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BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm --- Week 2: Plant defenses and herbivore feeding Slide 1

BIOS 5970: PLANT-HERBIVORE INTERACTIONS

•  B. PLANT DEFENSES AND HERBIVORE FEEDING •  Week 2.

•  1. The world is green: •  Darwin “noted that sheep of different breeds have

different susceptibilities to plant poisons.” •  “It is no surprise to an evolutionary ecologist that insects

quickly evolve resistance to insecticides. Long evolutionary history has given insects the ability to detoxify a myriad of natural plant poisons, and the potential to evolve resistance to artificial toxins similar to those with which they can naturally cope.”


2. Evolutionary trade-offs:

•  Trade-off between defense and growth •  For example:

•  Allele differences at two loci determine whether clover (Trifolium repens) is cyanogenic and produces cyanide.

•  Cyanogenic plants grow more slowly than acyanogenic plants

•  But this cost is more than compensated for by effective defense against insect and snail herbivores.


3a. Testing hypotheses to explain observations (patterns to processes): •  Initial observation:

•  Woodland ants carry seeds of violets (Fig. 1.1) and Dutchman's breeches (Fig. 2.1) - why?

•  a) Comparative method: •  Ants take seeds to nests (some eaten, some

survive). •  Dicentra has seeds with nutrient-rich

elaiosomes. •  Inconclusive.


3b. Testing hypotheses to explain observations (patterns to processes): •  b) Observation >> hypotheses:

•  Initial observations plus library research suggested 3 alternative hypotheses:

•  (i) Ants are seed predators. •  (ii) Ants remove seeds for nutrient-rich elaiosomes

but are not effective dispersal agents. •  (iii) Ants disperse seeds.

•  Then see that ants keep Dicentra seeds in caches inside their nests.

•  Seeds in ant nests are intact with elaiosomes chewed off. •  However, ants abandon nests frequently.


3c. Testing hypotheses to explain observations (patterns to processes): •  c) Experimental method:

•  Test whether seed survivorship (per capita) is higher for seeds taken by ants than seeds left alone.

•  Need replicated and controlled experiments.

•  Thus a combination of observational, comparative, and experimental evidence is most valuable to answer the original question.

•  see Table 2-1 for the results of an experiment in which ants enhance seed survivorship during germination.


4. Why is the Earth so green?

•  Bottom-up (plant defense) versus top-down control (natural enemies).

•  Adaptation and counteradaptation: •  Plants use a variety of devices to protect roots,

stems, leaves, and seeds (flowers?). •  For example:

•  Cellulose roughage slows digestion. •  Exotic amino acids interfere with protein formation.


5. Herbivore counteradaptation to plant defenses:

•  Herbivores counter-defend with ploys such as: •  Behavioral avoidance.

•  Digestive chemicals that dismantle lethal plant

molecules.


6. Tactics versus strategies:

•  Tactics can respond to particular interactions within strategic, evolutionary, phenotypic constraints.

•  Strategies determine the operation range of various tactics. •  Thus specialization is a feeding strategy, but

alkaloid detoxification is a tactic.


7. Herbivory:

•  Herbivory is not simply the consumption of plants by animals, it is a process that describes the interaction between plant defense and herbivore foraging (Fig.20.1 from Malcolm, 1992).

•  450 million years of evolution has produced huge diversity in both plants and herbivores.


8. Interactions among 3 trophic levels:

•  Tritrophic interactions: •  Like herbivory,

predation is a process that describes the interaction between defense and foraging (Fig. 20.1: Malcolm, 1992)


9a. Plant defenses:

•  (1) Mechanical protection on the plant surface: •  Includes spines, trichomes, glandular hairs (Fig. 3.2).

•  (2) Complex polymers or silica crystals to reduce plant digestibility: •  Digestibility Reducers (Table 3-1):

•  Dose-dependent or quantitative, because the more that are present the less nutritional resource a herbivore receives.

•  Includes cellulose, hemicellulose, and pectin as complex polysaccharides (Fig. 3-3) that can be 80-90% of plant dry weight.

•  As well as lignins, tannins and silica.


9b. Plant defenses:

•  Omnivores & carnivores cannot digest nutrients in the presence of digestibility reducers.

•  So many herbivores require symbiotic microbes associated with digestive modifications. •  Also lignins (complex phenolic polymers) bind to

polysaccharides; waxes or cutins and tannins (also polyphenols but not bound to polysaccharides).

•  Condensed tannins bind to protein and reduce digestion by: •  (i) Blocking the action of digestive enzymes, or, •  (ii) Binding to proteins being digested, or •  (iii) Interfering with protein activity in the gut wall.


9c. Plant defenses:

•  (3) Toxins that kill or repel herbivores at low concentrations:

•  Secondary compounds with a defensive rather than a metabolic function

•  Secondary metabolites or allelochemicals (see Fig. 3-4 for metabolic sources).

•  Qualitative toxins are poisonous and are very diverse (Table 3-1 and Fig. 3-5).

•  Include alkaloids, terpenoids and HCN (Fig. 3-6 common in almonds and cherries etc.) which blocks cytochrome oxidase and hence cellular respiration.

•  See Table 9-2 for evolution of toxic chemicals in plants.


10. Constitutive versus Inducible defenses

•  Constitutive: •  Permanent protection always present:

•  e.g. spines and trichomes as well as many chemicals that reduce digestibility and also function as structural support.

•  They could also include some toxins.

