1. Biotic and Abiotic Stress Physiology – Introduction
HORT 301 – Plant Physiology
November 19, 2007
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

2. Plant Defense against Insect Herbivores and Pathogens
HORT 301 – Plant Physiology
November 19, 2007
Taiz and Zeiger, Chapter 13 (p )
Additional materials – Taiz and Zeiger, Chapter 13 (p ); 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

3. 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

4. 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)

5. 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

6. Nitrogen-containing compounds – backbone structure are aliphatic or aromatic amino acids
Alkaloids – contain heterocyclic ring, human drugs and insect toxins

7. Cyanogenic glycosides – release cyanide, inhibitor of respiration

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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. Plant Defense against Insect Herbivores and Pathogens
HORT 301 – Plant Physiology
November 19, 2007
Taiz and Zeiger, Chapter 13 (p )
Additional materials – Taiz and Zeiger, Chapter 13 (p ); 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

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

25. 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

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