Chemical Ecology & Secondary Metabolism

iChip Technology & Novel Antibiotics

• Uncultured bacteria make up 99% of all species in external environments, representing an untapped source of new antibiotics.
• iChip Technology: Cultures bacterial species within their soil environment using tiny diffusion chambers.
  • The soil is diluted (in agar, nutrients) -> only a single cell grows in a well.
  • The chip is enclosed in a semipermeable membrane and buried in soil to allow in nutrients.
  • Limitations: * Does not allow synergistic partners to grow.
    • Requires a substantial amount of water to prevent drying.
    • Creates anoxic conditions on the chip sometimes, due to the buildup of sediments.

Teixobactin

• Discovered from 10,000 isolates from iChip cultures.
• Isolates were put in plates overlaid with S. aureus to check for antimicrobial activity.
• beta-proteobacteria E. terrae showed good activity. Its genome was sequenced using 16S DNA and in silico DNA-DNA hybridization. It belongs to Aquabacteria (Gram-negative).
• The partially purified active fraction underwent 1242 Da mass spec isolation to yield Teixobactin.
• Mechanism & Target:
  • Has extensive activity against Gram-positive bacteria, including drug-resistant strains.
  • Inhibits peptidoglycan synthesis, but lacks resistant mutations.
  • The target could be a lipid (not a protein).
  • Shows dose-dependent binding with lipid I, II, and III (undecaprenyl-pyrophosphate).

Clovibactin

• Sourced from unculturable bacteria.
• Prevents peptidoglycan synthesis and kills Gram-positive bacteria.
• The method was found out using: biochemical assays, solid-state NMR, and atomic force microscopy.
• Utilizes an unusual hydrophobic interface to tightly wrap around pyrophosphate -> bypasses variable elements.

Chemical Signalling & Quorum Sensing

• Types of Signalling:
  1. Paracrine: Acts locally between neighbouring cells.
  2. Autocrine: Signalling cells can bind to the ligand, target cell can be similar/same.
  3. Endocrine: Acts on distant cells (from endocrine organs).
  4. Gap Junction: Connections between plasma membranes allowing diffusion of intracellular mediators.

Quorum Sensing (QS)

• Cell-to-cell communication to coordinate behaviours in an environmental and cell-density-dependent manner.
• Uses secreted autoinducers (AI) for intra-group communication. These range from small hydrophobic molecules (acyl homoserine lactone - AHL) to larger peptide-based molecules. AHLs, for example, are transcriptional regulators.
• The type of QS depends on cell wall physiology:
  • Thin cell walls (Gram-negative): Hydrophobic N-acyl-homoserine lactone (passive diffusion).
  • Thick cell walls (Gram-positive): Large peptides (active transport through ABC Transporters).
• Mechanism Loop: Autoinducers move out of the cell -> bacteria reproduce -> autoinducer hits critical mass -> Extracellular conc. > Intracellular conc. -> negative feedback loop & other functions.

Examples of Quorum Sensing

1. Aliivibrio fischeri & the Bobtail Squid

• Bacteria reside in the squid’s light organ -> produces light.
• Day -> Night cycling renders the squid invisible.
• The gene regulation of light production is under control of a regulon (operon).
• Binding of LuxR at the site => gene expression (Luciferase -> Light).

2. Vibrio cholerae

• Uses QS for virulence; builds biofilms to help transport nutrients.
• Regulates responses to autoinducers via LuxO.
• Low cell density: Autoinducer receptors act as kinases -> Transfers phosphate to LuxO -> LuxO~P -> promotes expression of biofilm-forming proteins.
• High cell density: [Extracellular AI] > [Intracellular AI]. AI receptor turns from kinase to phosphatase. LuxO~P becomes inactivated LuxO -> biofilm genes get inactivated.

Autoinducer Identification & Mechanism

• Identification: AI is released only in high cell density. Conditioned media was collected from high cell density culture -> Fractionation -> Each fraction tested in diff. cultures with low cell density -> Culture that induced light production -> isolation of compound & structure determination: homoserine lactone.

• Mechanism: DNA from V. fischeri lysed -> pieces of DNA transferred into E. coli plasmids -> check for glowing E. coli (recombinants).

• Isolation of fragment from glowing E. coli -> sequence to find genes. It coded several genes: (1) enzyme that makes AI (LuxI), (2) receptor for AI (LuxR), (3) light-producing proteins.

