Lecture 31 Plant Defense and Secondary Metabolites Alkaloids and Flavonoids

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Lecture 31 Plant Defense andSecondary
Metabolites Alkaloids and Flavonoids
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Three Groups of Secondary Metabolites
Limonoids Saponins Pinene
Flavonoids Anthocyanins Salicylic acid Lignin
Alkaloids Nicotine Morphine Cocaine Caffeine Gluco
sinolates
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Alkaloids

• Low Molecular weight Nitrogenous compounds
• 20 of plant species contain alkaloids
• Mainly involved in plant defense against
  herbivores and pathogens
• Led to rapid utilization by humans
• Synthesized from lysine tryptophan tyrosine
• Most are alkaline
• Increase in response to herbivore attack
• Alkaloid-containing plants lupines larkspur
  tobacco coffee

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Uses of Alkaloids
Pharmaceuticals Stimulants
Narcotics Nicotine Cocaine Caffeine Morphine
Coffee
Cocoa
Tobacco
Opium Poppy
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Uses of Alkaloids
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Some Examples of Alkaloids and Their Structures
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Nicotine
Anti-herbivory Causes addiction Decreases
appetite Induced upon wounding/insect
attack Expensive for plant to make
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Nicotine and Herbivory
Induced defense upon herbivory - Nicotine
synthesis in roots - Transport to shoot - JA
mediates signal transduction
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Some herbivores can become adapted to tolerate
plant defense substancesAn example
Pyrrolizidine
Cinnabar moth
Non-toxic form stored in plants
Toxic form passes through membranes
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Phenols
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Role of Phenolics in Plants

• Nearly 10000 individual compounds
• Large chemical diversity
• Defense compounds against herbivores and
  pathogens (Tannins Isoflavanoids)
• Mechanical support (Lignin)
• Attract pollinators and fruit dispersers
  (Anthocyanins)
• Absorb harmful UV radiation (Flavonoids)

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Phenolics are synthesized in different ways
Roundup
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Biosynthesis of Phenols from Phenylalanine
PAL - important regulatory step in formation of
many phenolic compounds PAL is increased by
environmental stress (nutrient deficiency light
fungal infection) transcriptional regulation
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Biosynthesis of Phenols from Phenylalanine
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Some Phenolic Compounds
- Activated by light phototoxic - level may
increase 100fold in stressed plants -
causing skin rashes on humans
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Flavonoids
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Anthocyanins are Colored Flavonoids that Attract
Animals
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Effects of Ring Substitutes on Anthocyanidin Color
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Flavones and Flavonols absorb UV light
Black-eyed Susan (Rudbeckia sp.) as
seen by humans and by honeybees
UV-absorbing flavonols are found in inner parts
of the rays but not in the tip