CHEMICAL DEFENSES IN PLANTS AGAINST INSECT HERBIVORES
I. Introduction
A. Significance
B. General Attributes of Plant Biochemicals affecting their roles as “Allelochemicals”
1. Chemical Structure
2. Physical Properties
3. Distribution:
4. Biological Activity
5. Inducibility
6. Biosynthesis
7. Analysis
II. Classes of Plant Allelochemicals: Overview
A. Nitrogen containing compounds
1. Non protein amino acids
2. Cyanide and Cyanogenic glycosides
3. Glucosinolates
4. Proteinase Inhibitors
5. Lectins
6. Alkaloids
B. Shikimic Acid Pathway Derivatives
1. Phenolics & Quinones
2. Tannins & Lignins
3. Coumarins
4. Flavonoids
C. Mevalonic Acid Pathway Derivatives: Terpenoids
1. Monoterpenes
2. Sesquiterpenes & sesquiterpene lactones
3. Diterpenes & diterpene acids
4. Triterpenoids
5. Cardenolides
6. Iridoid Glycosides
7. Precocenes & Polyacetylenes
8.
Nitrogen containing compounds:
Non protein amino acids
Chemical Structure R-C-@-H-(NH2)-COOH
Distribution:
Biological Activity
Active at very low doses
Neurotoxins & Antimetabolites
Increase susceptibility to pathogens
Very broad spectrum
Analysis:
Cyanide and Cyanogenic glycosides
Chemical Structure: Contain HCN
Physical Properties
Distribution:
Biological Activity
Inducibility: Damage - released cleavage
Biosynthesis: Derived from phenylalanine, tyrosine, related amino acids
Analysis
Glucosinolates
Chemical Structure: Glucose linked S + N compounds
S - Glucose
/
R - C = N - O SO3Physical Properties
Distribution: Small group @100 compounds; Crucifers:
Variation
Biological Activity: Broad spectrum:
Toxic at very low doses
Inducibility: Rupture of plant tissue - release of hydrolases - toxic components
Biosynthesis: Derivatives of amino acid metabolism (source of R group)
Analysis
Proteinase Inhibitors
Chemical Structure: small proteins @20,000 Daltons
Biological Activity
Inhibit protein digesting enzymes (trypsin- or chymotrypsin- like specificities
Reduce growth, usually non toxic
Active at very high concentrations
Inducibility: Protein Inhibiting Inducing factor (PIIF)
Lectins
Chemical Structure: Very poorly defined from a chemical perspective
Proteins with 2-4 subunits, each of which contains a specific sugar-binding
site. Most contain metals; Most are glycoproteins
Distribution: Universally distributed throughout plant kingdom
Mostly in legumes
Mostly in cotyledons of seeds; Somewhat in roots & leaves
Biological Activity
Active at very low concentrations
Broad spectrum
Insects: Seed predators
Bind to epithelial lining of midgut: Prevent nutrient absorption
Agglutinate insect erythrocytes
Maintains specificity of legume nodule - bacteria relationships
Inducibility: Mostly constitutive
Analysis: very difficult
Alkaloids
Chemical Structure: Cyclic compound containing N in a negative oxidation state
Categories
Benzylisoquinloine Alk; Monoterpenoid Indole Alk; Pyrrolizidine
Alk; Quinolizidine Alk; Alkaloid Glucosidase Inhibitors
Physical Properties
Distribution: Very widely distributed:
Mostly in seeds & roots, growing tissues
Biosynthetic Regulation
Biological Activity
Broad pharmacological effects
Active at low concentrations
Inducibility:
Analysis:
Shikimic Acid Pathway Derivatives: Phenolics
Simple Phenolics & Quinones
Tannins
Chemical Structure: Polymers
Categories: Condensed tannins; Hydrolyzable tannins
Physical Properties
Distribution: Throughout plant kingdom:
Temporal patterns
Biological Activity: Binds proteins
Broad spectrum; Effective at high doses
Inducibility: Increased PAL activity; Induced by broad range of agents
Analysis: Relatively difficult - specific to each plant or phenolic group
Coumarins
Chemical Structure: 2 H - 1 Benzopyran -2-one nucleus
Furanocoumarin: Additional 5- or 6- C ring
Physical Properties
Distribution:
Biological Activity
Very broad spectrum; Toxic at very low doses
Bind to DNA base
Photo-activated: Release oxygen free radicals
Inducibility
Elicited by exposure to UV light;
Analysis
Fungi; herbivory
Flavonoids
Chemical Structure: Aromatic Heterocyclic compounds derived from flavone.
Based on C15 skeleton; Ether as part of ring; Polyphenolic; Most occur as glycosides
Classification: Anthocyanins, Flavones, Flavanols, Isoflavonoids
Physical Properties
Distribution:
Biological Activity: Inhibit digestive enzymes
Analysis
Phenolic Glycosides
Mevalonic Acid Pathway Derivatives: Terpenoids
General: Isoprene Derivatives
Specialized resin ducts and glands
Inducibility
Analysis
Monoterpenes
Chemical Structure
Distribution
Biological Activity
Sesquiterpenes & sesquiterpene lactones
Chemical Structure
Distribution
Biological Activity
Diterpenes & diterpene acids
Chemical Structure
Distribution
Biological Activity
Triterpenoids
Chemical Structure
Distribution
Biological Activity
Cardenolides
Chemical Structure: Steroid with additional 5 member ring; Usually as glycosides
Distribution: 12 families, all angiosperms, mostly perennial herbs
Mostly in phloem (Latex), Mostly in cell vacuoles
Biological Activity:
Broad spectrum
Active at very low doses
Neurotoxin: Inhibit Na/K pump Interfere with ATP ase activity
Biosynthesis:
Analysis
Iridoid Glycosides
Chemical Structure
Terpene derived Cyclopentadoid, O - linked Glycosides
Distribution: 57 plant families (all angiosperms)
Biological Activity: Broad spectrum
Analysis
Precocenes & Polyacetylenes
Precocenes: Juvenile hormone mimics;
Polyacetylenes: Anti - JH activity
========================================
III. Synergists & Additives
A. Synergists
B. Additives
IV. General Sources of Variation in Plant Allelochemicals
A. Interspecific
B. Intraspecific
1. Genetic Vs. Environmental
2. Temporal
V. Induction of Plant Allelochemicals
A. Introduction: Types of chemical changes; Phytolalexins
B. Elicitation
C. Case Histories