6.3 Plant-virus interactions and disease symptoms

Plant viruses are sneaky invaders that mess with plants' inner workings. They slip past cell walls, hijack plant machinery, and spread through tiny tunnels between cells. Plants fight back with immune responses and RNA interference, but viruses have tricks to dodge these defenses.

Viral infections cause all sorts of weird plant symptoms. Leaves might get spotty or curly, plants can end up stunted, and fruits may develop funky patterns. These symptoms happen because viruses disrupt normal plant processes, throwing off growth and messing with hormones.

Plant-Virus Interactions

Viral Entry and Movement

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• Plant viruses overcome physical barriers (cell wall, plasma membrane) to enter plant cells and initiate infection
• Viruses utilize mechanisms for cell-to-cell movement
  • Plasmodesmata modification
  • Production of movement proteins
• Viral replication complexes form in association with host cell membranes
  • Lead to reorganization of cellular structures (endoplasmic reticulum, Golgi apparatus)

Plant Immune Responses

• Plant immune system recognizes viral components through pattern recognition receptors (PRRs)
  • Initiates PAMP-triggered immunity (PTI)
  • Activates defense-related genes (pathogenesis-related proteins)
• RNA interference (RNAi) serves as a defense mechanism against viruses
  • Targets and degrades viral RNA
  • Limits viral replication and spread
• Plant hormones mediate antiviral responses and systemic acquired resistance
  • Salicylic acid induces expression of defense genes
  • Jasmonic acid activates wound response pathways

Viral Countermeasures

• Viruses employ suppressors of RNA silencing to counteract plant's RNAi defense
  • P19 protein of tombusviruses sequesters siRNAs
  • HC-Pro of potyviruses inhibits RISC assembly
• Some plant viruses manipulate host gene expression
  • Create favorable environment for viral replication
  • Suppress host defense responses (downregulation of resistance genes)
• Viral proteins interact with host factors
  • Disrupt normal cellular processes (protein synthesis, cell cycle regulation)
  • Enhance viral replication and movement (recruitment of host proteins)

Plant Viral Symptoms

Leaf and Stem Symptoms

• Mosaic patterns develop on leaves
  • Irregular patches of light and dark green or yellow areas
  • Caused by uneven distribution of chlorophyll (Tobacco Mosaic Virus)
• Leaf deformation occurs due to altered cell growth and division
  • Curling, crinkling, or distortion of leaves
  • Examples include leaf curl in tomatoes (Tomato Yellow Leaf Curl Virus)
• Chlorosis results from chlorophyll degradation or inhibited synthesis
  • Yellowing of leaves, often starting between veins
  • Observed in plants infected with Cucumber Mosaic Virus
• Vein-related symptoms appear in vascular tissues
  • Vein clearing: lightening of leaf veins
  • Vein banding: darkening of leaf veins
  • Common in plants infected with Potato Virus Y

Whole Plant and Fruit Symptoms

• Stunting reduces overall plant growth and development
  • Smaller plant size and reduced yield
  • Often seen in cereals infected with Barley Yellow Dwarf Virus
• Necrosis causes localized death of plant tissues
  • Brown or black lesions on leaves, stems, or fruits
  • Example: local lesions caused by Tobacco Necrosis Virus
• Ringspots form circular patterns on leaves or fruits
  • Chlorotic or necrotic tissue in ring-like formations
  • Characteristic symptom of Tomato Spotted Wilt Virus

Disease Development in Plants

Cellular and Metabolic Disruptions

• Viral interference with chloroplast function and structure leads to chlorosis and mosaic patterns
  • Disrupts photosynthesis and chlorophyll production
  • Alters thylakoid membrane organization
• Virus-induced changes in carbohydrate metabolism affect plant growth
  • Redirects nutrients to support viral replication
  • Impairs sugar transport from source to sink tissues
• Viral proteins interact with host factors involved in cellular processes
  • Disrupt chloroplast function (inhibition of RuBisCO)
  • Alter protein synthesis (hijacking of host ribosomes)
  • Interfere with cell cycle regulation (induction of cell division in mature tissues)

Hormonal and Developmental Effects

• Altered hormone balances caused by viral infection result in abnormal growth patterns
  • Contributes to leaf deformation and stunting
  • Disrupts apical dominance and branching patterns
• Viral movement proteins accumulate in plasmodesmata
  • Affects cell-to-cell communication
  • Causes developmental abnormalities (leaf malformation, stem fasciation)
• Viral suppression of RNA silencing disrupts normal gene expression patterns
  • Alters developmental timing (delayed flowering)
  • Affects organ formation and differentiation

Plant Defense Responses

• Programmed cell death (PCD) triggered by hypersensitive response (HR) forms necrotic lesions
  • Restricts viral spread to infected areas
  • Can lead to systemic necrosis in severe cases
• Systemic acquired resistance (SAR) induces defense responses throughout the plant
  • Upregulates pathogenesis-related (PR) proteins
  • Primes uninfected tissues for future infections

Environmental Influence on Plant-Virus Interactions

Abiotic Factors

• Temperature fluctuations affect viral replication rates and plant defense responses
  • Higher temperatures can increase symptom severity (Potato Virus Y)
  • Lower temperatures may suppress viral symptoms (Tobacco Mosaic Virus)
• Light intensity and photoperiod impact photosynthesis and plant metabolism
  • Low light conditions can exacerbate chlorosis symptoms
  • Altered day length affects viral movement in some plant species
• Soil nutrients and water availability affect plant vigor and stress levels
  • Nutrient deficiencies can increase susceptibility to viral infections
  • Drought stress may enhance viral symptom expression

Biotic and Environmental Stressors

• Humidity levels influence survival and spread of viral vectors
  • High humidity favors aphid reproduction, increasing virus transmission
  • Low humidity can reduce vector populations, limiting disease spread
• Air pollution and other abiotic stresses weaken plant defenses
  • Ozone exposure can increase susceptibility to viral infections
  • Heavy metal contamination may exacerbate viral symptoms
• Presence of other pathogens or pests creates synergistic or antagonistic effects
  • Co-infection with multiple viruses can lead to more severe symptoms
  • Some bacterial infections may enhance plant resistance to viruses

Climate Change Impacts

• Climate change-induced alterations affect virus and vector distribution
  • Warmer temperatures expand geographical range of certain viruses
  • Changes in precipitation patterns influence vector populations
• Extreme weather events impact plant-virus interactions
  • Increased frequency of storms can damage plants, facilitating viral entry
  • Extended drought periods may increase plant susceptibility to viruses