Herbivory and medicinal plants

Question 1

Plant defence strategies

Answer 1

Mechanical protection on surface of plant
- Spines = cacti, thistles
- Thorns = brambles, rose
- Both deter large animals e.g mammals
- Trichomes (hairs) including glandular hairs, deter small animals e.g insects
- Stinging nettle = sharp stinging hairs act like hypodermic syringe releasing substances into the skin which causes irritation
Complex polymers or silica crystals; reduction in digestibility
- Dosage-dependent substances (quantitative effects)
- Cellulose and hemicellulose
- Pectin
- Lignins
- Cutins and waxes
- Tannins
- Silica
Phytotoxins
- Not particularly dosage dependent (qualitative effects)
- Active substances are secondary metabolites
- Biochemical and angiosperm coevolution
- Presence of compounds characterises groups and even whole families

Question 2

Primary and secondary metabolism

Answer 2

Primary metabolism = carbon and nitrogen assimilation
Secondary metabolism = lipids (steroids, terpenoids), sugars, nucleic acids, amino acids (alkaloids, nitroaliphatics, cyanogenic compounds), proteins

Question 3

Nitrogen based phytotoxins

Answer 3

Non-protein amino acids – canavanine in legumes
Cyanogenic glycosides – form hcn
Glucosinolates – n and s (Brassicaceae)
Alkaloids – e.g morphine from poppies
Peptides – i.e as short as polymers
Proteins – e.g ricin from castor oil
Nitroaliphatics (legumes)
Non-nitrogen based phytotoxins
Iridoids – intermediates in alkaloid synthesis
Terpenoids – menthol, Taxol, rubber, cannabinoids
Cardiac glycosides – e.g digoxin in foxglove
Saponins – natural detergents, form soaps
Furanocoumarins – e.g phytochemicals in parsnip and hogweed
Isoflavonoids – e.g phytoestrogens
Quinones – derived from phenolic compounds
Polyacetylenes – e.g in asteraeae

Question 4

Some plant toxins

Answer 4

Nicotine – alkaloid, solanaceae
Strychnine – alkaloid, poison
Cyanogenic glycosides – release hcn when leaf is damaged
- Milkweeds produce several cardiac glycosides = very bitter, very toxic
- Monarch butterfly caterpillars ingest milkweed tissue and sequester the toxins into their own tissues, toxins survive in tissues into the adult butterfly stage
- Adult butterfly has aposematic colour warning pattern, blue jays learn to avoid monarchs by recognising pattern = only 50% of monarch population needs to be toxic to effect 100% protection to population
- Tiger moths eat the leaves of foxglove and sequester cardiac glycosides = an individual moth may contains both alkaloid and cardiac glycoside in same body
- Moths have aposematic warning colouration
- Some danaid butterflies consume pyrrolizidine alkaloids in addition to milkweed toxins in order to synthesise pheromones
3-nitropropionic acid – inhibits mitochondrial activity
Digitoxin – cardiac glycoside
Ricin – protein, deadly in tiny amounts, inhibits protein synthesis

Question 5

Plant transparency theory

Answer 5

Long lived, late successional species use quantitative plant defences e.g polymers, lignins, tannins, silica
Apparent plants are easy to find by herbivores that employ generalist strategies
Short-lived, early successional species use qualitative toxins e.g alkaloids, terpenes, glucosinolates
These plants are not particularly easy to find and herbivores employ specialist strategies

Question 6

Cyanogenesis

Answer 6

A polymorphic trait in populations of white clover and birds-foot trefoil
Two genes
- G controls synthesis of the cyanogenic glycoside (metabolite)
- E controls synthesis of the glucosidase (enzymes)
4 possible genotypes
- GE = cyanogenic
- gE = not
- Ge = not
- ge = not
Frequency of cyanogenesis in clover is correlated with the mean January temp
Freq = 70-95% when mean temp >5 degrees c
Freq = 25-50% in mid Europe
Freq = 0% in Russia

Question 7

Plants and medicine

Answer 7

70% of pharmaceuticals now in use are derived from natural products

Question 8

Important secondary metabolites in plants

Answer 8

Alkaloids = caffeine, nicotine, morphine
Steroids = hormones and cortisones
Glycosides = e.g salicin related to aspirin
Oils and resins = laxatives and essential oils
Polyacetylenes = potential anticancer
Organic acids = salicylic acid – willow bark
Amino acids, peptides, proteins = ricin
Flavonoids = antioxidant vitamin p

Question 9

Some plant based medicines

Answer 9

Cough mixtures
Laxatives
Inhalers
Painkillers
Powerful drugs extracted directly from plants
Atropine for pupil dilation
Ephedrine for chronic bronchitis
Digitoxin/digoxin for cardiotonic
Morphine for pain
Quinine as an antimalarial
Reserpine as a tranquiliser
Scopolamine as a sedative
Vincristine for anti-tumour
Taxol/paclitaxel for anti-tumour

Question 10

Vincristine and vinblastine

Answer 10

Rosy periwinkle = native to rainforests of se and e Madagascar, powerful anti-cancer properties
Vinblastine has helped increase the chance of surviving childhood leukaemia from 10% to 95%
Vincristine is used to treat Hodgkins’ disease

Question 11

Strategies for surveying the plant world for medicines

Answer 11

Screen everything
- Probs impossible
- Very expensive
- US screened over 35000 tropical rainforest plants since 1956 and US national cancer institude has identified over 1400 substances as potential anti-cancer drugs = few have proven to be commercially successful so far except periwinke
Systemic approach = screen productive taxonomic groups
- Based on phytochemistry and chemosystematics
- Certain types of chemicals are only found in certain taxa of plants e.g digitoxin
- Many people feel this approach unlikely to uncover new drugs
Ethnopharmacological approach based on ethnobotany

Question 12

Important plant families

Answer 12

• Apocynaceae (Periwinkles)
• Asclepiadaceae (Milkweeds)
• Berberidaceae (Mandrake or May Apple)
• Dioscoriaceae (Yams)
• Liliaceae (Colchicum)
• Moraceae (Cannabis)
• Nyssaceae (Camptotheca or Happy Tree)
• Papaveraceae (Poppies)
• Rubiaceae (Cinchona)
• Rutaceae (Rues)
• Scrophulariaceae (Foxglove)
• Solanaceae (Nightshades)
• Taxaceae (Yew)
  Biochemical co-evolution = presence of compound characterises groups and whole families

Question 13

Ethnopharmacology and ethnobotany

Answer 13

WHO has listed 21000 plants used for medicinal purposes worldwide
Estimated that 80% of people in less developed countries rely on traditional medicines for primary healthcare
Study of plant used by ethnic cultures= ethnobotany
Study of drugs derived directly from natural sources used by ethnic cultures = ethnopharmacology
e.g arrow poisons as muscle relaxtant, ordeal poisons used in eye disorders and Alzheimers, ethnic medicine such as prostratin as an anti-viral could potentially treat hiv, cocaine as anaesthetic, steroidal sapogenins for oral contraceptives, cortisone and hydrocortisone