13 (p. 334-344)Additional materials – Taiz and Zeiger, Chapter 13 (p. 315-

334); Web Essays 13.1, 13.2, 13.6, 13.7 and 13.8, Ryan et al. (2007) Curr Microbiol 9:1902

Plant defensive responses against insect herbivores

Plant defensive responses against phytopathogens

Plant defense against herbivores and pathogens - both constitutive (constantly functioning) and induced responses

Induced defensive responses – “activated” in plants based on pest or pathogen recognition and/or herbivore activity or infection


Constitutive defense – secondary products that are “toxins” or insecticides

Insecticidal secondary metabolites - products of biochemical pathways that result in the synthesis of terpenes (terpenoids), phenolics or nitrogen-containing compounds (see Chapter 13, p. 315 to 334)

13.4 A simplified view of the major pathways of secondary-metabolite biosynthesis

isopentenyl diphosphate

Terpenes (terpenoids) – isoprene units (five carbon molecules, isopentenyl diphosphate), mevalonic acid pathway is the most characterized pathway

Terpenoids - growth and development, hormones - cytokinins, gibberellins, brassinosteroids and abscisic acid (ABA)

Essential oils (mint oil, fragrances)

Volatile signaling molecules

Defensive molecules against herbivores, e.g. pyrethoids (natural insecticides), limonoids (bitter taste in citrus), saponins (disturb insect cell membranes), phytoecdysones (natural analogs of insect morphogenesis hormones)

Phenolic compounds – backbone structure derived via the shikimic acid (phenylalanine and tyrosine biosynthesis) or malonic acid pathways

Function in cell wall tensile strength (lignin, retards insects predation), pigmentation (flavanoids, e.g. anthocyanins and other pigments)

Defense against insects (coumarins - simple phenolics, tannins – insect anti-feedants)

13.4 A simplified view of the major pathways of secondary-metabolite biosynthesis

isopentenyl diphosphate

Nitrogen-containing compounds – backbone structure are aliphatic or aromatic amino acids

Alkaloids – contain heterocyclic ring, human drugs and insect toxins

Cyanogenic glycosides – release cyanide, inhibitor of respiration

13.20 Enzyme-catalyzed hydrolysis of cyanogenic glycosides to release hydrogen cyanide

Glucosinolates – mustard oil glycosides, release pungent volatiles that are highly reactive, destroy host molecules

13.21 Hydrolysis of glucosinolates to mustard-smelling volatiles

Induced plant defense against insects – responses are initiated based on detection and herbivore activity

Three types of insect herbivores:Phloem feeders – aphids and white flies, typically damage is minimal unless there is an extreme infestation; however, these insects are vectors for viruses

Cell content feeders – mites, thrips, cause intermediate cellular damage

Chewing insects (herbivores) – larvae of moths and butterflies (lepidopteran insects), grasshoppers and beetles

Plants respond to insect damage more substantially than to equivalent mechanical wounding, i.e. there is also chemical elicitation of plant defense

Elicitors – chemical substances produced by the insect and “sensed” by plants, e.g. chitin, volicitin

Volicitin – elicitor in herbivore insect saliva, fatty acid (plant)–amino acid (insect) conjugates

Fatty acids - obtained by the insect during digestion of plant material and is conjugated to an insect-produced amino acid in the gut

13.23 Concerted biosynthesis of elicitors from plant and insect precursors

Insect ingests plant material, fatty acid (plant) conjugated to amino acid (insect) and processed (hydroxylation to C17) in the gut

Volicitin - N-(17-hydroxylinolenoyl)-L-glutamine

Volicitin stimulates plant defensive responses against insects without mechanical wounding

Plant defensive responses to insects – “activated” by detection of herbivore (elicitation) and damage, production and action of the plant hormone jasmonic acid (JA)

Jasmonic acid (JA) is biosynthesized from linolenic acid by the octadecanoid signaling pathway

13.24 Steps in the pathway for conversion of linolenic acid (18:3) to jasmonic acid (Part 1) 13.24 Steps in the pathway for conversion of linolenic acid (18:3) to jasmonic acid (Part 2)

Octadecanoid pathway – JA synthesis

Jasmonic acid (JA) regulates transcription of genes that encode insecticidal proteins that are ingested by insects during herbivory

-amylase inhibitors – block the starch degrading enzyme -amylase in the insect gut, reduces sugar assimilation

Lectins – bind to carbohydrates and glycoproteins (carbohydrate-containing proteins) in the epithelial lining of insect gut and disturb nutrient absorption

Proteinase inhibitors – serine and cysteine proteinase inhbitors that block the function of protein digestive enzymes

Lipoxygenases – degrade lipids, particularly those in the membranes

Systemic (whole plant) resistance – localized herbivore damage leads to induced plant defense throughout the plant

Solanaceae family (e.g. tomato, potato) – peptide hormone systemin activates jasmonic acid (JA) biosynthesis (octadecanoid pathway) in the phloem companion cells

JA is transpored to the sieve elements (phloem conducting cells)

JA is transported through the phloem to other areas of the plant where it activates plant defensive responses, insecticidal protein gene expression

Insect damage induces synthesis of prosystemin (200 amino acids) in the phloem parenchyma cells, processed to systemin (18 amino acids) by proteolytic cleavage

13.25 Proposed systemin signaling pathway

It is presumed that, yet undiscovered, peptide hormone cascades are involved in insect defense by plants in other families

Systemin is transported to apoplast, interacts with a receptor in phloem companion cells to activate the octadecanoid pathway resulting in jasmonic acid (JA) biosynthesis, JA is transported into sieve elements and through the phloem to “activate” insectidal protein gene expression, plant defense

Tritrophic interaction of herbivores, plants and predatory insects – plants detect herbivores and signal parasitic wasps that are herbivore predators

The tritrophic interaction among host plants, herbivores and natural enemies

Plants respond to herbivore feeding by producing volatile compounds that attract carnivorous/parasitic natural predators of the plant pests.