• Important Assumption: All genes were assumed to be present on the same fragment—or that there was no dependence on other genes to regulate. But what if there are additional genes?

• Transposon Mutagenesis: Used to induce random mutations into V. fischeri genome. Loss-of-function mutations would identify regulating genes & also characterise LuxI and LuxR.

  • Assumption: regulation would be binary.
  • Results showed that all genes that had Transposon insertions were light-synthesising enzymes, but never LuxI/LuxR.
  • This made identification of LuxR difficult, but LuxR had to be present or high cell density control wouldn’t function.
  • Decided to look for mutants that were dim but not dark. These mutants were not defective in luciferase genes, but were defective in proper regulation of light production.
  • V. harveyi was used for this process, since they are non-symbionts & can be cultured.

• Types of AI: Different AIs for communication encode different info about neighbours (e.g., ratio of Intraspecies pop to Interspecies).

  • AI-1: intra-species
  • CAI-1: intra-genus
  • AI-2: inter-species

• Communication Disruption: Certain bacteria secrete enzymes that disrupt others’ communication (consuming, clipping, or inactivating competitor’s autoinducers) to increase growth.

  • Example: Certain species of Bacillus -> secretes lactonase -> cleaves carbon rings of homoserine lactone AI. The AiiA gene can be used to protect from plant pathogens.

Microbial Volatile Organic Compounds (MVOCs)

• VOCs (Volatile Organic Compounds): alkenes, ketones, alcohols, benzenes, pyrazines, sulfides, terpenes.
• Act as novel communication infochemicals. Volatility allows faster transmission over longer distances -> much more important than non-volatile infochemicals. Found in Bacteria, Plants, Protists, Fungi.

An Example: P. aeruginosa

• Autoinducer: 2-aminoacetophenone (2-AA). Small VOC, sweet grape-like odor.

(Annotation: Structure shows a benzene ring with an attached acetyl group (-C(=O)CH3) and an adjacent amino group (-NH2).)

• Usually present in difficult to treat P. aeruginosa infections. Is it involved in antibiotic tolerance?
• Mechanism: 2-AA modulates translational capacity of the cell -> reprogrammes metabolism toward low-energy, stress tolerant state -> leads to increase in the number of persister cells -> increased survival during antibiotic exposure without inducing genetic resistance.
  • (Annotation: Persister cells are subpopulations of cells that resist treatment by changing into a state of dormancy = antimicrobial tolerant).
• Host Effects: Mice treated with 2-AA before infection showed 90% survival vs. 10% in control. However, CFU loads (Log CFU) in 2-AA mice remain higher/persist longer in untreated animals.
  • 2-AA dampens lethal inflammatory response without clearing bacteria -> shifts cytokine balance -> promotes host tolerance & pathogen persistence.
• MVOC signal host attractant: P. aeruginosa -> secretes 2-AA -> facilitates attraction to food for several fly species. Long-term colonisation of fly intestine disseminates bacteria to new locations.

Chemical Communication & Semiochemicals

• Advantages: Effective over long dist., works day & night, longer-lasting, metabolically cheaper.
• Disadvantages: Requirement of specialised receptors for sensing, not effective in upward wind direction.
• Semiochemicals: Infochemicals that convey messages between organisms of same/diff species. Secondary (2-degree) metabolites produced as byproducts from primary metabolism.

Classification

1. Intraspecific: Trail, alarm, interactions, marking.
2. Interspecific:
   • Kairomones: Chemical signal that provides adaptive advantage to receiver. (e.g., 1-octen-3-ol from bovine host attracts parasites).
   • Synomones: Chemical signal that benefits both receiver and emitter. (e.g., strigolactones in pollination).
   • Allomones: Adaptive advantage to emitter. (e.g., bombardier beetle).

Interaction Flowchart Annotations:

• Sender (—) –[Kairomone]–> Receiver (+++)
• Sender (+++) –[Synomone]–> Receiver (+++)
• Sender (+++) –[Allomone]–> Receiver (—)

Communication Signals

• Signals can be Auditory, Chemical, Tactile, Visual.
• Communication: Sender to receiver -> causes a change in receiver (inter-specific or intra-specific).
• Medium (if chemical): Long range, slow, flows around barriers, low cost, high specificity.
• Purpose of a signal: Communicating quality -> alters receiver’s beliefs/behaviours in ways that benefit the sender.

