Biotic and Abiotic Stress Physiology – Introduction HORT 301 – Plant Physiology November 19, 2007 paul.m.hasegawa.1@purdue.edu 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 Pathogens HORT 301 – Plant Physiology November 19, 2007 paul.m.hasegawa.1@purdue.edu 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

Plant defensive responses against insect herbivores 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) 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) 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

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

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

Volicitin - N-(17-hydroxylinolenoyl)-L-glutamine 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 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 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 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 It is presumed that, yet undiscovered, peptide hormone cascades are involved in insect defense by plants in other families

Courtesy of Keyan Zhu-Salzman Tritrophic interaction of herbivores, plants and predatory insects – plants detect herbivores and signal parasitic wasps that are herbivore predators Preditory wasp “attractants” are volatile products of plant secondary metabolism, aldehydes, alcohols and esters, that are likely specific for each herbivore species Courtesy of Keyan Zhu-Salzman

Courtesy of Keyan Zhu-Salzman Summary of induced plant responses to herbivore attack Defense gene regulation Wound signals (local/systemic) Volicitin oral secretion Octadecanoid- jasmonate signal Ethylene complex Volatile attractant Defense gene expression -amylase inhibitors lectins proteinase inhibitors lipoxygenases Courtesy of Keyan Zhu-Salzman Courtesy of Keyan Zhu-Salzman

Plant Defense against Insect Herbivores and Pathogens HORT 301 – Plant Physiology November 19, 2007 paul.m.hasegawa.1@purdue.edu 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

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

Courtesy of Ji-Young Lee SA and JA induction of plant defensive gene (anti-microbial proteins) expression 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