Phytoalexins LUQMAN ADIL BKUC

1. PHYTOALEXINS
 Greek word“phyto-plant”, “alexin-warding off compound”
Toxin like substances whichare formedonly post infectionally (Muller &
Boerger)
 Phytoalexins are lowmolecular weight toxic substancesproducedby
plants in response toinfectionby parasites, physiochemical stresses and
confer protectionagainst the infecting parasite.
 Term coinedby: Muller &Boerger (1940)
 Hundreds of Phytoalexins have beencharacterized, mostly widespreadin
Fabaceae and Solanaceae families andless reportedinfamilies like
poaceae and cucurbitaceae.
 Two hypotheses were put forwardtoexplainthe mechanismof the
inductionof Phytoalexins:
 De novo synthesisof enzyme.
 Activationor activity control of enzymes already formed.
 Phytoalexins are fungistatic rather thanfungicidal They are chemically
diverse , but large number of themare products of the shikimic acid
pathway.
Synthesis:
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 Luqman adilMscbotany 4th semester

2. PHYTOALEXINS
 Activatedonly whenhost cell come in contact withthe pathogen
 Developedonly after fungal infection.
 Basis of differentiationbetweenresistant &susceptible hostsis basedon
speedof formation.
 Resistant state of the host is not inherited. The sensitivity of host cell
which determinesthe speedof host reactiontoproduce phytoalexin, is
specific andgenetically determined. (Cruickshank-1966
Silent features:
 Broad based anti-fungal compounds
 Not translocated&rarely persistent
 Stimulant has no role in determining the chemical nature of phytoalexin,
characteristic of host plants.
 Microbial agent serves as maininducer innatural conditions
 Quantity producedmay be influencedby age of host, inoculum
concentration, temperature etc.
 the inhibitory material is achemical substance & may be regardedas a
product of necrobiosis of the host cell.
 Non specific inits toxicity.
Types of Phytoalexinsin plants:
 Ipomoeamarone:
 Sesquiterpinoidinducedinsweet potatotissueinfectedwithblack rot
fungus Ceratocystis fimbriata. Inhibitory effect onthe fungus evenat 0.1%
concentrations.
Luqman adilMsc botany 4thsemester

3. Pisatin:
 It is a phenolic ether having chromocoumarinring. Producedby the
exposedendocarpof the detachedpeain response toinoculationwith
many fungi or injury. Productionof pisatinby pea pods inoculatedwith
Moniliafructicola.
 It is a weak antibiotic withbroad spectrumactivity
Phaseollin:
 It is similar topisatinin chemistry andfunctionprducedby beans. It is
fungicidal at highconcentrations andfungistatic at low concentrations
against S.fructigena. A no. of compounds suchas phaseollidin,
phaseollinisoflavanand kievitone whichare structurally similar to
phaseollinhave beenidentified.
Trifolirhizin:
 It is a glucoside whichhas beenisolatedfromthe roots of redcloves. Its
structure indicates that it is chemically closely relatedtopisatin. It has the
formula C16H12O5
Rishitin:
 Muller andBoerger (1940) werethe first toshowthat the potatotubers
carying the gene R1 for late blight resistance respondedwheninoculated
withavirulent race of P.infestans by producing a phytoalexinthat
inhibitedthe development of avirulent race. It is a bicyclic non-
sesquiterpine alcohol.
Luqman adil Mscbotany 4th semester

4. Gossypol:
 It is an ether soluble phenol producedby cotton. It is alsoproducedin
diseases like black spot of rose (Diplocarponrosa), leaf spot of wheat
(Septoriatritici).
Xanthotoxin:
 Isolatedfromparsniproot discs inoculatedwithC.fimbriataInoculation
withother non pathogens resultedinproductionof xanthotoxin.
Capsidiol:
 it is a sesquiterpene phytoalexinproducedinpepper fruits inoculated
withfungi. Producedconcentrations are sufficient toinhibit thesefungi in
vitro.
Medicarpin:
 Alfalfa(Medicagosativa) inoculatedwithaseries of pathogens and non
pathogens have beenstudied. The antifungal compound was isolatedand
identifiedas Medicarpin
Glyceollin:
 producedin soybeanplants infectedwiththe fungus Phytophthora
megaspermaf.sp.glycinea. Inoculationof fungal races resultedinhigher
concentrations inincompatible host cultivarsthanininoculations of
fungal races on compatible cultivars.
Luqman adil Mscbotany 4th semester