Types of Signals

• Index Signals: Directly related to physical characteristics and cannot be faked. Tightly constrained by earlier developmental conditions or current physical state. Physically impossible for lower-quality individual to produce signal associated with higher-quality individual.
• Handicap Signals: Communicative ads are reliable because they are costly and wasteful to produce. Can be produced by senders in poor condition—but not repeatedly, or at the intensity required to be effective; often reduces survival chances. Honesty is maintained because the cost of signalling is disproportionately higher for low-quality individuals than for high-quality individuals.
• Referential Signals: Specific communications associated with distinct objects, events, external stimuli. Convey specific information about referents (e.g., a specific type of predator), allowing the receiver to react with a functionally appropriate behaviour, even if they cannot see the threat themselves.

Examples: Trail Pheromones in Pharaoh Ants

• Poison gland -> Trail pheromone -> guides nestmates to food sources.
• Dufour’s gland -> Nest relocation pheromone -> guides nestmates to new nest site.
• Both are chemically different; one leads to food & other leads to new nest. In this case, receivers respond differentially to sender’s signals depending on the nature of the signal.

Cue vs. Signal

• Signal: A perceivable feature that has evolved specific characteristics conveying information about the signaler or signaler’s environment. Feature -> used as signal -> because receiver uses it as a guide.
• Cue: Any feature that an organism can use as a guide to display particular behaviour. Feature -> used because receiver has info on sender.
• Dual Functionality: A feature may function as a signal as well as a cue.
  • Example: Root exudate is a class of carotenoid-derived molecules called strigolactones (SL) that adapt shoot & root architecture to env., and enable root-parasites & symbionts to detect hosts. Used as a signal by fungal symbiont, but used as a cue by parasite.
• Exaptation: Non-communicative compounds (unintentional unavoidable metabolic byproducts) can be exapted into attractants/signals.
  • They remain honest signals because: (1) production is constrained by metabolic processes, or (2) production is condition-dependent (costly/hard to fake).
  • Condition-dependent signalling tells us cheating can happen, but it cannot stabilise in a population because the marginal cost is disproportionately high for low quality individuals. It demands abnormally high quantities of compound without underlying metabolic state & associated costs.
  • Signals originally start out as cues without receiver-specific info, then slowly become signals that carry receiver-specific info and alter behaviour. Sender’s precursors can also become signals (e.g., defense volatiles become alarm pheromones for ants; isoprenyl acetate from sting gland).
  • Exaptation can also mean amplification of an existing internal communication system. Example: plants release VOCs as intra-specific defense system to herbivory -> interspecific defense signal.
  • Mechanisms of change: production in larger quantities, addition of volatile compounds, addition of behavioural compounds, modification of time & place of emission to coincide with receiver.

Plant Pollinator Interactions

Plants rely on animals for pollination & seed dispersal. Strategies to attract specific pollinators must be discussed.

• Bee Visual Ecology Adaptations: Process images 5x faster, red-blind but BG/UV, detects polarised light for directionality, daytime land on flowers to collect pollen.
• Bee Eye Structure: Daytime eyes or apposition eyes.
  • Single lens with 3 smaller eyes on centre-top of head (ocelli) that helps navigate, maintain stability, judge light intensity and stay oriented.
  • Ommatidia made of thousands of ommatidium. Each is a tiny eye containing: (1) corneal lens (facet), (2) crystalline cone (focuses light), (3) photoreceptor cells (retinula cells), (4) Rhabdom (light-sensitive rod formed by photoreceptors), (5) Pigment cells (isolate light from neighbouring ommatidia).
• Superposition eye: Light from many neighbouring facets onto a single rhabdom -> increases sensitivity, but decreases resolution (moths, beetles, crustaceans).
• Bat Pollination: Investment is significantly higher since they are large pollinators needing a large quantity of nectar. In Agave, which produces large flowers with 0.5-0.7 ml of nectar/night, flowering is so costly that the plant dies shortly after.

Visual Processing (Rhodopsin)

Colour is due to the differential reflection of some but not all wavelengths of light.

