Phytoalexins

Presented By Jadhav Murlidhar SPh.D Biotechnologymurligeneticeng@gmail.com

• Phytoalexins are defined as "low molecular weight,

anti-microbial compounds that are both synthesized

and accumulated in plants after exposure to

microorganisms or abiotic agents“

• The term phytoalexin is derived from Greek- phyto

meaning plant and alexin means warding off

compound

• The concept was formalized by Müller & Börger

(1941)

• Müller (1958) demonstrated the detection of a

chemical entity as a phytoalexin while working

with the hypersensitive response of bean tissue

to the soft-fruit pathogen Monilinia fructicola

Concept of Phytoalexins

• Mueller and Borger concept of Phytoalexins and their

conclusions :

• a) phytoalexin is formed only when the host cells come into

contact with the parasite.

• b) the defence reaction occurs only in the living cells.

• c) the inhibitory material is a chemical substance & may

be regarded as a product of necrobiosis of the host cell.

Contd………

• d) phytoalexin is non-specific in its toxicity.

• e) the resistant state is not inherited.

• f) the defence reaction is confined to the tissue

colonized by the fungus and its immediate

neighbourhood.

Chemical Nature

• They are broad spectrum inhibitors and are chemically

diverse with different types characteristic of particular

plant species.

• Phytoalexins tend to fall into several classes including

terpenoids, glycosteroids and alkaloids

• Derivatives simple phenylpropanoid pathway, Shikimic

acid pathway, Trp pathway and mevalonic acid pathway

(Hammerschmidt, 1999)

• Derived from one or more primary biosynthetic

pathway

Capsidol-MVA

pisatin- shikimic A and acetate -malonate pathway

• Much diverse in chemical structure, phytoalexins produced by many plant families fall into the same class

• Used to examine chemotaxonomic relationship

• Phytoalexin production is often associated with a

widespread but poorly understood plant disease defense

reaction called the hypersensitive reaction (HR).

• observed after a few hours or a few days following infection

by an incompatible race or species of plant pathogen as the

death or d i s o r g a n i z a t i o n of the plant c e l l s

immediately adjacent to the infection site concomitant with

or preceeding the r e s t r i c t i o n of pathogen

development

Phytoalexins in Health

• Indole phytoalexins (Camalexin) have antioxidant,

anticarcinogenic and cardiovascular protective activities of

Brassica vegetables

• Peanut (Arachis hypogea) phytoalexins have antidiabetic,

anticancer and vasodilator effects

• Glyceollin, a soybean (Glycine max) have antiproliferative and

antitumor actions

• The sorghum (Sorghum bicolor) phytoalexins, 3-

deoxyanthocyanins, might be useful in helping to reduce

incidence of gastrointestinal cancer

• The phytoalexin resveratrol from grapevine (Vitis vinifera) has

anti-aging, anticarcinogenic, anti-inflammatory and antioxidant

properties

(Ahuja et al, 2011)

• To evaluate the importance of phytoalexins in defence the

following criteria are used:

• 1. The restriction of the pathogen development must be

associated it phytoalexin production,

• 2. Phytoalexins must accumulate to antimicrobial levels at

the infection site in resistant plants or cultivars that could

result the cessation of the pathogen growth

• 3. There must be strong evidence that the phytoalexins

have vital importance in resistance, and absence of these

compounds would result enhanced susceptibility

(Merk-Turk,

2002)

TYPES OF PHYTOALEXINS:

• Ipomoeamarone:

• It is an abnormal sesquiterpinoid induced in sweet potato

tissue infected with black rot fungus Ceratocystis

fimbriata. It has a striking inhibitory effect on the fungus

even in 0.1% concentrations. More phytoalexin is

produced in the resistant varieties than in susceptible

ones.

(Ahuja et al, 2012)

• Pisatin:

▫ It has the chromocoumarin ring system and is a

phenolic ether. produced in pea in response to

inoculation with many fungi or injury.

▫ Production of pisatin by peapods inoculated with

Monilia fructicola , a non pathogen is reduced at

high temperature &on anaerobic storage. It is a

weak antibiotic with broad spectrum

contd….

