Detailed description on the mode of actions of various phytoalexins, mechanisms involved phtyoalexin formation, various types of phytoalexins, its functions

1. PHYTOALEXINS
Prepared by,
SOUNDARYA KASIRAMAN
Ph.D.(Ag.)Plant pathology.
Faculty of Agriculture,
ANNAMALAI UNIVERSITY

3. Protection of plants from pathogens by
 Pre formed structures, chemicals, toxins and secondary
metabolites
 Response of immune system thus by reducing the virulence of
pathogen (growth, reproduction and Sporulation)
PLANT DISEASE RESISTANCE

4. Non host
resistance
True resistance Apparent
resistance
Plant Defense mechanism
Vertical Resistance Horizontal resistance Disease
escape
Disease Tolerance
Pre infectional defense mechanism Post- Infectional defense mechanism
Morphological and structural
defense mechanism
Bio chemical defense
mechanism
Morphological and
structural defense
mechanism
Bio chemical
defense mechanism

5. STRUCTURAL DEFENSE MECHANISM
 The surface of the plant or host is first line of defense against the pathogen.
 The pathogen must adhere to the surface and penetrate , if it is to cause infection
Structural defense mechanism are mainly two type :
a. Pre existing structural defense mechanism
b. Post inflectional or induced structural defense mechanism

6. PRE EXISTING STRUCTURAL DEFENSE
INDUCEDSTRUCTURAL DEFENSE
 Wax
 Thick cuticle
 Thickness and toughness of the outer wall of epidermal cells
 Stomata
 Sclerenchyma cells
 Lenticel
 Cellular defense structure :
Hyphal sheathing
 Histological defense structure :
a) Formation of corky layer
b) Formation of Abscission layer
c) Formation of tyloses
d) Deposition of gums

7. PRE EXISTING STRUCTURAL DEFENSE
It includes,
• Amount and quality of wax and cuticle
• shapes , size and locations of natural openings
• Presence of thick walled cells in the tissues of the plant that hinder the advance of pathogen
WAX
• long chain of apolar lipid
• protective coating on plant leaves
and fruit
• Extremely hydrophobic
• Ex., Presence of wax on
Amaranthus leaf surface prevent
white blister caused by Albugo
candida (Mario serrano 2014)

8. CUTICLE & EPIDERMAL CELL
Example:
Disease resistance in Barberry species infected with Puccinia graminis tritici has been
attributed to the tough outer epidermal cells with a thick cuticle .
In linseed , Cuticle acts as a barrier against Melampsora lini

9. SCLERENCHYMA CELLS
 It composed of thickened walls of
lignin.
 Present in stem and leaf veins
 Brittle cells help in mechanical
support of the plant
 Effectively block the spread of some
fungal and bacterial pathogens that
cause angular leaf spot.

10. Ex: Beet varieties – Cercospora beticola
STRUCTURE OF NATURALOPENINGS
 Most of pathogen enter plants through
natural opening.
 Some pathogen like stem rust of wheat
(Puccinia graminis f.sp.tritici) can enters its
host only when the stomata are open
 Structure of stomata provides resistance to
penetration by certain plant pathogenic
bacteria
A. Stomata:

11. LENTICELS
 Lenticels are opening on fruit,
stem and tubers that are filled
with loosely connected cells that
allow the passage of air
 Shape and internal structure of
lenticels can increase or decrease
the incidence of diseases.
 Ex. Small and suberized lenticels
will offer resistance to potato scab
pathogen, Streptomyces scabies.
(Sanjeet kumar 2013)

12. POST INFECTIONAL / INDUCED STRUCTURALDEFENSE MECHANISM
• Most pathogen manage to penetrate their hosts through wounds and natural
opening and to produce various degree of infection.
• Pathogen penetration through the host surface induced the structural
defense mechanism in the host cells
These may be regardedas :
 Histological defense barriers (Cork layer, Abscission layers and
Tyloses formation)
 Cellular defense structures (Hyphal sheathing )