• In light, rhodopsin (11-cis-retinal + opsin) -> all-trans-retinal.
• Opsin changes shape -> metarhodopsin II.
• Activated receptor acts on G-proteins -> initiates neural signals.
• Reaction: R-C(=O)H + H2N-(CH2)4 -> R-C=N-(CH2)4 -> R-C=N-(CH2)i1-opsin

Floral Pigments

Largely due to pigments in the vacuole / chromoplast of floral tissues.

• Anthocyanins (mostly cyanic, purple, red-blue)
• Carotenoids (mainly yellow, some orange & red)
• Quinones (brown, red, yellow)
• Chlorophyll (green)
• Betaine alkaloids (yellow, red)

Carotenoids

• Tetra terpene pigments orange/red/yellow conjugated str. Absorb 400-500 nm reflect in yellow, orange, red.
• Functions: Absorb light energy for photosynthesis, photoprotection by quenching. Carotenoids that contain unsubstituted beta-ionone rings can be converted into retinol by body -> vitamin A activity.
• Quenching Pathway: Light -> Chlorophyll I -> IChl* (high energy/short-lived singlet).
  • Can go via photochemical quenching via resonance energy transfer.
  • Or decay via intersystem crossing -> 3Chl* (half-life, energy) -> quenched by Carotenoids (Non-photochemical quenching) to prevent energy transfer to other molecules. Plants form singlet oxygen species (an oxidant) during photosynthesis; carotenoids quench them.
• PsbS is a pH-sensing protein of PSII that detects lumen acidification under excess light -> induces conformational changes in antenna complexes, enabling non-photochemical quenching of excitation energy as heat.
• Blue light filtering: Lutein, meso-zeaxanthin & zeaxanthin absorb 90% of blue light due to density at macula -> protects internal str. from light-induced oxidative damage by short wavelength (age-related eye disease link).

Anthocyanins

• C6-C3-C6 structure.

(Annotation: Shows a flavylium cation core (three rings) with R1, R2 functional groups and various OH groups.)

• Methylation causes a red to blue shift (reddening effect on colour) + increases stability of chromophore. All anthocyanidin glycosides are called anthocyanins. Rarely, loss of 3’OH group -> shift to shorter wavelength (deoxy anthocyanidins).
• Colour Shifts based on Acidity: Anthocyanins function as weak diacids (because of electron-withdrawing pyrilium) and as a result the OH groups are fairly acidic. As pH increases, de-protonation occurs, absorption maxima shifts towards higher wavelengths.
  • pH < 3: Flavylium cation (red to orange colour).
  • pH 6-7: Quinoidal anhydrobases (violet colour).
  • pH 7-8: Anionic quinones (blue).
  • pH > 11: Unstable dianionic forms (blue-green).
• Blue colour mechanism: Anthocyanin alone never exhibits blue in neutral/weakly acidic environments -> remains purple & loses colour rapidly. In heavenly blue flowers, this is brought about by increase in vacuolar pH (6.6-7.7), caused by a tonoplast-located Na+-H+ exchanger.

Copigmentation

• Non-coloured chemicals enhance colour of pigments in sol. E.g., complexation of red anthocyanins with Al3+ -> blue in blueberries. Tanning can also affect colour.
• Anthocyanins are large planar compounds, form non-covalent interactions with co-pigments to stabilize coloured species by blocking hydration of flavylium cation.
• Intermolecular: E.g., cyanidin 3-O-glucoside & quercetin. Ratio of anthocyanin:flavonol in petals -> diff colours. Affinity: p-coumarate < caffeic acid < ferulic acid < sinapic acid.
• Intramolecular: Hydroxycinnamic acid (HCA) decoration disables hydration reaction. HCA attachment position directs aggregation.
• Fuzzy Metal Complexes / Metalloanthocyanins: Anthocyanins + multivalent metal cation stabilised by external co-pigmentation interactions -> blue only in aqueous sol. Fixed ratio of 6:6:2 -> six anthocyanins : six flavone molecules : two metal cations.

Other Pigments

• Betalains: Betacyanins -> water-soluble pigments in vacuoles. Different from anthocyanins, found in Caryophyllales.
• Anthochlors: Secondary plant metabolites (chalcone / aurone). UV nectar guides. UV-absorbing petal zones -> orientation value to pollinating insect.
• Chromoplasts: Produce, store, display pigments, chiefly carotenoids in fruits, plants, vegetative tissue.
  • Types: Globular (mangoes - carotenoids in spherical droplets/plastoglobuli), Membranous (daffodils - concentric layers), Crystalline (carrots - solidified into distinct crystals), Fibrillar (tulips - dense bundles of fibrils & tubules).

