Lesson 5: Plant Viruses of Agriculture and Ecosystems importance

Plant viruses play a significant role in agriculture and natural ecosystems, impacting both crop yield and biodiversity. They are classified into various families and genera based on their genome structure, particle morphology, and replication strategies.

1. Tobacco Mosaic Virus (TMV)

• Family: Virgaviridae
• Genus: Tobamovirus
• Virus: Tobacco Mosaic Virus (TMV)
• Transmission: Mechanically transmitted through direct contact, infected plant sap, or contaminated tools; can also be seed-borne.

Replication Cycle

• TMV enters host cells through wounds, then uncoats to release RNA.
• Viral RNA is directly translated by the host ribosomes, and the virus replicates in the cytoplasm.
• TMV movement protein facilitates spread to adjacent cells through plasmodesmata.

Pathogenesis and Clinical Signs

• Infects tobacco and many solanaceous plants (e.g., tomatoes, peppers).
• Causes mosaic or mottling patterns on leaves, stunted growth, and leaf curling.
• Severe infections can lead to necrotic lesions on leaves and stems.

Prevention

• Use of resistant varieties and virus-free seeds.
• Sanitation practices, including sterilization of tools.
• Avoid planting susceptible plants near infected crops.

2. Cucumber Mosaic Virus (CMV)

• Family: Bromoviridae
• Genus: Cucumovirus
• Virus: Cucumber Mosaic Virus (CMV)
• Transmission: Primarily spread by aphid vectors in a non-persistent manner; also transmitted through seeds and mechanically.

Replication Cycle

• Upon entry into the host cell, CMV releases RNA, which is translated into proteins.
• RNA replication occurs in membrane-bound vesicles within the cytoplasm.
• CMV moves cell-to-cell with the help of movement proteins and systemically through the phloem.

Pathogenesis and Clinical Signs

• Infects cucumbers, melons, tomatoes, and other plants.
• Symptoms include mosaic patterns, yellowing, stunted growth, and distorted leaves and fruits.
• Causes severe yield loss in crops due to poor fruit quality.

Prevention

• Use of aphid management practices, including insecticides and reflective mulches.
• Removing infected plants and controlling weeds that may harbor the virus.
• Planting virus-resistant or tolerant varieties when available.

3. Tomato Yellow Leaf Curl Virus (TYLCV)

• Family: Geminiviridae
• Genus: Begomovirus
• Virus: Tomato Yellow Leaf Curl Virus (TYLCV)
• Transmission: Transmitted by whiteflies (Bemisia tabaci) in a persistent manner.

Replication Cycle

• TYLCV enters host cells via whitefly feeding.
• The single-stranded DNA genome is replicated in the nucleus using host DNA polymerases.
• TYLCV moves through plasmodesmata into neighboring cells and systemically via the phloem.

Pathogenesis and Clinical Signs

• Causes severe symptoms in tomato plants, including upward leaf curling, yellowing, and stunted growth.
• Leads to significant yield losses due to reduced fruit production.

Prevention

• Use of insect-proof nets and reflective mulches to deter whiteflies.
• Application of insecticides and biological controls to manage whitefly populations.
• Planting resistant varieties and practicing crop rotation.

4. Barley Yellow Dwarf Virus (BYDV)

• Family: Luteoviridae
• Genus: Luteovirus
• Virus: Barley Yellow Dwarf Virus (BYDV)
• Transmission: Spread by aphids (e.g., Rhopalosiphum padi) in a persistent, circulative manner.

Replication Cycle

• BYDV replicates in the cytoplasm of host cells after aphid-mediated entry.
• Viral RNA serves as both a template for protein translation and replication.
• Virus spreads through plasmodesmata and systemic transport within the phloem.

Pathogenesis and Clinical Signs

• Affects cereals such as barley, wheat, and oats.
• Symptoms include yellowing, stunting, and reduced tillering, leading to poor grain yield.
• Infected plants often have decreased root and shoot growth.

Prevention

• Control aphid vectors with insecticides and integrated pest management (IPM).
• Early planting and selection of tolerant crop varieties.
• Use of resistant cultivars and avoidance of overlapping planting times with aphid peaks.

5. Rice Dwarf Virus (RDV)

• Family: Reoviridae
• Genus: Phytoreovirus
• Virus: Rice Dwarf Virus (RDV)
• Transmission: Transmitted by leafhoppers (Nephotettix spp.) in a persistent, propagative manner.

Replication Cycle

• Virus particles enter host cells and release double-stranded RNA, which is replicated in the cytoplasm.
• Viral RNA segments are used to synthesize proteins and replicate the genome.
• Virus moves cell-to-cell with the help of viral movement proteins.

Pathogenesis and Clinical Signs

• Affects rice plants, causing stunting and reduction in tiller formation.
• Leaves may show white streaks and rolling, leading to poor grain formation and yield loss.

Prevention

• Use of resistant rice varieties.
• Control leafhopper populations with insecticides and IPM.
• Early planting to avoid peak leafhopper populations.

6. Papaya Ringspot Virus (PRSV)

• Family: Potyviridae
• Genus: Potyvirus
• Virus: Papaya Ringspot Virus (PRSV)
• Transmission: Spread by aphids in a non-persistent manner and through infected seeds.

Replication Cycle

• Virus enters host cells and uncoats to release RNA, which is translated and replicated in the cytoplasm.
• Movement proteins facilitate local spread, and the virus moves systemically via the phloem.

Pathogenesis and Clinical Signs

• Infects papaya and other cucurbits (e.g., squash, cucumbers).
• Causes ringspot patterns on fruit, leaf mosaic, and distortion, leading to poor fruit yield and quality.

Prevention

• Use virus-free seeds and resistant cultivars.
• Control of aphid vectors using IPM practices.
• Remove and destroy infected plants to prevent spread.

Summary Table