phytoalexins
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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
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