Q50. Phytoalexins play important role in plant defense against pathogens.
Choose the INCORRECT option related to phytoalexins.
(A) Phytoalexins belong to secondary metabolites.
(B) Phytoalexins have antifungal activity.
(C) Phytoalexins are abundant in plants under normal condition.
(D) Different hosts produce phyallexins of varying chemical nature.
The incorrect option is (C) Phytoalexins are abundant in plants under normal condition. Phytoalexins are inducible defense compounds that accumulate only after stress or pathogen attack, not abundantly under normal, healthy conditions.
Introduction
Phytoalexins in plant defense against pathogens are low-molecular-weight secondary metabolites that are synthesized de novo in response to biotic and abiotic stresses. These antimicrobial compounds accumulate near infection sites and help restrict the growth of fungi, bacteria and other pathogens, making them critical components of induced plant immunity.
Correct answer and concept check
-
Correct option (INCORRECT statement): (C) Phytoalexins are abundant in plants under normal condition.
-
Reason: Phytoalexins are not constitutively abundant; they are stress‑induced antimicrobial secondary metabolites produced after pathogen attack or other elicitor signals.
Option-wise explanation
Option (A): Phytoalexins belong to secondary metabolites.
This statement is correct. Phytoalexins are products of plant secondary metabolism, usually lipophilic, low‑molecular‑weight antimicrobial compounds derived from pathways such as phenylpropanoid, terpenoid or alkaloid metabolism. They are not essential for basic growth and development but are crucial for ecological functions like defense, which is the hallmark of plant secondary metabolites.
Key points:
-
Derived from secondary metabolic pathways (e.g., isoflavonoids, stilbenes, terpenoids).
-
Accumulate as stress metabolites at infection sites rather than functioning as primary metabolic intermediates.
Therefore, (A) correctly links phytoalexins to plant secondary metabolites.
Option (B): Phytoalexins have antifungal activity.
This statement is correct. Phytoalexins are defined as antimicrobial stress metabolites, and a major documented role is inhibition of fungal growth in vivo and in vitro. Many classic phytoalexins, such as resveratrol, pisatin, phaseollin, camalexin or kauralexins, show strong antifungal effects by inhibiting spore germination, germ tube elongation and mycelial growth.
Mechanisms include:
-
Disruption of membrane integrity and leakage of cellular contents.
-
Inhibition of fungal enzymes and interference with pathogen metabolism and reproduction.
Thus, (B) is a correct statement because antifungal activity is a central functional attribute of many phytoalexins.
Option (C): Phytoalexins are abundant in plants under normal condition.
This statement is incorrect and is therefore the required answer. Phytoalexins are characterized as inducible defense compounds that are synthesized and accumulate only after exposure to biotic (pathogens, herbivores) or abiotic stresses (UV, wounding, chemicals), not under normal, unstressed conditions. In healthy, uninfected tissues, phytoalexin levels are typically low or undetectable; upon infection, their concentration at the infection site can rise to levels inhibitory to pathogen development.
Important features:
-
Produced de novo in response to elicitors from pathogens or damaged host cells.
-
Part of induced resistance along with other responses like ROS production, cell wall lignification and pathogenesis-related proteins.
Since the defining feature of phytoalexins is their stress‑induced, not constitutive, nature, statement (C) is wrong.
Option (D): Different hosts produce phytoalexins of varying chemical nature.
This statement is correct. Phytoalexins are defined by their function (inducible antimicrobial plant metabolites) rather than by a single chemical structure, and different plant species, or even different tissues, produce chemically diverse phytoalexins. They can belong to several structural classes such as isoflavonoids, stilbenes, terpenoids, alkaloids, and others, and are often considered host‑specific because each plant species has its own characteristic set of phytoalexins.
Examples:
-
Legumes: isoflavonoid phytoalexins like medicarpin and phaseollin.
-
Grapevine: stilbene phytoalexin resveratrol and its derivatives.
-
Brassicaceae: indole‑derived phytoalexin camalexin in Arabidopsis.
-
Maize: diterpenoid phytoalexins called kauralexins.
Hence, (D) is correct because different hosts indeed synthesize phytoalexins with varying chemical structures.
Key exam takeaways
-
Phytoalexins are inducible secondary metabolites with antimicrobial (often antifungal) roles in plant defense.
-
They are not abundantly present under normal, unstressed conditions; their hallmark is stress- or pathogen-induced accumulation at infection sites.
-
Chemically, phytoalexins are diverse and host-specific, spanning isoflavonoids, stilbenes, terpenoids, alkaloids and related classes.
Therefore, for the given MCQ, the incorrect statement and correct choice is (C).
