Question

Suggest how a virus-free healthy plant can be obtained from a diseased sugarcane plant.

Hint:

Even if a plant is completely riddled with a virus, its growing tips (apical and axillary meristems) are always clean. Cut the tip, grow it in a sterile test tube, and you get a completely healthy clone.

Answer 1

Concept: Even when an entire plant body is systematically infected with virulent pathogens such as viruses, certain localized localized cellular zones remain entirely virus-free. In plant biology, the specific tissue regions that are consistently free of viral particles are the active apical and axillary meristems. By exploiting this anatomical phenomenon via in vitro plant tissue culture techniques, scientists and agriculturalists can successfully regenerate completely healthy, virus-free clonal progeny from a heavily diseased parent plant.

Step 1: Scientific Explanation for Meristem Immunity to Viruses.
The zone at the absolute tip of the shoot (apical meristem) and the regions in the leaf axils (axillary meristems) remain virus-free due to several physiological factors:
• Absence of Vascular Connectivity: Viruses travel long distances through the plant body primarily via the phloem sieve tubes and cell-to-cell through plasmodesmata. The active meristematic zone consists of tightly packed cells that lack mature, functional vascular tissues (xylem and phloem connectivity), effectively cutting off the virus's primary transit route.
• Rapid Cell Division Rate: The cells within the meristem divide actively and continuously at a rate that typically outpaces the rate of viral replication and intracellular movement.
• High Endogenous Auxin Concentration: The very high physiological levels of specific phytohormones (like auxins) localized in shoot apices actively inhibit the enzymatic replication machinery of many plant viruses.

Step 2: Practical Methodology using Plant Tissue Culture (Meristem Culture).
To recover a virus-free sugarcane plant, the following step-by-step scientific protocol is performed in a sterile laboratory environment:
• Explant Excision: A microscopic piece containing the apical or axillary meristem dome along with a few primordial leaves ($0.1$ to $0.5\text{ mm}$ in size) is carefully excised from the infected sugarcane stalk using sterile surgical micro-scalpels.
• Surface Sterilization: The isolated meristematic explant is surface sterilized using chemical disinfectants such as dilute sodium hypochlorite or mercuric chloride solutions to eliminate superficial fungal or bacterial contaminants, followed by multiple rinses with sterile double-distilled water.
• Inoculation: The sterile explant is transferred under an aseptic laminar airflow hood onto a synthetic culture medium (such as Murashige and Skoog medium) fortified with essential macronutrients, micronutrients, vitamins, a carbon energy source (sucrose), and balanced growth regulators (an optimal ratio of auxins and cytokinins).
• Incubation and Regeneration: The culture tubes are kept in a temperature- and light-controlled growth chamber. The balanced phytohormones stimulate the meristem cells to undergo organized organogenesis, directly regenerating into a micro-shoot without undergoing an unorganized callus stage (to maintain genetic stability).
• Rooting and Hardening: The regenerated virus-free micro-shoots are transferred onto a specialized rooting medium rich in auxins to initiate root development. Once a complete plantlet is formed, it is gradually acclimated to ambient outdoor environmental conditions (hardened) in green-houses before being transplanted directly into field soil.