​A researcher had inoculated the bottom leaves of a wild-type tobacco plant with tobacco ringspot virus (TRSV) and made the following statements regarding the disease after 23 days post-inoculation.

A. Strong ringspot symptoms develop on the lower leaves.
B. The ringspot symptoms are higher on the upper leaves.
C. The top leaves have no viral symptoms.
D. The top leaves are immune to secondary infection by the same virus.

Choose the option with all correct statements:

Correct option is A

​Explanation:

• A. Strong ringspot symptoms develop on the lower leaves(Correct)
• Since the virus was initially introduced to the lower leaves, these areas exhibit stronger symptoms compared to the upper regions.
• B. The ringspot symptoms are higher on the upper leaves(Incorrect)
• Typically, viral symptoms reduce as the infection moves upwards. The upper leaves may have mild symptoms, but the highest severity is observed in the lower leaves.
• C. The top leaves have no viral symptoms (Correct)
• In many cases, the virus does not reach the topmost leaves, or plant defense mechanisms prevent visible symptoms from appearing.
• D. The top leaves are immune to secondary infection by the same virus(Correct)
• This phenomenon is called Systemic Acquired Resistance (SAR), where plants develop resistance to further infections after an initial viral attack.

Information Booster:

1. Tobacco Ringspot Virus (TRSV) is a plant pathogenic virus belonging to the Nepovirus group.
2. It primarily affects tobacco, soybean, and various other crop species.
3. The virus is transmitted by nematodes (Xiphinema spp.), infected seeds, and mechanical means.
4. Symptoms include necrotic rings, stunting, and leaf malformations, primarily in lower leaves.
5. Systemic Acquired Resistance (SAR) provides immunity to secondary infections.
6. TRSV control methods include vector control, crop rotation, and resistant cultivars.

Additional Knowledge:

• Ringspot symptoms include necrotic lesions, chlorosis, and leaf deformation.
• TRSV spreads systemically but shows its primary impact in the inoculated region.
• Some upper leaves may show mild chlorosis or stunting, but not severe ringspot symptoms.
• Many plant viruses show limited systemic movement, meaning the top leaves may remain asymptomatic.
• Some plants produce antiviral defense compounds to limit symptom development.
• Systemic Acquired Resistance (SAR) is a well-documented defense mechanism in plants.
• Once infected, plants activate defense pathways that protect against further viral attacks.
• SAR involves salicylic acid and pathogenesis-related (PR) proteins, enhancing resistance.