Phytoalexins are broad-spectrum antimicrobial compounds produced by plants in response to pathogen infection. They are chemically diverse and fall into classes like terpenoids, steroids, and alkaloids. Phytoalexins are synthesized de novo from primary metabolites through pathways like the shikimic acid and mevalonic acid pathways after plants detect elicitors from pathogens. They function as toxins to disrupt the metabolism and reproduction of pathogens. Their production is part of plants' general short-term response to infection and their long-term systemic acquired resistance which protects the entire plant.

1. Phytoalexin

2.  Phytoalexins are antimicrobial and often antioxidative substances
synthesized by plants that accumulate rapidly at areas of pathogen
infection.
 They are broad spectrum inhibitors and are chemically diverse with
different types characteristic of particular plant species.
 Phytoalexins tend to fall into several classes including terpenoids,
glycosteroids and alkaloids

3. Phytoalexins
Low molecular mass antimicrobial metabolites
synthesized de novo from primary metabolites in
response to infection
1. Structurally diverse group of metabolites with the
isoflavonoids can be an example
2. The isoflavonoids phytolaexins are synthesized from
the flavonoids branch of the phenylpropanoid
pathways
(Phyto = “plant” and alexin = “to ward off/”)

4.  Production of phytoalexins may be stimulated by
certain compounds called elicitors.
 High molecular weight substances found in the cell
wall such as glucans, glycoprotein, or other
polysaccharides
 Gases such as ethylene (C2H4)
 In susceptible plants, a pathogen may prevent the
formation of phytoalexins, by the action of
suppressors produced by the pathogen
 The suppressor also can be a glucan, a glycoprotein,
or a toxin produced by the pathogen
PRODUCTION OF PHYTOALEXINS

5. How are Phytoalexins Formed?
 Shikimic acid pathway (phenylpropanoids)
– Hydroxycinnamic acids
– Coumarins
– Hydroxybenzoic acids
 Mevalonic acid pathway (Isoprenoids)
– Carotenoids
– Terpenoids
 Combination of Pathways Shikimic-Polymalonic)
– Flavonoids and anthocyanins
O
O
H
OH O
OH
OH
OH
O
H
HOOC
OH
OH
OOCCH C
H
OH
OH
The “-noids”

6. Signaling Cascade for Defense Responses
Molecular nature of elicitors:
1. Cell wall proteins (e.g., Harpin)
2. Intracellular proteins (defined genetically in a
bacterium by cloning avirulent loci)
3. Peptide derived from a larger protein
(from a fungus)
4. Heptaglucan (small oligosaccharide)

7. Model derived mostly
from studies in cell
culture using specific
elicitors.
However, there is
evidence for
induction in intact
plants by R genes.
Some aspects are also
constitutive and help
block most microbes
(non-host resistance).
Signaling Cascade for Defense
Responses

9. Function
 Phytoalexins produced in plants act as toxins to the attacking organism.
They may puncture the cell wall, delay maturation, disrupt metabolism or
prevent reproduction of the pathogen in question.
 When a plant cell recognizes particles from damaged cells or particles
from the pathogen, the plant launches a two-pronged resistance: a general
short-term response and a delayed long-term specific response.
 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, or
programmed cell death, in order to prevent the spread of the pathogen to
the rest of the plant.

10. Hypersensitive Response

11. Long-term resistance, or systemic acquired resistance (SAR),
 Involves communication of the damaged tissue with the rest of the plant using
plant hormones such as jasmonic acid, ethylene, abscisic acid 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, neighboring plants also manufacture phytoalexins in response.

12. Systemic Acquired Resistance

13. trans-resveratrol
Vitis vinifera
Allixin (3-hydroxy-5-
methoxy-6-methyl-
2-pentyl-4H-pyran-
4-one)
a product induced in plants by continuous
stress
against the growth of fungal pathogens such as
Botrytis cinerea
delta-viniferin fungal infection by Plasmopara viticola
phytoalexins

14. Pinosylvin is a pre-infectious stilbenoid toxin
heartwood of Pinaceae
fungitoxin protecting the wood from fungal
infection.
Sakuranetin is a flavanone
Polymnia fruticosa
rice
acts as a phytoalexin against spore germination
of Pyricularia oryzae

15. papaya fruit
Eucalyptus sideroxylon
Danielone
high antifungal activity against Colletotrichum
gloesporioides
Stilbenes
pathogens attacks

16. against herbivory
geranium
produces a unique chemical compound in its petals
to defend itself from Japanese beetles. Within
30 minutes of ingestion the chemical paralyzes
the herbivore
Alkaloids
nicotine, caffeine, morphine, cocaine, colchicine, ergolines, strychnine
 Some alkaloids can inhibit or activate enzymes, or alter carbohydrate and
fat storage by inhibiting the formation phosphodiester bonds involved in their
breakdown.
 Certain alkaloids bind to nucleic acids and can inhibit synthesis of proteins
and affect DNA repair mechanisms.
 Alkaloids can also affect cell membrane and cytoskeletal structure causing
the cells to weaken, collapse, or leak, and can affect nerve transmission.
 Although alkaloids act on a diversity of metabolic systems in humans and
other animals, they almost uniformly invoke an aversively bitter taste

17. Nicotine
caffeine
morphine
colchicine
strychnine

18.  Cyanogenic glycosides stored in inactive forms in plant vacuoles. They
become toxic when herbivores eat the plant and break cell membranes
allowing the glycosides to come into contact with enzymes in the cytoplasm
releasing hydrogen cyanide which blocks cellular respiration.
 Glucosinolates are activated in much the same way as cyanogenic glucosides,
and the products can cause gastroenteritis, salivation, diarrhea, and irritation
of the mouth.
 Benzoxazinoids, secondary defence metabolites, which are characteristic
for grasses (Poaceae), are also stored as inactive glucosides in the plant
vacuole. Upon tissue disruption they get into contact with β-glucosidases from
the chloroplasts, which enzymatically release the toxic aglucones. Whereas
some benzoxazinoids are constitutively present, others are only synthesised
following herbivore infestation, and thus, considered inducible plant defenses
against herbivory

19. The terpenoids, sometimes referred to as isoprenoids, are organic chemicals
similar to terpenes, derived from five-carbon isoprene units. There are over
10,000 known types of terpenoids.
Monoterpenoids, continuing 2 isoprene units, are volatile essential oils such as
citronella, limonene, menthol, camphor, and pinene. Diterpenoids, 4 isoprene
units, are widely distributed in latex and resins, and can be quite toxic.
Diterpenes are responsible for making Rhododendron leaves poisonous.
Plant steroids and sterols are also produced from terpenoid precursors,
including vitamin D, glycosides (such as digitalis) and saponins (which lyse red
blood cells of herbivores).
citronella limonene menthol camphor pinene

20.  Phenolics, sometimes called phenols, consist of an aromatic 6-carbon ring
bonded to a hydroxy group.
 Some phenols have antiseptic properties, while others disrupt endocrine
activity.
 Phenolics range from simple tannins to the more complex flavonoids that give
plants much of their red, blue, yellow, and white pigments.
 Complex phenolics called polyphenols are capable of producing many
different types of effects on humans, including antioxidant properties.
 Some examples of phenolics used for defense in plants are: lignin, silymarin
and cannabinoids.
 Condensed tannins, polymers composed of 2 to 50 (or more) flavonoid
molecules, inhibit herbivore digestion by binding to consumed plant proteins and
making them more difficult for animals to digest, and by interfering with
protein absorption and digestive enzymes.
 Silica and lignins, which are completely indigestible to animals, grind down
insect mandibles