Phytoalexins are toxic compounds produced by plants in response to infection by pathogens or exposure to stresses. They help confer resistance against infecting parasites. The document discusses that phytoalexins are induced after infection and inhibit fungal growth. It provides examples of specific phytoalexins produced by different plants like pisatin in peas and gossypol in cotton in response to various fungi. The document also explains the role of phytoalexins in plant defense and resistance.

2. Adm.NO ; 20329
Course ; DISEASE RESISTANCE
PHYTOALEXINS
SAJAD UN NABI

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

4. Greek word “phyto-plant”, “alexin-warding off
compound”
Toxin like substances which are formed only post
infectionally (Muller & Boerger )
A substance that inhibits the development of a fungus
on hypersensitive tissue formed when host plant cells
come in contact with the parasite.
 Phytoalexins are low molecular weight toxic substances
prduced by plants in response to infection by parasites
,physiochemical stresses and confer protection against the
infecting parasite.
Term coined by:Muller & Boerger (1940).

5. 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.

6. Shikimic acid pathway

7. Host plant surface+ water
Diffusion of host nutrients
Host lechates +Fungal spores
Germination & growth of fungus
Mycoantigen +viable host cells
Stimulation of host metabolism
Production of phytoalexin

8.  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. 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.

11. 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.

12. 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.

13. 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.

14. Refrences
Plant pathology (George N Agrios) pp
236-37
Introductory microbiology & plant
pathology:disease resistance in plants
314-16
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