PHYTOALEXINS By Irda Safni

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

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

Constitutive : Morphological/Structural Characters

Constitutive : Biochemical defences Onion smudge disease susceptible resistance

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

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

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

(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.

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

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

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.

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

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)

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

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

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

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.

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

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

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.

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.

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.

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