Question

How are virus-free plants produced from virus-infected plants? State any two advantages of virus-free plants.

Hint:

Meristem tip culture is widely used to maintain and propagate vegetatively reproduced crops like potato, sugarcane, banana, and strawberry, which are highly susceptible to accumulating viral diseases over successive generations.

Answer 1

Step 1: Scientific Principle of Virus Exclusion
Even in a plant heavily infected with systemic viruses, the apical and axillary meristems remain completely virus-free. This occurs for several reasons:
1. Vascular Independence: Meristems lack mature vascular tissues (xylem and phloem), preventing systemic viruses from traveling into the growing tip.
2. Division Speed: The rapidly dividing meristematic cells outpace the rate of viral replication and cell-to-cell movement (via plasmodesmata).
3. Hormonal Environment: The high concentration of endogenous auxins in apical buds inhibits viral replication.


Step 2: Meristem Tip Culture Procedure
To produce virus-free plants:
1. Excision: The apical or axillary bud is removed, and the outer leaves are carefully dissected under a sterile laminar airflow hood.
2. Isolation: A tiny meristematic dome ($0.1 - 0.5\text{ mm}$ in size, containing the meristem and $1-2$ leaf primordia) is surgically excised using a sterile micro-scalpel.
3. Cultivation: This micro-explant is placed on a sterile MS agar medium supplemented with balanced cytokinins and auxins to stimulate shoot development.
4. Acclimatization: Once shoots develop, they are transferred to a rooting medium, hardened, and grown into complete, virus-free plants.


Step 3: Two Advantages of Virus-Free Plants
1. Substantial Yield and Quality Increase: Viral infections can cause leaf chlorosis, stunted growth, and systemic decline. Virus-free plants grow vigorously, significantly increasing crop yields and quality.
2. Safe International Germplasm Trade: Many countries have strict quarantine regulations that block the import of plant materials to prevent introducing viral pathogens. Certified virus-free plants easily clear these biosecurity screens, facilitating global trade and breeding.
Final Answer: Virus-free plants are produced using meristem tip culture, which exploits the virus-free nature of apical and axillary meristems. Culturing these tiny tissues on MS medium regenerates healthy, virus-free clones. Advantages include (1) significantly higher crop yields and (2) safe international trade that bypasses strict quarantine restrictions.