6.1 Major plant virus families and their characteristics

Plant viruses come in diverse families, each with unique characteristics. From the rod-shaped Tobacco mosaic virus to the twinned particles of Geminiviridae, these pathogens showcase a variety of structures and genome types. Understanding their differences is key to grasping their impact on agriculture.

This topic dives into major plant virus families like Potyviridae and Bromoviridae, exploring their particle morphology and genome organization. We'll also look at replication strategies, classification systems, and how these viruses interact with their host plants, causing diseases that affect crops worldwide.

Plant Virus Families

Major Plant Virus Families

Top images from around the web for Major Plant Virus Families

• 

  Frontiers | Small RNA-Omics for Plant Virus Identification, Virome Reconstruction, and Antiviral ... View original

  Is this image relevant?

• 

  Frontiers | Control of Plant Viruses by CRISPR/Cas System-Mediated Adaptive Immunity View original

  Is this image relevant?

• 

  Frontiers | Viral Vectors for Plant Genome Engineering | Plant Science View original

  Is this image relevant?

• 

  Frontiers | Small RNA-Omics for Plant Virus Identification, Virome Reconstruction, and Antiviral ... View original

  Is this image relevant?

• 

  Frontiers | Control of Plant Viruses by CRISPR/Cas System-Mediated Adaptive Immunity View original

  Is this image relevant?

1 of 3

Top images from around the web for Major Plant Virus Families

• 

  Frontiers | Small RNA-Omics for Plant Virus Identification, Virome Reconstruction, and Antiviral ... View original

  Is this image relevant?

• 

  Frontiers | Control of Plant Viruses by CRISPR/Cas System-Mediated Adaptive Immunity View original

  Is this image relevant?

• 

  Frontiers | Viral Vectors for Plant Genome Engineering | Plant Science View original

  Is this image relevant?

• 

  Frontiers | Small RNA-Omics for Plant Virus Identification, Virome Reconstruction, and Antiviral ... View original

  Is this image relevant?

• 

  Frontiers | Control of Plant Viruses by CRISPR/Cas System-Mediated Adaptive Immunity View original

  Is this image relevant?

1 of 3

• Potyviridae stands as the largest plant virus family
  • Characterized by flexuous filamentous particles
  • Contains single-stranded positive-sense RNA genome
  • Examples include Potato virus Y and Turnip mosaic virus
• Geminiviridae viruses feature unique twinned icosahedral particles
  • Possess single-stranded circular DNA genome
  • Transmitted by whiteflies
  • Cause significant crop losses worldwide (cassava mosaic disease, tomato yellow leaf curl)
• Bromoviridae family encompasses viruses with varied morphology
  • Exhibit icosahedral or bacilliform particles
  • Contain tripartite genome of positive-sense single-stranded RNA
  • Examples include Brome mosaic virus and Cucumber mosaic virus

Unique Plant Virus Families

• Caulimoviridae viruses stand out among plant viruses
  • Contain double-stranded DNA genome
  • Replicate via reverse transcription
  • Known as plant pararetroviruses
  • Examples include Cauliflower mosaic virus and Banana streak virus
• Tombusviridae family comprises small icosahedral viruses
  • Possess positive-sense single-stranded RNA genome
  • Often transmitted through soil or water
  • Examples include Tomato bushy stunt virus and Carnation mottle virus
• Luteoviridae family consists of phloem-limited viruses
  • Transmitted persistently by aphids
  • Contain single-stranded positive-sense RNA genome
  • Examples include Barley yellow dwarf virus and Potato leafroll virus
• Closteroviridae family includes largest known plant RNA viruses
  • Feature long, flexuous particles
  • Possess complex genome organization
  • Examples include Citrus tristeza virus and Beet yellows virus

Plant Virus Structure and Genome

Virus Particle Morphology

• Plant virus particles (virions) exhibit diverse shapes and sizes
  • Icosahedral (spherical) particles measure ~20-30 nm in diameter (Tombusvirus)
  • Rod-shaped particles range from 300-500 nm in length (Tobacco mosaic virus)
  • Filamentous particles can reach up to 2000 nm in length (Potyvirus)
• Capsid composition plays crucial roles in virus function
  • Constructed from multiple copies of one or few types of coat proteins
  • Protects viral genome from environmental degradation
  • Facilitates virus transmission between hosts
  • Mediates interactions with host cellular components