• Phaseollin:

▫ It is similar to pisatin in chemistry and function. It

is fungicidal at high concentrations and fungistatic

at low concentrations against S. fructigena.

▫ A no. of compounds such as phaseollidin ,

phaseollinisoflavan and kievitone which are

structurally similar to phaseollin have been

identified.

contd….

Glyceollin:

produced in soybean plants infected with the

fungus Phytophthora megasperma f.sp.glycinea.

Inoculation of fungal races resulted in higher

concentrations in incompatible host cultivars than

in inoculations of fungal races on compatible

cultivars.

Due to reduced biodegradation rather than

increased biosynthesis.

contd….

• Isocoumarin:

▫ isolated from carrot root tissues inoculated with a

fungus non-pathogenic to carrot, Ceratocystis

fimbriata.

▫ It can also be produced in response to a no.of non-

pathogenic microorganisms such as C.ulmi,

Helminthosporium carbonum, Fusarium oxysporum

f.sp.lycopersici & Thielaviopsis basicola.

▫ chemically related to the pterocarpan phaseollin.

contd….

▫ Trifolirhizin: It is a new glucoside which has been

isolated from the roots of red cloves. Its structure

indicates that it is chemically closely related to pisatin.

▫ Rishitin: Muller and Boerger(1940) were the first to

show that the potato tubers carying the gene R1 for late

blight resistance responded when inoculated with

avirulent race of P.infestans by producing a phytoalexin

that inhibited the development of a virulent race.

▫ It is a bicyclic non-sesquiterpine alcohol

• Gossypol:▫ It is an ether soluble phenol . It is produced in diseases

like black spot of rose (Diplocarpon rosa),leaf spot of

wheat (Septoria tritici).

•Xanthotoxin: ▫ Isolated from parsnip root discs inoculated with C.

fimbriata Inoculation with other non pathogens resulted

in production of xanthotoxin

Capsidiol:

it is a sequisterpene phytoalexin produced in pepper fruits

inoculated with a non – pathogenic fungi. Produced

concentrations are sufficient to inhibit these fungi in vitro.

 Medicarpin:

Alfalfa (Medicago sativa) inoculated with a series of

pathogens and non pathogens have been studied.

The antifungal compound was isolated and identified as

Medicarpin

• Camalexin:

▫ an indolic secondary metabolite, is a major phytoalexin in

Arabidopsis thaliana. Its synthesis is stimulated by a variety of

microorganisms

▫ including Pseudomonas syringae, Alternaria brassicicola, and

Botrytis cinerea and by

▫ some abiotic stresses, such as AgNO3 and amino acid starvation,

and it has been shown to inhibit the growth of fungal pathogens.

▫ However, the signaling pathway connecting pathogen infection to

camalexin biosynthesis is not completely known

Induction of phytoalexin biosynthesis

• induction of a mitogen-activated protein kinase (MAPK)

cascade involving MPK3 and MPK6.

• Camalexin induction in Arabidopsis infected with P. syringae

is dependent on the transcription factor WRKY33, which

binds directly to the promoter of the camalexin biosynthesis

gene PAD3 (Qiu et al., 2008).

• MPK3/ MPK6 signaling leads directly to phosphorylation of

WRKY33, and this drives camalexin production in

Arabidopsis challenged by pathogens. (Kishi-Kobashi, 2010)

• This work establishes a direct link between MPK3/MPK6 and

WRKY33, demonstrating that WRKY33 is a target of MPK3/ MPK6

signaling

• and is necessary for the induction of camalexin biosynthesis in

Arabidopsis following infection by the necrotrophic fungus B.

cinerea.

Nancy A. Eckardt

Conclusion

• Phytoalexins are only one components of the

complex mechanisms for disease resistance in plants

• Most of them regulated through MAP kinase

signalling pathway

• Health promoting effect

• Challenge is to decipher and identify the complete

biosynthetic pathway and the key enzyme to employ

transgenic strategy in disease resistance

Thank you