Q.49 Although Pseudomonas syringae infection in plants is actively inhibited by the
endogenous salicylic acid (SA) of host origin, a successful infection is still
established because the bacterium secretes coronatine, an effector molecule.
Which one of the following best describes the mode of action of coronatine?
(A) Coronatine inhibits SA biosynthesis.
(B) Coronatine promotes the biosynthesis of jasmonic acid (JA), and JA signaling in
turn inhibits SA response.
(C) Coronatine is a structural analogue of SA, which binds to the SA receptor and
inhibits its function.
(D) Coronatine is a structural analogue of jasmonic acid (JA), which activates JA
signaling to inhibit SA response.

Coronatine acts as a structural analogue of jasmonic acid (JA), specifically mimicking JA-isoleucine (JA-Ile), to activate JA signaling pathways that antagonize salicylic acid (SA)-mediated plant defenses.​

This enables Pseudomonas syringae to overcome host SA defenses despite active inhibition by endogenous SA.​

Correct Answer

(D) Coronatine is a structural analogue of jasmonic acid (JA), which activates JA signaling to inhibit SA response.​

Option Analysis

Option (A) Coronatine inhibits SA biosynthesis.
COR indirectly suppresses SA accumulation by activating JA signaling, which represses SA synthesis genes like ICS1 and induces metabolism genes like BSMT1 via NAC transcription factors (ANAC019/055/072). It does not directly block SA biosynthesis enzymes.​

Option (B) Coronatine promotes the biosynthesis of jasmonic acid (JA), and JA signaling in turn inhibits SA response.
COR mimics JA-Ile rather than inducing JA production; it binds COI1 to degrade JAZ repressors, activating JA-responsive genes that antagonize SA without elevating JA levels.​

Option (C) Coronatine is a structural analogue of SA, which binds to the SA receptor and inhibits its function.
COR structurally resembles JA-Ile, not SA; it targets the JA receptor complex (COI1-JAZ), with ~1000-fold higher affinity than JA-Ile, suppressing SA via crosstalk, not direct SA receptor binding.​

Option (D) Coronatine is a structural analogue of jasmonic acid (JA), which activates JA signaling to inhibit SA response.
This accurately describes COR’s mechanism: it functions as a potent JA-Ile mimic, promoting COI1-JAZ interaction, MYC2 activation, NAC TF induction, and subsequent SA suppression for P. syringae virulence in stomata, apoplast, and systemically.​

Coronatine’s mode of action enables Pseudomonas syringae to establish infection despite plant salicylic acid (SA) defenses. This phytotoxin, a structural analogue of jasmonic acid (JA), activates JA signaling to inhibit SA responses, crucial for CSIR NET Life Sciences.​

Molecular Mechanism

Coronatine binds the JA receptor COI1 with high affinity, promoting JAZ repressor degradation and MYC2 release. MYC2 induces NAC TFs (ANAC019/055/072), which repress ICS1 (SA synthesis) and activate BSMT1/SAGT1 (SA metabolism), reducing SA levels.​

This cascade reopens stomata, enhances apoplastic growth, and induces systemic susceptibility.​

JA-SA Antagonism

JA signaling and SA pathways mutually antagonize; coronatine exploits this by hyperactivating JA via JA-Ile mimicry (~1000x potency), overriding SA-mediated immunity without direct SA interaction.​

Exam Relevance

For CSIR NET, recognize coronatine as JA analogue (not SA) activating signaling for SA inhibition—distinguishes from direct biosynthesis blocks. Mutants like coi1 confirm COI1-dependence.​