1. PHYTOALEXINS
By Irda Safni

2. Multitrophic Interactions
Plant Defense:
Multitrophic Interactions
Secondary carnivores
Carnivores
Herbivores
Pollinators
Shoots and flowers
Competitor plants
Pathogens
Aboveground
Belowground
Roots
Pathogens
Parasitic plants
Herbivores
Symbionts
Carnivores
Modified from Bruinsima & Dicke 2008

4. Constitutive vs. Induced Defenses
Constitutive defense - always present
Induced defense - synthesized in response to challenge

5. Constitutive : Morphological/Structural Characters

6. Constitutive : Biochemical defences
Onion smudge disease
susceptible
resistance

7. Induced: Structural defences
Abscission layer Cork layer
Xanthomonas pruni- shot hole
Cork layer
Rhizoctonia solani- canker in potato

8. Tyloses formation : are overgrowths of living cells that protrude via pits into xylem vesels blocking the vascular system.

9. Induced: Biochemical defences
HR- hypersensitive response
Biochemical inhibitors produced in
response to injury by pathogen
– Phenolics ( Chlorogenic acid, caffeic acid)
– Oxidation products of plants ( floretin,
hydroquinonesm hyroxytyromine)
Phytoalexins

10. (Phyto = “plant” and alexin = “to ward off/”)
Phytoalexins
(Phyto = “plant” and alexin = “to ward off/”)
Low molecular mass antimicrobial compounds that are produced by plants as a response to biotic and abiotic stresses
Many phytoalexins have been isolated from plants (>20 families).
E.g. Leguminosae, Solanaceae, Malvaceae, Graminae, Compositae, Umbelliferae, Chenopodiaceae.

11. Best studied phytoalexins are:
Legumes
– Phaseollin; Common beans
– Vestitol: Red clover
– Phaseollidin: Common bean
– Medicarpin; alfalfa, lucern, beans, chickpea
– Pisatin: Pea
Solanaceae
– Rishitin: potato, tomato
– Capsidol: chilli/ pepper
– Phytubrin: potato
Graminae
– Avenalumin-I,II, III : Oats
– 6-mehtoxymellin: carrot
Orchidaceae
– Orchinol: Orchid

13. Characteristics of phytoalexins
Should be fungistatic & bacteristatic and active at verylow concentration.
Produced by the host in response to infection or
metabolic bye products of micro-organisms and stimuli.
Absent in healthy cells or present in very minute quantity.
Usually remain close to the site of their production

14. Produced relatively quickly in cells after infection
Produced in quantities proportionate to the size of inoculum
Produced in large quantity in response to weak pathogen or non pathogen than virulent one.
Produced relatively quickly in cells after infection
Host specific rather than pathogen specific.

15. Example of attack and counterattack
Phytoalexins are synthesized in
response to
pathogen attack
Some pathogens
counterattack
producing enzymes
that degrade the
phytoalexin

18. PRODUCTION OF PHYTOALEXINS
Production of phytoalexins may be stimulated by certain compounds called elicitors.
High molecular weight substances found in the cell wall such as glucans, glycoprotein, or other polysaccharides
Gases such as ethylene (C2H4)

19. In susceptible plants, a pathogen may prevent the formation of phytoalexins, by the action of suppressors produced by the pathogen
The suppressor also can be a glucan, a glycoprotein, or a toxin produced by the pathogen

21. How are Phytoalexins Formed?
The “-noids”
Shikimic acid pathway (phenylpropanoids)
Hydroxycinnamic acids
Coumarins
Hydroxybenzoic acids
Mevalonic acid pathway (Isoprenoids)
Carotenoids
Terpenoids
Combination of Pathways Shikimic-Polymalonic)
Flavonoids and anthocyanins

22. Mode of Action of Phytoalexins
Phytoalexins are more than antimicrobial compounds.
Many phytoalexins exhibit toxicity toward higher plants and animals.

23. Mode of Action of Phytoalexins
Disruption of membrane integrity and function
Example:
- Camalexin  disrupts membranes of Pseudomonas syringae pv. maculicola
- Desoxyhemigossypol  disrupt membranes of Verticillium dahliae
- 6-Methoxymellein  disrupt membranes of Candida albicans
- Xanthotoxin  acts on mitochondrial membranes of onion root cells. Inhibits oxygen uptake.

24. 2. Inhibition of electron transport system
Example:
- Glyceolin  electron transport in isolated soybean mitochondria
- Ipomeamarone  inhibits electron transport and oxydative phosphorylation
- Pisatin  serves as an uncoupler

37. Phytoalexins in Recent Research
Allixin (3-hydroxy-5-methoxy-6-methyl-2-penthyl-4H-pyran-4-one), a non-sulphur containing compound having g-pyron skeleton structure, was the first compound isolated from garlic as a phytoalexin, a product induced in plants by continuous stress.
Shown to have unique biological properties, such as: a) anti-oxidative effects,
b) anti-microbial effects,
c) anti-tumor promoting effects,
d) inhibition of aflatoxin B2 DNA binding, and
e) neurotrophic effects

38. Allixin showed an anti-tumor promoting effect in vivo, inhibiting skin tumor formation in mice
Analogs of this compound have exhibited anti tumor promoting effects in in vitro experimental condition
Here in, allixin and/or its analogs may be expected useful compound for cancer prevention or chemotheraphy agents for other diseases.

39. Sorghum produces two distinct phytoalexins belonging to the 3-deoxyanthocyanidin chemical group, apigeninidin and luteolinidin.
In maize, phytoalexins are represented by members of the terpenoid class (zealexins, kauralexins and benzoxazinoids)
Phytoalexins from sorghum and maize are applied in disease resistance, health and biomedicine.
Phytoalexin-like compounds from the genus Tephrosia (Leguminosae family), such as polyphenolics (flavones, flavonols, flavononols, flavans, isoflavones and chalcones), triterpenoids and sesquiterpenes are used as estrogenic, antitumor, antimicrobial, antiprotozoal and antifeedant agents.

40. CONCLUSIONS
Phytoalexins are only one components of the complex mechanisms for disease resistance in plants.
Most of them regulated through melanovic acid pathway (MAP) kinase signalling pathway.
Challenge is to decipher and identity the complete biosynthetic pathway and the key enzyme to employ transgenic strategy in disease resistance.

41. Phytoalexins have found many applications in human health and disease
To increase the fungitoxicity of phytoalexins, design and synthesis of more active phytoalexin
derivatives is needed.
Another limitation in our knowledge of phytoalexins is the difficulty in analyzing the events occurring between the plant and the pathogen under natural conditions