Pollinator-Induced Adaptation & Speciation

• Groups of genes function to regulate transcriptional switch in pigment biosynthesis. Pollinator-induced adaptations can cause speciation.
• M. cardinalis vs. M. lewisii:
  • YUP gene determines colour. YUP switches off red pigment by suppressing carotenoid accumulation in petals.
  • M. cardinalis: Inactive YUP -> carotenoids found throughout petals & flower -> red -> bird pollinators.
  • M. lewisii: Active YUP -> carotenoid decreases -> pink flowers -> bee pollinators.
  • Causes assortative mating influenced by pollinators.

Floral Scent & Deceptive Pollination

• Relatively low conc. of odours work to attract pollinators; directed production for maximum efficiency.
• Diurnal variations: maximum scent when pollen is ripe & ready for pollination. Reached at noon for day-pollinators, at dusk for night-pollinators.
• Pollinators are often specialised for a particular plant, & hence, colour is often not sufficiently specific to serve as identification. (Compounds like Cinammaldehyde, eugenol, citronellal, geraniol).
• Functions: distance attractant, landing cues, repellant, feeding cue, nectar guide.
• The Trade-Off: Floral scents are very rarely detectable only by mutualists; they often attract antagonists, such as herbivores or florivores. VOCs often evolve under selection for both reproduction & survival. Enhanced floral fragrance increases the attraction of detrimental florivores, which decreases reproduction.

(Graph Annotations: X-axis = VOC emission, Y-axis = Fitness. If herbivores are rare, fitness increases as VOC increases. If herbivores are abundant, fitness decreases as VOC increases.)

Context-Specific Cues & Sensory Bias

• Primary metabolites like CO2 may also func. as pollinator attractant. In Datura, high CO2 during the first 30-60 mins coincides with nectar availability. Floral CO2 + nectar used to evaluate cues.
• Context-dependent Information: Methyl-salicylate (MeSA) -> anti-aphrodisiac; mated females acquire it from male Pieris napi. Also initiates hunting behaviour in carnivorous mites (% predator attracted to beta-myrcene & MeSA).
• Odor-mediated Push/Pull in Australian Cycads: Cones are thermogenic, synchronize mid-day heat production with dosage-dependent pollinator attraction & repellance by same volatile compounds (pollinated by Cycadothrips with (E)-beta-ocimene). Pulls pollinators via visual/odor cues, and increased scent emission repels pollinators after pollination -> depart cones.
• Sensory exploitation suggests pollinators select for VOCs that are primarily used in ecological contexts other than flower visitation. Deciphered via pre-existing bias: 4 methoxylated aromatic compounds act as sex/aggregation pheromones as well as floral attractants. Explained by sensory drive: pollinators select for volatiles for which olfactory receptors and/or preferences have not evolved in pollination context, since pheromone functions are evolutionarily older.
• Example: Plants release herbivore-induced plant volatiles (HIPVs) like GLVs -> Social wasps use it to hunt caterpillars -> plants co-opted them to recruit these wasps as pollinators. Might reward visitor with nectar, or sometimes no reward (Epipactis releases terpenoids/alarm pheromone of aphids to attract hover flies to lay eggs on flowers; they feed on aphids + pollinate rewardless flowers).

Deceptive & Nursery Pollination

• Deceptive Pollination: Sexual mimicry offers false mating opportunity to potential pollinators. Can be sex-pheromone mimicry (alkenes, long-chain dienes & trienes) in Ophrys speculum.
• Nursery pollination: Involves pollinators that lay eggs on the flowers they pollinate & brood feeds on flower parts / developing ovules. Floral VOCs used to attract pollinators. E.g., fungus gnats in cup-shaped male flowers (hard to escape) and female flowers (impossible to escape).
• Fig-Wasp Mutualism Flow:
  1. Pollen-laden female enters fig through ostiole.
  2. Female lays eggs in some of the flowers & pollinates some female flowers in the syconium.
  3. Ovaries with larvae form enclosing gall-like str., others produce seed for the fig plant.
  4. As fig matures, male wasps emerge first & travel through syconium & fertilise still enclosed females. Male flowers mature by the time female wasps emerge.
  5. Wingless males dig escape tunnels for females & die.
  6. After collecting pollen from males, females escape through funnel.
  7. Mated female wasps enter another fig tree’s syconium & die.
• Each fig wasp interaction is unique in terms of the VOC profile + style length adjusted for successful oviposition. Partners depend on each other for survival; association between specific partners must be renewed at each gen.

