Virus

• Plant Viruses


  Obligate parasites that lack machinery for their own reproduction, with shapes ranging from spherical to filamentous, consisting of segments of double or single-stranded RNA or DNA encased in protein structures

  
• Virus Components


  • Protein coat: Provides a protective sheath for the nucleic acid, facilitates movement from cell to cell, determines vector transmissibility and symptoms
  • Nucleic acid: Infective part of the virus, most plant viruses have single-stranded RNA (ssRNA)

  
• Classes of viruses based on Genome Type and Replication Strategy


  • Rolling Circle Replication of single-stranded DNA virus

  
• Common Virus Symptoms


  • Ring spots, Leaf Distortion, and Fruit Deformation (Papaya ringspot virus)
  • Mottling and Mosaic pattern (Tobacco mosaic virus)
  • Yellowing, Leaf Curling (Tomato leaf curl virus)
  • Stunted Growth, Twisting of Leaves (Banana bunchy top virus)

  
• Alterations in Plant Cellular Processes


  Viruses can alter various cellular processes in plants

  
• Environmental Factors


  Influence virus-induced symptoms in plants
• Modes of Virus Transmission


  • Infected pollen, seeds, and other planting materials
  • Mechanical
  • Vectors (Nematodes, Soil-borne fungi, Insects)

  
• Virus-Vector Relationship


  • Acquisition Phase: Vector feeds on infected plant and acquires virus
  • Latent Period: Vector has acquired virus but cannot transmit it
  • Retention Period: Length of time vector can transmit virus to healthy host

  
• Plant Virus Transmission Strategies in Insect Vectors


  • Non-persistent (Stylet-borne) transmission
  • Semi-persistent (Foregut-borne) transmission
  • Persistent Circulative, non-propagative transmission
  • Persistent Circulative, propagative transmission

  
• Rice Tungro Disease


  • Caused by Rice tungro baciliform virus (RTBV) and Rice tungro spherical virus (RTSV), transmitted by leafhopper Nephotettix virescens
• Viroids are small, single-stranded, circular RNAs that infect higher plants and can induce specific diseases

  
• Symptoms caused by Viroids


  • Potato spindle tuber viroid (PSTVd): Erect, spindly, and dwarfed plants, small and erect leaves, elongated tubers
  • Citrus exocortis viroid (CEVd): Cracked and scaly bark, yellow blotches, leaf and stem epinasty, darkening of leaf veins and petioles
  • Coconut cadang-cadang viroid (CCCVd): Slow development of symptoms over 8-15 years
• Cellular Inclusions


  • Produced by plant cells in reaction to virus infection, can help identify the virus

  
• Methods in Identifying Plant Viruses and Viroids


  • Symptomatology and Host Range
  • Physical Properties (Thermal Inactivation Point, Longevity, Dilution End Point)
  • Serological Tests (ELISA)
  • Detection through PCR

  
• PCR is an essential and routine tool in most biological laboratories, used to amplify or copy specific segments of DNA exponentially
• General Steps in PCR


  • Denaturation
  • Annealing
  • Extension
• Control Measures for Virus Diseases

  • Preventive Measures (Quarantine, Certification, Use of virus-free materials)
  • Eradicative Measures (Roguing and destruction, Hot water treatment)
  • Protective Measures (Vector control)
  • Cross Protection
  • Genetic Engineering
  • Early detection