13. HISTOLOGICAL DEFENSE MECHANISM
 These cork cells inhibits the further invasion by
the pathogen beyond the initial lesion and also
blocks the spread of toxin substance secreted by
the pathogen
 It also stop the flow of nutrients and water from
the healthy to the infected area and deprive the
pathogen of nourishment
CorkY layer
Ex: potato tubers infected by Rhizoctonia
(Sanjeet kumar 2013)

14. ABSCISSIONLAYERS
 An abscission layer consists of a gap formed between infected and healthy cells of leaf surrounding the
locus of infection
 Due to the disintegration of middle lamella of Parenchymatous tissue.
 Gradually , infected area shrivels , dies and sloughs off , carrying with it the pathogen
 Abscission layers are formed on young active leaves infected by Fungi, bacteria or viruses.
Ex: Xanthomonas pruni on peach leaves. (Young et al. 2002)

15. TYLOSES
 Tyloses are the overgrowths of the Protoplast of adjacent living parenchymatous cells, which protrude into
xylem vessels through pits.
 Tyloses have cellulosic walls
 It formed quickly ahead of the pathogen and may clog the xylem vessels completely blocking the further
advance of the pathogen in resistant varieties
Ex: Tyloses form in vessels of most plants under invasion by most of the vascular wilt pathogens
(Sauban Musa Jibril et al. 2016)

16. GUM DEPOSITION
 Various types of gums are produced by many plants around lesions after infection by pathogen or
injury
 Gums secretion is most common in stone fruit trees
 Generally these gums are exudated by plant under stressed condition
 Gummosis is the process in which gum produced by the plants and trees
(Sauban Musa Jibril et al. 2016)

18. BIOCHEMICAL DEFENSE MECHANISM
Pre existing chemical defense :
1. Inhibitors :
Released by plant in it’s environment
Ex. Chick pea - Malic acid – Ascochyta rabiei
Citrus - Cutin acid – Gleosporium limetticola
2. Present in plant cells before infection
1. Phenolics :
Tannins - Botrytis sp.
2. Phytoanticipin - Tomatin in tomato , Avinasin in oats
3. Hydrolytic enzymes -Glucanase , Chytinase
4. Lectins
Induced chemical defence :
 Hypersensitivity response (HR)
 Production of Antimicrobial substrates
i. Phytoalexins
ii. PR proteins (Agrios 2005)

19. PHYTOALEXINS
Phytoalexins are antimicrobial and often antioxidative substances that accumulate
rapidly at areas of pathogen infection.
Derived from Greek word, Phyto- ‘plants’; Alexin- ‘to ward off’
DEFINITION: “Low molecular weight antimicrobial compounds that are produced by plants as a
response to biotic and abiotic stresses”. This concept was formalized by Müller & Börger
(1941)
Fungistatic action rather than fungicidal activity.
It is chemically diverse , but large number of Phytoalexin products are produced by
Shikimatic acid pathway.
 Müller (1940) 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

21. MAIN FUNCTIONS
• Phytoalexins act as toxins to the attacking organism.
• They may puncture the cell wall, delay maturation, disrupt
metabolism or prevent reproduction of the pathogen.
• Their importance in plant defense is indicated by an increase
in susceptibility of plant tissue to infection when phytoalexin
biosynthesis is inhibited.

22. CHARACTERISTICS OF PHYTOALEXINS
• Fungitoxic and bacteriostatic at low concentrations.
• Produced in host plants in response to stimulus (elicitors) and
metabolic products.
• Absent in healthy plants.
• Remain close to the site of infection.
• Produced in quantities proportionate to the size of inoculum.
• Produced in response to the weak or non-pathogens than pathogens.
• Produced within 12-14 hours reaching peak around 24 hours after
inoculation.
• Host specific rather than pathogen specific.

