Phytoalexins are antimicrobial compounds produced by plants in response to pathogens like fungi and bacteria. Some key phytoalexins include ipomeamarone from sweet potato, pisatin from pea pods, phaseollin from bean pods, and glyceollin I from soybeans. Phytoalexins are synthesized after the plant recognizes molecules from the pathogen. They function to inhibit pathogen growth through mechanisms like disrupting cell membranes or inhibiting energy production. Their production is part of the plant's defense response and helps contribute to resistance against diseases.

1. Phytoalexins
Vaishali S.Patil
Professor, Department of Botany
Shri Shivaji College of Arts, Commerce & Science Akola

2. Introduction
Phyto=plant, alexin=warding off compound
•Phytoalexins are toxic antimicrobial and often antioxidative
substances synthesized ‘de novo’(in the begining) in the plants
in response to injury, infectious agents.
• The term phytoalexin was first used by the two
phytopathologists Muller and Borger (1940) for fungi static
compounds produced by plants in response to mechanical or
chemical injury or infection.
•Cruickshank and Perrin (1960) crystallized and chemically
characterized the first phytoalexin; this was pisatin, a
pterocarpan derivative produced by pods. Other examples
are Phaseolus vulgaris, pterocarpans (e.g. phaseollin) on
fungal inoculation; Convolvulaceae (ipomeamarone from
infected sweet potato) and of the Orchidaceae (orchinol from

3. orchid tubers).
• All phytoalexins are lipophilic compounds and were first
detected after a study of late blight of potato caused by
Phytophthora infestans. It is a compound which inhibits the
development of the fungus in hypersensitive tissues and is
formed or activated only when the host plants come in
contact with the parasite.
•Some fungi have the capacity to further metabolize and
detoxify phytoalexins.
•Phytoalexins are believed to be synthesized in living cells but
surprisingly necrosis(death of tissue) follows very quickly.
•A metabolite of the host plant interacts with specific receptor
on the pathogen’s membrane resulting in the secretion of
“phytoalexin elicitor” which enters the host plant cells and
stimulates the phytoalexin synthesis.

4. •Phytoalexins are considered to stop the growth of pathogens
by altering the plasma membrane and inhibiting the oxidative
phosphorylation.
•Phytoalexins have been identified in a wide variety of species
of plants such as Soyabean, Potato, sweet potato, barley,
carrot, cotton etc.
• Classes are terpenoids, glycosteroids and alkaloids.
•Some common phytoalexins are Ipomeamarone, Orchinol,
Pistatin, Phaseolin, Medicarpin, Rishitin, Isocoumarin,
‘Gossypol’ Cicerin, Glyceolin, Capisidiol etc.
•The inhibitory agent is a discrete chemical substance, a
product of the host cell.
•They are non-specific in its toxicity towards fungi; however,
fungal species may be differentially sensitive to it.

5. •The basic response in both resistant and susceptible cells is
the same, the basis of differentiation between resistant and
susceptible hosts being the speed of formation of the
phytoalexin.
•The defence reaction is confined to the tissue colonized by
the fungus and its immediate neighbourhood.
•The resistant state is not inherited; it is developed after the
fungus has attempted infection. The sensitivity of the host cell
which determines the speed of the host reaction is specific and
genotypically determined.
•In all phytoalexin is a substance that is produced by plant
tissues in response to contact with a parasite and
specifically inhibits the growth of that parasite.

6. Types-The following Table gives a list of phytoalexins, chemical nature, the host and the
pathogens in response to which these are produced:

7. 1. Ipomeamarone:
Ipomeamarone is a furanosesquiterpene ketone (tetrahydrofuran,
C15H22O3; mel.wt. 230) and is synthesized in the roots of sweet potato
(Ipomoea batata) infected with the fungus Ceratocystis funbriata
causing black rot disease. Biosynthesis of ipomeamarone may involve
mevalonic acid as a precursor and could perhaps also be induced by
poisonous chemicals and pectinase enzyme.
It functions as an inhibitor of electron transport and energy transfer
reactions and thus prevents mycelial growth, sporulation, and protein
synthesis of the pathogen. Ipomeamarone is also produced in roots of
sweet potato in response to infection by violet root fungus
Helicobasidium mompa, and in presence of toxic chemicals. However,
ipomeamarone is toxic to both nonpathogenic and pathogenic fungi and
to some bacteria, and there seems to be no correlation between its
inhibitory effect and pathogenecity.

