• 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)