24. How are phytoalexins formed
• The Shikimic acid pathway is related to the metabolism of
carbohydrates and aromatic amino acids.
• It consists of seven steps, starting with the condensation
of Phosphoenolpyruvate and Erythrose-4-phosphate. Their
condensation and cyclization lead to the formation of Shikimic acid
and end with the synthesis of chorismic acid
• The shikimic acid pathway provides an alternative route
to aromatic compounds, particularly the aromatic amino acids. Various
metabolites, including alkaloids, phenolics and mycotoxins, are
derived from aromatic acids

25. Reactions, pathway intermediates and enzymes of the shikimate pathway

30. TYPES OF PHYTOALEXINS
IPOMOEAMARONE
Sweet potato- Black rot fungus
Ceratocysis fimbriata

31. ISOCOUMARIN
• CARROT FRUIT ROT- Ceratocystis fimbriata

32. GOSSYPOL
• Black spot of rose- Diplocarpan rosa

33. GLYCEOLLIN
• SOYBEAN- Phytophthora megasperma

34. PISATIN
• Detached pea- Monilia fructicola

35. CAPSIDIOL
• PEPPER FRUIT ROT
• Nicotiana
tabacum after
infection by the
oomycete water-
mold Phytophthora
capsici

36. CAMALEXIN
• Arabidopsis thaliana- Pseudomonas syringae

37. ALLIXIN
GARLIC ROT-
Fusarium proliferatum

38. AVENANTHRAMIDES
• (Anthranilic acid amides) are a
group of phenolic alkaloids found
mainly in oats (Avena sativa), also
isolated from fungal-infected
carnation (Dianthus caryophyllus)
• The name “Avenanthramides” was
coined by Collins
• Oat kernel extracts with
standardized levels of
Avenanthramides are ideal for
skin, hair, baby and sun care
products.

39. DANIELONE
• Papaya fruit.
• high antifungal activity
against Colletotrichum
gloesporioides
-Fernando echeverr et al. 1997

40. MOMILACTONE B
• An allelopathic agent produced from the
roots of rice (Oryza sativa L.) has also
been induced in response to infection by
(Magnoporthe grisea) Blast of rice.
• More recently it has been shown to be a
potential chemotherapeutic agent
against human colon cancer

41. SAKURANETIN
• A flavan-on found in
Polymnia fruticosa and
rice
• Phytoalexin against
spore germination
of Pyricularia oryzae

42. PTEROSTILBENE
• Found in almonds, blueberries, grape
vines and red sandalwood tree
(Pterocarpus santalinus)
• chemically related to resveratroL, In
plants, it serves a
defensive phytoalexin role
• It is considered to be a corrosive
substance, dangerous upon exposure
to the eyes, and is an
environmental toxin, especially
to aquatic life.

46. PHYTOALEXIN IN HEALTH
• Indole phytoalexins (Camalexin) have antioxidant, anti-carcinogenic and
cardiovascular protective activities of Brassica vegetables
• Peanut (Arachis hypogea) phytoalexins have antidiabetic, anticancer and
vasodilator 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,
anti-carcinogenic, anti-inflammatory and antioxidant properties
(Ahuja et al. 2011)

47. SUMMARY
• Phytoalexins are only one components of the complex mechanisms
for disease resistance in plants.
• Phytoalexins tend to fall into several classes including terpenoids,
glycosteroids and alkaloids.
• They act against fungi pathogens especially.
• They are synthesized de novo by plants that accumulate rapidly at
areas of pathogen infection.
• Health promoting effect.
• They have susceptible host and present in living cells only.

48. REFERENCES
 Mehrotra R S '' Plant Pathology '' First Edition (1980), Tata McGraw Hill
Publishing Company limited, New Delhi.
 Panday B P " Plant pathology (pathogen and palant diseases) '' First Edition
(1982) , S.Chand publication ,New Delhi.
 Yadav Dinesh Kumar "Plant Pathology"First Edition (2013), Shree Publishers
and Distributors, New Delhi.
 http://bcs.whfreeman.com/webpub/Ektron/Hillis%20Principles%20of
%20Life2e/Animated%20Tutorials/pol2e_at_2801_Signaling_between_
Plants_and_Pathogens/pol2e_at_2801_Signaling_between_Plants_and
Pathogens.html