Lecture 21 Plant responses to enemies

Context & Lecture Framing

• Sixth lecture in the “Plants” series: focus on how plants respond to their enemies (herbivores, parasites, competitors)

• Lecturer stresses value of face-to-face interaction and broadening introductory material to conceptual, research-level ideas

• Personal research interests of lecturer heavily inform examples

Why Defenses Matter

• Herbivores consume \approx 20\% of global annual plant productivity

• Paradox: despite heavy consumption pressure, the terrestrial world remains visibly green (the “green world hypothesis”)

  • Indicates widespread, effective plant defenses + predator/herbivore control feedbacks

• Historical extreme: Rocky Mountain locust outbreaks

  • Swarms > 100{,}000\ \text{km}^2 (\approx half the area of Victoria, Australia)

  • Threatened 19th-century U.S. agriculture; species went extinct \sim 20 yr after settlers accidentally built cities on breeding riverbanks (anthropogenic habitat destruction)

Categories of Plant Defenses

• Physical / Structural

  • Cacti spines (modified leaves) deter large mammals; cost = reduced photosynthetic surface

  • Trichomes (leaf hairs in e.g., Daphnia experiment): create barriers; cost = lower photosynthetic efficiency

• Chemical Defenses (secondary metabolites)

  • Polyphenols (e.g., tannins)

  • Protein-binding; cause nutrient starvation in herbivores; taste = astringency in tea/wine; mitigated by milk proteins

  • Alkaloids (e.g., nicotine, caffeine, strychnine)

  • Nicotine doubles in tobacco leaves within < 1/2 day of attack; lethal at high doses; insecticidal & vertebrate-toxic

  • Glycosides (e.g., digitalis, 1080/fluoroacetate)

  • Interfere with cardiovascular or respiratory muscles; tiny doses lethal to mammals

  • Cyanogenic compounds in Trifolium repens (white clover)

  • Hydrolysis releases HCN gas; deters snails, mammals; frost-sensitive genotypes \rightarrow altitudinal frequency decline

• Induced vs. Constitutive

  • Constitutive defenses always present; induced defenses synthesized only when risk is perceived \rightarrow saves resources in herbivore-free periods

  • Costs of constitutive expression illustrated by cactus (photosynthesis loss) & cyanogenic clover (frost mortality)

Phenotypic Plasticity as “Plant Behaviour”

• Definition: the ability of a genotype to express different phenotypes in response to environmental/internal stimuli

• Examples previously covered in course (connections):

  • Shade vs. sun morphology (stem elongation)

  • Vessel diameter adjustment under water stress (narrow vessels reduce cavitation risk)

  • Root:shoot allocation shifts in Acacia implexa seedlings (more roots under nutrient/water stress)

  • Diaspore morphology in daisies (larger pappus : seed ratio under poor conditions \rightarrow greater dispersal distance)

Perception of Attack

• Wounding Chemicals / Hormones

  • Jasmonates (jasmonic acid, methyl-jasmonate) \rightarrow rapid up-regulation of defense genes; external application mimics herbivory

  • Ethylene gas: released by touch/wounding; triggers cascade & diffuses to neighbours

• Herbivore-Specific Cues

  • Saliva constituents (e.g., amylase from snails) added to wounds reproduce full induction vs. razor-blade cuts alone

  • Acoustic signals: plants exposed to audio playback of caterpillar chewing sounds increase chemical defenses (mechanism unknown)

• Genotype-Specific Gene Expression

  • Arabidopsis thaliana experiments show distinct transcriptional profiles for chewing larvae, leaf miners, sap-sucking aphids \Rightarrow plants discriminate attacker identity

Tritrophic Interactions (Plant \rightarrow Herbivore \rightarrow Parasitoid/Predator)

• Tobacco detects five-spotted hawk-moth (Manduca) caterpillars and emits volatile blend that attracts specialist Trichogramma wasps \rightarrow parasitoid kills caterpillar

• Tomato example: derivative of jasmonic acid attracts specialist parasitic wasp of beet armyworm

• Egg recognition: Brassica nigra distinguishes egg species on leaf surface

  • Raises volatiles summoning either generalist or specialist egg parasitoids depending on moth species (Pieris vs. Mamestra)

