History & Principles of Plant Pathology

• The Phenomenon of Cross-Protection: Cross-protection (or pre-immunization) is a systemic plant phenomenon in which prior systemic infection of a plant with a mild strain of a pathogen protects the host from subsequent severe disease induced by a challenge infection with a severe, related strain.
• Group of Pathogens: Cross-protection is predominantly and classically observed in plant viruses (e.g., Citrus tristeza virus, Papaya ringspot virus, Tobacco mosaic virus).
• Mechanism: The mild viral strain accumulates coat protein or produces specific viral siRNAs via RNA silencing that immediately target and degrade incoming closely related viral genomes upon entry.
• Conclusion: Cross-protection is most common in viruses (Option 2).

Correct option: (2) Viruses

• Pathogen-Derived Resistance (PDR) in Transgenic Crops:
• Sanford and Johnston Hypothesis (1985): Concept that expressing pathogen-derived genetic sequences in a host plant confers durable genetic resistance.
• The Papaya Ringspot Virus Triumph: The most celebrated commercial success of PDR is Rainbow and SunUp papaya developed by Dennis Gonsalves and colleagues in Hawaii (1998) to manage devastating Papaya Ringspot Virus (PRSV) by engineering plants with the viral coat protein gene, which saved the Hawaiian papaya industry through RNAi-mediated protection.
• Conclusion: Papaya Ringspot is managed through Pathogen-Derived Resistance (Option 3).

Correct option: (3) Ring Spot of papaya

• Liposome-Mediated Gene Transfer (Lipofection) in Plants:
• Mechanism: Artificially synthesized phospholipid bilayer vesicles (liposomes) encapsulate recombinant plasmids or foreign DNA molecules.
• Target Cellular Material: Plant cells possess thick, rigid cellulose and pectin cell walls that act as an impermeable barrier to liposomes. Therefore, liposomes are introduced specifically into enzymatically isolated plant protoplasts (wall-less plant cells), where they fuse with the plasma membrane via endocytosis in the presence of PEG or calcium ions.
• Conclusion: Liposomes are used to introduce genetic material into plant protoplasts (Option 3).

Correct option: (3) Plant protoplasts