8. 2. Pisatin:
Pisatin is a weak phytoalexin with broad spectrum. It possesses
chromocoumarin ring system and is phenolic ether (C17H14O6, mol. wt.
314). It has been reported produced by the exposed endocarp of detached
pea pods in response to inoculation of many fungi (e.g., Monilinia
fructicola, Ascochyta pisi) or injury.
When high concentrations of pisatin are accumulated, as happens when
the storage temperature is normal and conditions are aerobic, the pea
pods show resistance to these fungi. Studies reveal that plants produce
high concentrations of pisatin as found in host against pathogenic fungi.
3. Phaseollin:
Phaseollin is similar to pisatin in chemistry and function. It was isolated
from detached, opened bean pods following inoculation with non-
pathogenic fungus Monilinia fructicola. It has also been detected in
bean leaves inoculated with the bacterium Pseudomonas phaseolicola.
More recently, a number of compounds such as ‘phaseollidin’,
‘phaseollinisoflavan’ and ‘kievitone’, which are structurally similar to
phaseollin, have been identified from diffusates or tissues inoculated

9. with fungi, bacteria, and viruses.
Phaseollin has been found important in determining resistance at variety
level to bean anthracnose caused by Colletotrichum lindemuthianum.
It accumulates more rapidly in bean hypocotyl inoculated with an
avirulent race of the pathogen as against much lower accumulation when
inoculated with a virulent race.
4. Medicarpin:
This antifungal compound was isolated from diffusate solutions of leaves
of Medicago spp. (alfalfa) when inoculated with many pathogenic and
non-pathogenic fungi such as Colletotrichum phomoides,
Helminthosporium turcicum. This antifungal compound has been
identified as (-) dimethyl-homopterocarpin and named ‘medicarpin’.
Medicarpin occurs very commonly in legumes and is subject to de-
methylation. Possibly significantly, demethylmedicarpin was found in
leaves of groundnut susceptible to leaf rust caused by Puccinia arachidis
but not in cultivars which were resistant.

10. 5. Rishitin:
Rishitin is a nonsesquiterpene alcohol and is produced by potato tubers
of Solatium tuberosum var. rishiri inoculated with zoospores of an
incompatible race of Phytophthora infestans; produced by potato tubers
inoculated with Fusarium solani f. sp. phaseoli.
6. Glyceollin I:
Glyceollin I have been isolated from soybean roots inoculated with the
fungus Phytophthora megasperma. f.sp. glycinea. Inoculation of
fungal races on susceptible host cultivars resulted in lower
concentrations of glyceollin I than in inoculations of fungal races on
resistant cultivars in which the concentrations were higher. Plant
pathologists suggest that the higher concentrations of the phytoalexin in
resistant cultivars are the result of reduced biodegradation rather than
increased biosynthesis.
Recently in 1991, when a resistant cultivar and a susceptible cultivar of
soybean were inoculated with the soybean cyst nematode, Heterodera
glycines, it was found that glyceollin I accumulated in the head region of
the nematode 8 hour after inoculation in the resistant cultivar, whereas

11. none was found in the nematode inoculated in the susceptible cultivar.
7. Orchinol:
Orchinol is a phenanthrene produced by tubers of orchids (e.g., O.
malitaris, O. mario) when infected with fungus Rhizoctonia repens. It
is a strong fungistatic and its production is extended over the whole
tuber. Antimicrobial spectrum of orchinol is broad and hardly specific.
8. Gossypol:
Gossypol is an ether soluble phenol and has been first isolated in 1967 by
Bell from xylem vessels of the excised stems of Gossypium species and
several cultivars of cotton inoculated with conidia of Verticillium
alboatrum and Rhizopus nigricans, heavy metal ions and various
metabolic inhibitors. He related gossypol and gossypol-like compounds
to host resistance and inversely to the virulence of the pathogen.
9. Cicerin:
Cicerin is composed of two fluorescent phenols or phenolic compounds
and has been isolated from Cicer (gram) diseased by Ascochyta blight.
This compound diffuses into inoculation droplets during the course of
interaction between pod tissues of Cicer arientinum and the pathogen.