Plant–Plant Signalling (“Screaming Trees”)

• 1980s honours thesis (Ian Baldwin) showed clipped sagebrush induced defense chemicals in neighbouring undamaged plants via shared air stream

• Follow-up sagebrush–tobacco field study:

  • Clipped sagebrush \uparrow ambient methyl-jasmonate \sim 4\times

  • Nearby tobacco leaves \uparrow polyphenol oxidase \sim 4\times

  • Result: 50\% reduction in grasshopper damage on “warned” tobacco

• Everyday implication: perfumes & body sprays rich in methyl-jasmonate may inadvertently signal “herbivore danger” to household plants

Parasitic & Competitive Interactions

• Parasitic Plant – Host

  • Cuscuta (dodder) seedlings orient toward hosts with >95\% accuracy using host volatiles (works even when host visually hidden)

  • Some tomato cultivars (Solanum pimpinellifolium) mount ethylene-mediated thickening/touch response preventing dodder attachment; commercial Heinz variety bred for resistance

• Allelopathy / Novel Weapons

  • Spotted knapweed (Centaurea stoebe) invades N. American grasslands; exudes catechin from roots upon contact with neighbour roots

  • \text{Catechin}{\text{root}} \xrightarrow{\text{seconds}} \text{ROS burst}{\text{target}} \rightarrow \text{cell death (days)}

  • Supports “Novel Weapons Hypothesis”: invaders succeed by producing toxins naive native species cannot detoxify

Community-Level Behaviour: Masting

• Exemplified by sugar maple (Acer saccharum) forests

  • Most years: low seed output \rightarrow squirrels consume nearly all

  • Mast years (synchronous across >10^5\ \text{km}^2): enormous seed rain satiates predators; many seeds escape; squirrel populations boom then crash next year

• Mechanism unresolved; environmental synchrony insufficient over huge range \Rightarrow likely long-distance chemical or signalling cue among trees

Manipulating Herbivore Development for Competitive Edge

• Some plants time defense escalation to coincide with caterpillar molt from 3rd\rightarrow4th instar (most damaging phase)

  • Caterpillars relocate to neighbouring plants, effectively transferring damage; defending plant gains competitive advantage

Costs, Trade-offs & Constraints

• Maintaining “open” defense pathways incurs metabolic cost (e.g., reduced growth, frost susceptibility)

• Adaptive plasticity framework: genotype with high inducibility may underperform in herbivore-free environments but outperform when attacked (reaction-norm graphs discussed)

Ethical, Practical & Real-World Implications

• Not all “natural” or “plant-based” compounds are safe; many are potent toxins at realistic doses (public misconception addressed)

• Biological control: understanding tritrophic signals can enhance pest management (e.g., deploying parasitoid-attracting volatiles)

• Crop breeding: ethylene-responsive or cyanogenic lines for pest resistance; dodder-resistant tomato as commercial case study

• Invasion ecology: early detection of allelopathic invaders and development of tolerant native genotypes or soil remediation

Numerical / Statistical Highlights

• Herbivores remove \approx 1/5 of terrestrial primary production annually

• Rocky Mountain locust swarm record >100{,}000\ \text{km}^2

• Tobacco nicotine doubling time after damage: <0.5 day (\approx 12 h)

• Dodder orientation success: >95\% accuracy

• Sagebrush–tobacco study: grasshopper damage reduced \sim 50\% in warned plants

• 1080 lethality: 4 small West-Australian Acacia leaves \rightarrow fatal human dose

Connections to Earlier Lectures

• Hormonal signalling (auxin, ethylene) \rightarrow parallels in defense signalling (jasmonates)

• Vessel diameter plasticity & hydraulic safety as another adaptive response to environmental stress

• Shade avoidance & resource allocation examples reinforce concept of phenotypic plasticity that underpins inducible defenses

Study Tips & Lab Opportunities

• Practical exams approaching; open labs with lecturers/demonstrators available all week for questions on lecture or lab material

• Encourage integrating lecture concepts with hands-on observations of structural and chemical defenses (e.g., trichome counts, cyanide assays)