Adm.NO ; 20329

Course ; DISEASE RESISTANCE

PHYTOALEXINS

SAJAD UN NABI

PHYTOALEXINS

DEFENCE SYSTEM

PRE -EXISTING POST EXISTING

STRUCTURAL BIOCHEMICAL

PHYTOALEXINS

Greek word “phyto-plant”, “alexin-warding offcompound”

Toxin like substances which are formed only postinfectionally (Muller & Boerger )

A substance that inhibits the development of a funguson hypersensitive tissue formed when host plant cellscome in contact with the parasite.

Phytoalexins are low molecular weight toxic substancesprduced by plants in response to infection by parasites,physiochemical stresses and confer protection against theinfecting parasite.Term coined by:Muller & Boerger (1940).

Hundreds of Phytoalexins have been characterized ,mostly widespread in Fabaceae and Solanaceae familiesand less reported in families like poaceae andcucurbitaceae.

Two hypotheses were put forward to explain themechanism of the induction of Phytoalexins: De novo synthesis of enzyme. Activation or activity control of enzymes alreadyformed.

Phytoalexins are fungistatic rather than fungicidal Theyare chemically diverse , but large number of them areproducts of the shikimic acid pathway.

Shikimic acid pathway

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

Activated only when host cell come in contact withthe 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. Thesensitivity of host cell which determines the speed ofhost reaction to produce phytoalexin, is specific andgenetically determined.

(Cruickshank-1966)

Broad based anti fungal compounds

Not translocated & rarely persistent

Stimulant has no role in determining the chemical natureof phytoalexin,charecteristic of host plants.

Microbial agent serves as main inducer in naturalconditions

Quantity produced may be influenced by age of host,inoculum concentration, temperature etc.

the inhibitory material is a chemical substance & may beregarded as a product of necrobiosis of the host cell.

Non specific in its toxicity

Phytoalexins in plants

Ipomoeamarone: Sesquiterpinoid induced in sweet potato tissueinfected with black rot fungus Ceratocystis fimbriata. Inhibitory effecton 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 toinoculation with many fungi or injury. Production of pisatin by peapods inoculated with Monilia fructicola . It is a weak antibiotic withbroad spectrum activity

Phaseollin: It is similar to pisatin in chemistry and function prduced

by beans. It is fungicidal at high concentrations and fungistatic at lowconcentrations against S.fructigena. A no. of compounds such asphaseollidin , phaseollinisoflavan and kievitone which are structurallysimilar to phaseollin have been identified.

Isocoumarin: isolated from carrot root tissues inoculated with afungus non-pathogenic to carrot, Ceratocystis fimbriata ,chemicallyrelated to the pterocarpan phaseollin

Trifolirhizin: It is a glucoside which has been isolated from theroots of red cloves. Its structure indicates that it is chemically closelyrelated 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 resistanceresponded when inoculated with avirulent race of P.infestans byproducing a phytoalexin that inhibited the development of avirulent race. It is a bicyclic non-sesquiterpine alcohol.

Gossypol: It is an ether soluble phenol produced by cotton . It isalso produced in diseases like black spot of rose(Diplocarponrosa),leaf spot of wheat (Septoria tritici).

Xanthotoxin: Isolated from parsnip root discs inoculated withC.fimbriata Inoculation with other non pathogens resulted inproduction of xanthotoxin

Capsidiol: it is a sesquiterpene phytoalexin produced in pepperfruits inoculated with fungi. Produced concentrations aresufficient to inhibit these fungi in vitro.

Medicarpin: Alfalfa (Medicago sativa) inoculated with a series ofpathogens and non pathogens have been studied. Theantifungal compound was isolated and identified as Medicarpin

Glyceollin: produced in soybean plants infected with the fungusPhytophthora megasperma f.sp.glycinea. Inoculation of fungalraces resulted in higher concentrations in incompatible hostcultivars than in inoculations of fungal races on compatiblecultivars.

RefrencesPlant pathology (George N Agrios) pp 236-37Introductory microbiology & plant pathology:disease resistance in plants 314-16www.google .com