Plant Viruses: An Introduction

Plant Viruses: Introduction

• Plant viruses were discovered by Martinus Beijerinck, who studied a virus infecting tobacco plants
• Ivanowski discovered the first virus in 1892
• Beijerinck discovered the Tobacco Mosaic Virus in 1898
• Bawden & Pirie revealed that the first virus contained RNA in 1937
• The first RNA virus was infected by in vitro transcripts from clones cDNA, Ahlquist in 1984
• Today, there are >1000 recognized plant virus species
• Plant diseases cause global food production losses of at least 10% each year
• Tulipomania was a historical event where infected tulip bulbs were traded for high prices

Disease Symptoms

• Disease symptoms include: dwarfing, leaf curling, reduced yield, fruit distortion, chlorosis (yellowing), color deviations, mosaic patterns, ring-shaped spots, wilting, necrosis, and scaling lesions

Virus Transmission

• Emerging and re-emerging RNA viruses are often transmitted by migrating insect vectors
• Rice viruses pose a major threat to rice production
• Plant viruses do not bind to surface receptors of plant cells which is the major difference between plant and animal viruses
• Horizontal transfer occurs when plants touch, while vertical transfer happens from parent to offspring through infected seeds
• Mechanical transmission is due to human or environmental damage and plants can be physically damaged by wind
• Wind can cause rubbing between healthy and infected leaves
• Soil transmission occurs when viruses in the soil transmit to leaves through wind-blown dust or rain-splashed mud, acting as an abrasive
• Soil-borne and waterborne viruses may also infect plants through slightly damaged roots

Vectors and Viral Entry

• Vector: piercing and chewing insects
• Vector: fungi, bacteria nematodes and parasitic plants
• Virus borne pollen and seeds: if pollen grains are infected the seedling will grow from that seed or may infect the plant through fertilized flower
• Viruses can transfer among fruit trees
• Vegetative propagation through cutting & grafting involves joining two young plants so that the best features from each combine to one plant
• A twig or scion is grafted onto a rootstock, developing into a new shoot like citrus and tomato

Diagnosis and Prevention

• Diagnosis involves techniques similar to those in animal and human viral infections
• Diagnosis includes: direct detection by electron microscopy, inclusion bodies by light microscopy, infectivity assays, serology (ELISAS), or DNA/RNA probes
• Prevention and control include managing insect vectors, removing alternate hosts, sanitation, virus-free seeds, resistant crops, and plant isolation

Morphology and Structure

• Most plant viruses are naked helical rods
• Plant viruses can be long, helical, flexuous (10nm x 480-2000 nm), rigid helical (15 x 300 nm), or short (bacillus-like) rods
• Polyhedron shaped plant viruses also exist- multipartite viruses, 17-60 nm in diameter
• Rhabdoviruses and tospoviruses are the only plant viruses that contain envelopes

Genomic Diversity and Viral Entry

• Plant virus genomes vary, dsDNA infects algae and lower plants only
• Majority of viruses are +ssRNA (90%), dsRNA and ssDNA
• Some viruses have segmented multipartite genomes
• Unlike animal viruses, multipartite plant viruses package each nucleic acid molecule into a separate virus particle which removes the requirement for accurate sorting
• Disadvantage of multipartite viruses: all individual genome segments must be packaged into separate virus particles and taken up by a single cell for effective infection
• Plant viruses enter through breaks in the cell wall/channels, known as plasmodesmata since plants have an impermeable cell wall
• Viruses use specialized movement proteins to traverse to neighboring cells through the plasmodesmata
• Plant viruses "channel" through the plant cell wall via movement proteins WITHOUT causing cellular LYSIS

Replication and Gene Function

• Once inside the cytoplasm, the viral genome is uncoated
• +ssRNA viruses encode their own RNA-dependent RNA polymerases for genome replication and use host factors for replicase complex formation
• The +ssRNA is copied into a -ssRNA intermediate, which then serves as a template for production of genomic +ssRNAs in new virions
• Most plant viruses have at least 3 genes that are associated with the replication of the viral genome and cell-to-cell movement of the virus (Movement protein)
• Movement proteins: associate with RNA/DNA viral genomes to form nucleoprotein complexes, dilate plasmodesmata microchannels, or form tubular structures for virus passage
• Movement proteins appear to be derived from host plant genes that encode for chaperonins, systematic plant infections may occur if the virus is transported long distances through the vascular system (phloem)
• One or more genes encode a structural protein that makes up the coat/capsid protein, plant virus exit: exit through modified plasmodesmata without causing cell lysis, differing from animal and human viruses that lyse cells or bud through the PM of cell

Systemic Viral Infection

• The virus enters through a wound and goes down the phloem, then to younger leaves at the top and down the plant

Tobacco Mosaic Virus (TMV)

• TMV has a helical shape, 300 nm x 18 nm in diameter
• TMV is one of the most stable viruses
• TMV has +ssRNA genome ~6500 nucleotides in length, with a protein to RNA ratio of 95%
• Martinus Beijerinck named TMV
• Rosalind Franklin showed that the TMV particle was hollow and its RNA genome was single stranded
• TMV can infect over 550 species of flowering plants
• TMV Infects commercial crops (tobacco, peppers, tomatoes, potatoes), and easily transmits from crop to crop
• Transmission occurs mainly through mechanical means, but not through insect vectors; infectivity can last 2 years in soil not exposed to freezing/drying

TMV Lifecycle

• TMV enters the plant cell via mechanical transmission
• