Question

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.

Hint:

Remember: Coronatine mimics {jasmonic acid}, not salicylic acid. Its virulence strategy is to hijack the antagonism between JA and SA pathways, shutting down the SA-dependent defense that normally stops {P. syringae}.

Answer 1

The Correct Option is D

Step 1: Plant immune signaling pathways.
Plants rely on two major defense hormones: salicylic acid (SA) and jasmonic acid (JA).
- SA is primarily responsible for resistance against biotrophic pathogens.
- JA is involved in defense against necrotrophic pathogens and herbivores.
Importantly, SA and JA signaling are antagonistic to each other.
Step 2: Role of coronatine.
Coronatine, secreted by Pseudomonas syringae, is a structural mimic of jasmonic acid (JA).
By mimicking JA, coronatine binds to the JA receptor and artificially activates JA signaling.
This suppresses SA-mediated defense signaling, thereby weakening the host immune response against the biotrophic bacterium.
Step 3: Elimination of wrong options.
(A) Incorrect — Coronatine does not directly block SA biosynthesis.
(B) Partly correct but inaccurate — Coronatine does not promote JA biosynthesis; it directly mimics JA.
(C) Incorrect — Coronatine is not an SA analogue.
(D) Correct — Coronatine is a structural analogue of JA that activates JA signaling and thereby inhibits SA response.
\fbox{Hence, option (D) is correct.}