Genome Types and Organization

• Plant virus genomes display diverse nucleic acid compositions
  • Single-stranded or double-stranded
  • DNA or RNA based
  • Segmented or non-segmented
• Positive-sense single-stranded RNA genomes predominate
  • Serve directly as mRNA for protein synthesis upon infection
  • Examples include Tobacco mosaic virus and Potato virus X
• Negative-sense RNA genomes require additional steps
  • Found in viruses like plant rhabdoviruses (Lettuce necrotic yellows virus)
  • Necessitate complementary RNA synthesis before protein production
• Genome organization maximizes coding capacity
  • Utilizes overlapping open reading frames
  • Employs polyprotein strategy (common in Potyviridae)
  • Produces subgenomic RNAs for efficient gene expression
• Movement proteins feature prominently in plant virus genomes
  • Essential for cell-to-cell movement through plasmodesmata
  • Examples include the 30K protein of Tobacco mosaic virus and the triple gene block of potexviruses

Plant Virus Replication Strategies

RNA Virus Replication

• RNA-dependent RNA polymerase (RdRp) plays a central role
  • Synthesizes complementary strands and new genomic RNA
  • Often encoded by the viral genome
• Positive-sense RNA viruses employ direct translation strategy
  • Genomic RNA serves as mRNA upon entering host cell
  • Produces viral proteins, including RdRp for genome replication
  • Examples include viruses from Potyviridae and Bromoviridae families
• Negative-sense RNA viruses require additional steps
  • Virion-associated RdRp synthesizes complementary positive-sense RNA
  • Resulting RNA serves as template for protein production and genome replication
  • Examples include plant rhabdoviruses (Sonchus yellow net virus)

DNA Virus Replication

• DNA plant viruses utilize varied replication mechanisms
  • Geminiviruses replicate in the nucleus using host DNA polymerases
  • Employ rolling circle mechanism for genome amplification
  • Examples include Maize streak virus and Tomato yellow leaf curl virus
• Caulimoviruses use unique reverse transcription strategy
  • Replicate dsDNA genome via pre-genomic RNA intermediate
  • Reverse transcriptase enzyme synthesizes new DNA strands
  • Examples include Cauliflower mosaic virus and Cassava vein mosaic virus

Replication Complexes and Strategies

• Many plant viruses form specialized replication complexes
  • Associated with cellular membranes (endoplasmic reticulum, chloroplasts)
  • Concentrate viral and host factors necessary for replication
  • Examples include the cylindrical inclusions of potyviruses
• Subgenomic RNA production serves as expression strategy
  • Allows expression of genes located internally on genomic RNA
  • Commonly used by viruses with larger genomes
  • Examples include Tobacco mosaic virus and Brome mosaic virus

Plant Virus Classification

Baltimore Classification System

• Groups plant viruses into seven classes based on genome and replication
  • Class I: dsDNA viruses (Caulimoviridae)
  • Class II: ssDNA viruses (Geminiviridae)
  • Class III: dsRNA viruses (Reoviridae)
  • Class IV: (+)ssRNA viruses (Potyviridae, Bromoviridae)
  • Class V: (-)ssRNA viruses (Rhabdoviridae)
  • Class VI: ssRNA-RT viruses (not found in plants)
  • Class VII: dsDNA-RT viruses (Caulimoviridae)
• System facilitates understanding of viral replication strategies
  • Provides insights into genome structure and expression mechanisms
  • Allows for comparison of viruses across different host types

Morphological Classification

• Categorizes plant viruses based on particle shape and structure
  • Icosahedral (spherical) viruses
  • Rod-shaped viruses
  • Filamentous viruses
• Icosahedral viruses feature symmetrical protein shells
  • Composed of multiple copies of one or few coat protein types
  • Examples include viruses from Bromoviridae and Tombusviridae families
• Rod-shaped viruses exhibit helical symmetry
  • Coat proteins arranged around RNA genome in spiral formation
  • Tobacco mosaic virus serves as classic example
• Filamentous viruses possess flexible, elongated particles
  • Coat proteins encapsidate RNA genome in long, thin structures
  • Potyviruses and closteroviruses exemplify this morphology

Unique Morphological Features

• Geminiviruses display distinctive twinned particle structure
  • Consist of two incomplete icosahedra joined together
  • Reflect their bipartite genome organization
• Some plant viruses exhibit complex structures
  • Rhabdoviruses feature enveloped, bacilliform particles
  • Contain additional structural proteins (nucleocapsid, matrix proteins)
  • Examples include Lettuce necrotic yellows virus and Potato yellow dwarf virus