12. 10. Isocoumarin:
Isocoumarin is produced in the tap root tissues of carrot (Daucus
carota) inoculated with Cyratocystis fimbriata, a non-pathogen of
carrot roots. This compound is also produced in some quantity when tap
root tissues of carrot are inoculated with fungal pathogens such as
Fusarium oxysporum f.sp. lycopersici, Rhizopus stolonifer,
Helminthosporium carbonum.
Besides the above described, some other phytoalexins have also been
isolated in recent years. Such phytoalexins are – Trifolirhizin (isolated
from the roots of red clover);
11.In Vitis vinifera grape, trans-resveratrol is a phytoalexin produced
against the growth of fungal pathogens such as Botrytis cinerea
and delta-viniferin is another grapevine phytoalexin produced
following fungal infection by Plasmopara viticola.
12.Pinosylvin is a pre-infectious stilbenoid toxin (i.e. synthesized prior
to infection), contrary to phytoalexins which are synthesized during
infection. It is present in the heartwood of Pinaceae.It is a fungitoxin
protecting the wood from fungal infection.

13. 13.Sakuranetin is a flavanone, a type of flavonoid. It can be found
in Polymnia fruticosa and rice, where it acts as a phytoalexin against
spore germination of Pyricularia oryzae.
14.In Sorghum, the SbF3'H2 gene, encoding a flavonoid 3'-hydroxylase,
seems to be expressed in pathogen-specific 3-deoxyanthocyanidin
phytoalexins synthesis, for example in Sorghum-Colletotrichum
interactions.
15.6-Methoxymellein is a dihydroisocoumarin and a phytoalexin
induced in carrot slices by UV-C, that allows resistance to Botrytis
cinerea and other microorganisms.
16.Danielone is a phytoalexin found in the papaya fruit. This compound
showed high antifungal activity against Colletotrichum gloesporioides, a
pathogenic fungus of papaya.
17.Stilbenes are produced in Eucalyptus sideroxylon in case of
pathogens attacks. Such compounds can be implied in the hypersensitive
response of plants. High levels of polyphenols in some woods can
explain their natural preservation against rot.

14. 18.Allixin (3-hydroxy-5-methoxy-6-methyl-2-pentyl-4H-pyran-4-one), a non-sulfur-
containing compound having a γ-pyrone skeleton structure, was the first compound
isolated from garlic as a phytoalexin, a product induced in plants by continuous stress.
This compound has been shown to have unique biological properties, such as anti-
oxidative effects, anti-microbial effects, anti-tumor promoting effects, inhibition
of aflatoxin B2 DNA binding, and neurotrophic effects. Allixin showed an anti-tumor
promoting effect in vivo, inhibiting skin tumor formation
by TPA in DMBA initiated mice. Herein, allixin and/or its analogs may be expected
useful compounds for cancer prevention or chemotherapy agents for other diseases
Other types- Wyerone acid (isolated from the leaves of broad bean infected with
fungus Botrytis fabae); hircinol (isolated from Loroglassum hircinum against outside
stimulus); capsidiol (isolated from diffusates from pepper, Capsicum frutescens,
inoculated with number of non-pathogenic fungi); xanthotoxin (isolated from the top
millimeter of parsnip root discs inoculated with Ceratocystis fimbriata, a non-
pathogen); camalexin from Arobidopsis thaliana; fungus Ascochyta rabie. casbene
from Ricinus communis; phytuberin from Solatium tuberosum; etc.

16. Synthesis of Phytoalexins
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

18. Function
1.Phytoalexins produced in plants act as toxins to the attacking organism.
2.They may puncture the cell wall, delay maturation, disrupt metabolism
or prevent reproduction of the pathogen in question.
3.They increase in susceptibility of plant tissue to infection.
4.When a plant cell recognizes particles from damaged cells or particles
from the pathogen, the plant launches a two-pronged resistance
i) a general short-term response ii) a delayed long-term specific
response.
i) short-term response, the plant deploys reactive oxygen species such
as superoxide and hydrogen peroxide to kill invading cells. In pathogen
interactions, the common short-term response is the hypersensitive
response, in which cells surrounding the site of infection are signaled to
undergo apoptosis,(cell suicide) or programmed cell death, in order to
prevent the spread of the pathogen to the rest of the plant.
ii) Long-term resistance, or systemic acquired resistance (SAR), involves
communication of the damaged tissue with the rest of the plant using

19. or salicylic acid. The reception of the signal leads to global
changes within the plant, which induce genes that protect from
further pathogen intrusion, including enzymes involved in the
production of phytoalexins.
•Often, if jasmonates or ethylene (both gaseous hormones) is
released from the wounded tissue, neighbouring plants also
manufacture phytoalexins in response.
•For herbivores, common vectors for disease, these and other
wound response aromatics seem to act as a warning that the
plant is no longer edible.