Courtesy of Keyan Zhu-Salzman

Preditory wasp “attractants” are volatile products of plant secondary metabolism, aldehydes, alcohols and esters, that are likely specific for each herbivore species

-amylase inhibitorslectinsproteinase inhibitorslipoxygenases

Summary of induced plant responses to herbivore attack


Defense gene regulation

Wound signals(local/systemic)

Defense gene expression

Volatileattractant

Volicitin oral secretion

Octadecanoid-jasmonate signal complexEthylen

e


Plant Defense against Insect Herbivores and PathogensHORT 301 – Plant Physiology


Taiz and Zeiger, Chapter 13 (p. 334-344)Additional materials – Taiz and Zeiger, Chapter 13 (p. 315-

334); Web Essays 13.1, 13.2, 13.6, 13.7 and 13.8, Ryan et al. (2007) Curr Microbiol 9:1902


Plant defensive responses against phytopathogens

Plant defense against herbivores and pathogens - both constitutive (constantly functioning) and induced responses

Induced defensive responses – “activated” in plants based on pest or pathogen recognition and/or herbivore activity or infection

Plant defensive responses against pathogens

Constitutive defense – structural chemical barriers and phytopathogenic “toxins”

Induced defense – hypersensitive response (type of programmed cell death) and systemic acquired resistance, including a type of innate immunity

Constitutive defense:

Cutin, suberin and waxes – fatty acid polymers that form barriers to pathogen infection

Secondary metabolites – terpenes, phenolics and nitrogen- containing compounds. phytoalexins

e.g. saponins – triterpenes glycosides that bind to sterols in fungal membranes and disrupt function

Induced defense:

Hypersensitive response – cells adjacent to the infected cell undergo programmed cell death, physically isolates the pathogen away from other living cells

Plant sense/recognize the phytopathogen leading to Ca2+-induced production of nitric oxide (NO) through the activation of nitric oxide synthase and reactive oxygen species through NADPH oxidase

13.26 Many types of antipathogen defense are induced by infection

Nitric oxide and reactive oxygen species (e.g. O2-, OH and H2O2) –

necessary for activation of the hypersensitive response

Plant hypersensitive response is mediated by phytoalexins, lignin, salicylic acid, hydrolytic enzymes and programmed cell death determinants that have not been characterized

Phytoalexins – antimicrobial molecules that are products of secondary metabolism, isolfavonoids, sesquiterpenes, tryptophan-derived camalexin

Hypersensitive response is induced by elicitors – components of bacterial and fungal cell walls, including glucans, chitin fragments, lipopolysaccharides, glycoproteins and proteins

Plants sense/recognize these molecules to induce defense against phytopathogens

A very specific and well characterized interaction is between the fungal elicitor referred to as the avirulence (Avr) protein and the plant “receptor” the R gene product



Avr and R interaction leads to hypersensitive response and systemic acquired resistance (SAR), localized infection that induces defensive responses at the infection site and throughout the plant

Systemic acquired resistance (SAR) – facilitated by the plant hormones salicylic acid and jasmonic acid13.26 Many types of antipathogen defense are induced by infection


Salicylic acid (SA) and jasmonic acid (JA) induce defensive gene expression, products are antimicrobial proteins

Salicylic acid (SA) – functions locally at the site of infection (in solution) and systemically through the phloem, and as a volatile

13.28 Initial pathogen infection may increase resistance to future pathogen attack

Jasmonic acid (JA) induces local and moves systemic defensive gene expression as described previously

SA and JA induction of plant defensive gene (anti-microbial proteins) expression

SA

NPR1

JA ET

EIN2

Defense response(PR genes)

Defense response(?)

Defense response( PDF1.2, Thi2.1,HEL, CHIB)

Erisyphesp.P.syringaeP.parasitica

X.campestris

(?)

wounding

Pythium sp.A.brassicicola

B.cinereaE.carotovora

JAR1

Biotropicpathogen Necrotropicpathogen

Courtesy of Ji-Young Lee

Innate immunity – pathogen recognition (elicitor detection) and plant defense

Pathogen recognition – plant defense is activated by recognition of pathogen-associated molecular pattern (PAMP), plant receptor detects PAMPs

PAMPs are presumed to be β-glucan elicitors, chitin fragments, lipopolysaccharides, glycopeptides and peptides that interact with a plant receptor

First identified PAMPs are small peptides – PEP-13 from Phytophora and bacterial flg22 and elf18

Ryan et al. 2007 illustration describes PAMP recognition, activation of plant defense via JA and SA, and auto-amplification by endogenous plant peptide ligands

Biotic and Abiotic Stress Physiology – Introduction HORT 301 – Plant Physiology November 19,...

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