Agricultural microbiology - Toxic Substances produced by Plants itself against infections.

1. Course : II yr,M.Sc., Microbiology
PHYTOALEXINS
By
Priyengha.R.S

2. PHYTOALEXINS
DEFENCE SYSTEM
PRE -EXISTING POST EXISTING
STRUCTURAL BIOCHEMICAL
PHYTOALEXINS

3. Greek word “phyto-plant”, “alexin-warding off compound”
Toxin like substances which are formed only post infectionally
(Muller & Boerger )
 Phytoalexins are low molecular weight toxic substances produced
by plants in response to infection by parasites ,physiochemical
stresses and confer protection against the infecting parasite.
Term coined by:Muller & Boerger (1940).
PHYTOALEXINS

4. Hundreds of Phytoalexins have been characterized ,
mostly widespread in Fabaceae and Solanaceae families
and less reported in families like poaceae and
cucurbitaceae.
 Two hypotheses were put forward to explain the
mechanism of the induction of Phytoalexins:
 De novo synthesis of enzyme.
 Activation or activity control of enzymes already
formed.
Phytoalexins are fungistatic rather than fungicidal They
are chemically diverse , but large number of them are
products of the shikimic acid pathway.

5. Shikimic acid pathway

6. • 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
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.

7.  Activated only when host cell come in contact with
the pathogen
 Developed only after fungal infection
 Basis of differentiation between resistant &
susceptible hosts is based on speed of formation.
 Resistant state of the host is not inherited. The
sensitivity of host cell which determines the speed of
host reaction to produce phytoalexin, is specific and
genetically determined.
(Cruickshank-1966)

9.  Broad based anti fungal compounds
 Not translocated & rarely persistent
 Stimulant has no role in determining the chemical nature
of phytoalexin,charecteristic of host plants.
 Microbial agent serves as main inducer in natural
conditions
 Quantity produced may be influenced by age of host,
inoculum concentration, temperature etc.
 the inhibitory material is a chemical substance & may be
regarded as a product of necrobiosis of the host cell.
 Non specific in its toxicity

10. To evaluate the importance of phytoalexins in defence the following
criteria are used:
 The restriction of the pathogen development must be associated it
Phytoalexin production.
 Phytoalexins must accumulate to antimicrobial levels at the infection site
in resistant plants or cultivars that could result the cessation of the
pathogen growth
 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)
Importance of Phytoalexins in Defence

11. Types of Phytoalexins in plants
Ipomoeamarone: Sesquiterpinoid induced in sweet potato tissue
infected with black rot fungus Ceratocystis fimbriata. Inhibitory effect
on the fungus even at 0.1% concentrations.
Pisatin: It is a phenolic ether having chromocoumarin ring. Produced
by the exposed endocarp of the detached pea in response to
inoculation with many fungi or injury. Production of pisatin by pea
pods inoculated with Monilia fructicola . It is a weak antibiotic with
broad spectrum activity
Phaseollin: It is similar to pisatin in chemistry and function prduced
by beans. 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.

12. Isocoumarin: isolated from carrot root tissues inoculated with a
fungus non-pathogenic to carrot, Ceratocystis fimbriata ,chemically
related to the pterocarpan phaseollin
Trifolirhizin: It is a glucoside which has been isolated from the
roots of red cloves. Its structure indicates that it is chemically closely
related to pisatin. It has the formula C16H12O5
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.

13. Gossypol: It is an ether soluble phenol produced by cotton . It is
also 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 sesquiterpene phytoalexin produced in pepper
fruits inoculated with fungi. Produced concentrations are
sufficient to inhibit these fungi in vitro.

14. 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
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.

15.  Indole phytoalexins (Camalexin)Produced by Brassica vegetables have
antioxidant, anticarcinogenic and cardiovascular protective activities.
 Peanut (Arachis hypogea) phytoalexins have antidiabetic, anticancer and vaso-
dilator 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)
Advantage of Phytoalexins

16. Summary -- Phytoalexins
 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.

17. Plant pathology (George N Agrios) pp 236-37
Introduction to Microbiology & Plant Pathology :-
Disease resistance in plants 314-16
Plant Pathology (P.D.Sharma) Pg no.,50-54
Plant Pathology (B.P.Pandey) Pg no.,99-100
Images from net and E-Book.
Reference

18. The END .