Seed Cues & Dispersal

• Seed dispersal is the movement of diaspores away from parent location (multiscale, multipartner process).
• Dispersal is costly (energy, time, risk, opportunity) and evolutionary pressures should select against it. However, dispersal is selected for due to: spatiotemporal variation in local env. conditions, sibling- or parent-offspring competition, conspecific competition, soil-borne pathogens specialised on maternal plant, and insect seed predators specialised on host plant species. Allows a plant to take advantage of ephemeral habitats.
• For flowering plants, dispersal mech is usually through visual cues of ripe fruits - esp. for mammals.

Mammalian Vision Evolution

• Old world monkey ancestor: Nocturnal earlier, night vision does not benefit from colours. Dichromatism: 2 out of 4 opsin genes lost (kept SWS1-blue and one M/L-greenish opsin).
• Trichromatism in Old World Primates: M/L opsin gene on X chromosome underwent accidental duplicative mutation of green-sensitive opsin gene. Further mutation -> shift of the new gene’s sensitivity to red.
• (Note: Tetrachromatic vertebrates have 4 opsin genes: SWS1-UV, SWS2-blue, RH2-green, LWS-red -> broad colour discrimination).

Dominant Sensory Cues by Disperser

• Birds (Ornithochory): Bright colour (UV/red) & shape.
• Fruit bats: Strong odour (night active).
• Primates: Odor + handling + moderate colour.
• Ungulates: Odor + bulk feeding.
• Rodents: Odor + handling.

Plant Manipulation & Chemical Filtering

• Plants manipulate frugivores using volatiles (e.g., terpenes) as attractants and sugar-rich pulp as reward. Emission rates peak before pollination & at ripening. Flowers, leaves, fruits have diff. VOC profiles (e.g., Mangoes have terpenes, octalactone, ethyl butanoate + urushiol group -> contact dermatitis).
• Sensory Bias: Inherent bias in perception & preference. Example: Aquilaria sinensis (canopy tree) capsules open high in canopy -> seeds suddenly exposed to sun, temp, humidity on ripening -> viability drops. Needs fast dispersal! Achieved through emission of highly volatile short chain compounds mimicking HIPV (herbivore-damaged leaves) to attract Vespa hornets. Olfactory signals dominate when time is critical.
• Chemical mediated filtering of vertebrate seed predators:
  • Capsaicin: Interacts with TRPV1 receptor -> pungency/temperature. Protects from microbial infections & invertebrate attack. Selectively discourages mammalian seed predators (who chew) and avoids deterring beneficial dispersers (directed deterrence hypothesis). Found only in fruits of Capsicum sp.. Structure: Fatty Acid Part + Vanilla-like Part.
  • Hypoglycin: Eliminates mammalian predators. Hypoglycin A -> keto analog -> MCPA-CoA (poison). Inhibits acetyl-CoA dehydrogenase. In litchi too, unripe fruit intake inhibits gluconeogenesis. MCPG -> MCPF-CoA. Acetogenins block NADH-ubiquinone reductase (complex I) of resp. chain, directly affecting electron transport in mitochondria -> decreases ATP -> apoptosis.
• Laxatives etc. might be modulated to change seed retention in gut (artificial fruit exp. with W. solanacea seeds).

Secondary Metabolites & Plant Defenses

• Plants are sessile. Abiotic & biotic stress -> defenses (secondary metabolite). Over 200,000 PSMs accumulate at increased levels on exposure to ensure competitiveness & survival.

• Direct Defence: Thorns, waxes, PSM.
• Indirect Defence: Volatiles, Extrafloral Nectar (EFN).
  • EFN is induced by herbivory -> secretes sugar/amino acid nectar -> attracts ants -> they disturb, kill, remove small arthropods/herbivores. EFNs are unrelated to pollination; functions include attracting bodyguards, luring ants away from floral nectar to prevent pollination interference, and resource provision prompts ants to build nests, enhancing plant nutrition. Ants are interested because EFN is easily located & persistently available.
  • HIPVs are usually C6-18 alcohols, aldehydes, esters, terpenoids, GLVs, phenylpropanoids, S/N-containing compounds. Without herbivory, very low amounts of VOC detected. VOC profile changes with type of herbivore.