•  Inducible: •  Responses by individual plants to tissue damage:

•  e.g. very widespread proteinase inhibitors: •  Polypeptides and proteins that block catalytic activity of proteolytic

enzymes by binding to the active site of the enzyme molecule.


11. Herbivore Foraging:

•  Scale: size range from aphids to elephants! •  Dan Janzen: “the plant world is not colored green;

it is colored morphine, caffeine, tannin, phenol, terpene, canavanine, latex, phytohemagglutinin, oxalic acid, saponin, and L-dopa.” •  Sensory modality for signal reception. •  “Why do different herbivores eat different

plants?” (page 40). •  Herbivores have mechanical, biochemical,

physiological, and behavioral countermeasures to plant defenses (Table 3-2).


12. Mechanical breakdown of plant food

•  To break open cells: •  Mammals use teeth:

•  low-crowned and high-crowned (Figs. 3-8 & 9-8).

•  Birds use beaks (cardinal) or gravel-filled gizzards (turkey, dodo).

•  Insects use chewing or sucking mouthparts (Fig. 3-9).


13. Microbial symbionts

•  Many herbivores have bacteria, flagellates and protozoans that can synthesize necessary vitamins, break down plant material, and detoxify allelochemicals through anaerobic fermentation.

•  Structural modifications to the gut: •  Foregut (sheep) and hindgut (horse) fermentors

(Table 3-3). •  Ruminants (Fig 3-10):

•  4-chambered stomach: rumen (+reticulum), omasum, and abomasum.


14. Herbivore gut ecosystems:

•  1 ml of sheep rumen fluid includes: •  16,100 x 106 bacteria, 106 flagellates and 3.3 x 105

ciliated protozoans. •  Whole sheep rumen holds 6L ! •  Digestion efficiency (Table 3-4):

•  Depends on volume, retention time and proportion of indigestible material in plant food.

•  Larger herbivores (bison at 450-1,350 kg take 80 hours to process fiber at about 70% efficiency) hold food longer than smaller herbivores.

•  White-tailed deer at 48-100 kg take 45 hours to process food at 56% efficiency.

•  Humans at 60 kg digest only 9% of alfalfa fiber eaten.


15. Figure 3-12: Digestibilities of different forages to mule deer.


16. Insect herbivores:

•  Cannot use large gut volumes and high retention times, so they specialize more and have a variety of ways to use symbiotic microbes (Table 3-5).

•  Insects often have: •  Long guts or elaborate cecae (Fig. 3-13), or, •  Intracellular symbionts in mycetocytes together as

mycetomes (Fig. 3-14), or, •  Fungal symbionts that are cultivated outside their bodies

(like leaf cutter ants and bark beetles).


17. Digestive enzymes:

•  Both general and specific enzyme systems are used.

•  The best known are: •  Mixed-function oxidases (MFOs).

•  These are membrane-bound enzymes that detoxify a wide range of poisons.

•  Vertebrates: •  Highest activity in the microsomes of the endoplasmic

reticulum of liver cells.

•  Insects: •  Mostly in fat bodies or midgut.


18. Characteristics of MFO systems:

•  (1) Catalyze oxidative reactions. •  (2) Nonspecific. •  (3) Easily induced by exposure to novel toxins.

•  They detoxify (Fig. 3-15) by: •  (1) Primary degradation to make water soluble

•  e.g. adding hydroxyl (-OH) groups) •  (2) Conjugation with sugars, amino acids, sulfates, phosphates, or

other molecules headed for excretion. •  This makes toxins soluble and excretable. •  There is generally more MFO activity in insects with

broader diets than those with narrower diets and generalists are better adapted for degrading novel toxins.


19. Choice and Avoidance:

•  Diet breadth spectrum: •  Polyphagous - many food species •  Oligophagous - few food species •  Monophagous - single food species

•  Variable diet breadth poses different sets of problems: •  Most mammals have to be polyphagous, or at

least oligophagous, because they are large. •  But most insects are small and less mobile and

need to be oligophagous or monophagous.


Figure 1.1: Formica podzolica ant holding violet seed by its elaiosome


Figure 2.1: Flowers of Dutchman’s breeches (Dicentra cucullaria) and seed with elaiosome


Table 2.1: Seedling emergence of violets in different treatments


Figure 3.2:

External protection of plants: (a) cactus spines; (b) hooked bean trichomes; (c) potato glandular hairs

a

bc


Table 3.1:


Figure 3.3:

Digestibility reducers in plants.


Figure 3.4: Biosynthetic origins of primary and secondary plant products


Figure 3.5:

Some toxic secondary compounds in plants: caffeine from coffee beans (Coffea), strychnine from Strychnos fruits, and the terpenes, pyrethrin from Chrysanthemum and glaucolide A from a sunflower.


Figure 3.6: Cyanide production by damaged cherry leaves (Prunus).


Table 9.2:


Table 3.2:


Figure 3.8: Low crowned tooth of omnivorous browsing mammal and high-crowned tooth of grazing mammal


Figure 9.8: Diversity of grazing and browsing Miocene horse genera of North America.


Figure 3.9: Insect mouthparts (a) chewing grasshopper, (b) seed-sucking milkweed bug.


Table 3.3:


Figure 3.10: Mammalian digestive tracts: fore & hind-gut fermentors and a carnivore


Table 3.4:


Table 3.5:


Figure 3.13: Grasshopper gut with expanded volumes and ceca for microflora


Figure 3.14: Mycetocytes in the midgut of a chrysomelid beetle


Figure 3.15: Mixed function oxidase (MFO) degradations of toxins: (a) hydrolysis of DDT, (b) N-oxidation of nicotine.