(Annotation: Herbivory -> Ca2+, ROS -> MAPK signalling -> Jasmonates -> Defence responses. The graph shows events rising over a timeline of seconds -> minutes -> hours -> days.)

Inducible vs. Constitutive Defense

• Induced defence: Expressed only in response to herbivory. Cost saving strategy; evolves only when herbivore pressure is variable & current/past attacks predict future ones. When pressure is variable, yields higher plant fitness.
• Constitutive defence: Under high & constant herbivore pressure, induced defense has lower fitness due to herbivore damage that occurs during delay in induction, and the costs associated with signalling cascades. Thus, constitutive defenses would be more adaptive.
• Risk Assessment: Plants assess risk of herbivory to make decisions. (1) When risk is high: early season herbivory causes them to become more resistant compared to others. (2) When risk is low: lack of early damage -> decreases risk of future damage. Sometimes herbivory indicates future lack of herbivory (e.g., scarlet gilia increase flowers/fruits after being grazed by elk). Plastic responses favoured if plants can respond appropriately to reliable info.

Defense Allocation Theories

• Apparency Theory: The more visible/apparent a plant is, the more heavily it is defended against generalist herbivores.
  • Easily visible -> Quantitative defense (cellulose, lignins, silica, phenolics, tannins).
  • Unapparent plants -> Qualitative defenses directed to specialists (alkaloids, cyanogens, Terpenes).
  • Biotic Stress (Greater lifespan = increased apparency) -> constitutive defenses. Abiotic Stress (lower resource availability) -> smaller sizes, delayed/early flowering -> reduced attractiveness to pollinators & herbivores -> inducible defenses.
• Optimal Defense Theory: Plant’s limited supply of defensive compounds is concentrated in regions where it would increase its fitness the most. E.g., stems & flowers are the most defended (100% constitutive), while roots rely on induced defenses.
• Resource Availability Hypothesis: Species adapted to resource-rich regions have faster growth rates, lower leaf lifetimes, lower amounts of constitutive defences, and support higher herbivory rates (invest more in inducible defenses). Slower growth/longer lifetimes in resource-poor environments allow high investment in constitutive defenses.
• Growth-Differentiation Balance Hypothesis: Trade off between growth (cell div) & differentiation (secondary metabolites).
• Carbon-Nutrient Balance Hypothesis: Defenses determined by available soil nutrients; High N -> N-based defenses.

PSM Toxicity & Insect Specialization

• Nitrogen enters plant metabolism via soil-nitrates or N2 -> hormones, GFs -> secondary PSMs / allelochemicals (alkaloids, amines).
• PSMs are usually highly toxic. Classes:
  • N-containing: Alkaloids, Cyanogenic glycosides, Non-protein amino acids, Glucosinolates (hydrolysed by myrosinase upon tissue damage -> volatile repellants).
  • S-containing: Phytoalexins, Glutathione.
  • Phenolics: Coumarin, Furanocoumarin, Flavonoids, Lignin.
  • Terpenes: Monoterpenes, Sesquiterpenes, Triterpenes.
• Insects can be polyphagous (any plant), oligophagous (few related sp), or monophagous (single plant sp). Host plants may be different in morphology but similar in PSMs. Insects often evolve to detoxify a repellant & convert it to an attractant, becoming completely dependent on it.
• Example - Bombyx mori (Silkworm): Monophagous on the mulberry plant. Attractants/biting factors include morin and isoquercitrin.
  • Plant releases deoxynojirimycin (DNJ). DNJ is an alpha-glucosidase inhibitor (AGI) that mimics alkaloids.

(Annotation: DNJ structure is a piperidine ring containing an -NH- group, substituted with several -OH and hydroxymethyl groups.)

* It is toxic to other insect caterpillars, but has no toxicity on the silkworm. Silkworms have a less affected beta-fructofuranosidase.
* DNJ derivatives used medically: Miglitol (N-hydroxymethyl derivative of DNJ) is a 2nd gen AGI for type-2 diabetes. Swainsonine is a competitive inhibitor of Golgi alpha-mannosidase II (metastasis inhibitor). Gymnemic acid blocks sweet taste receptors.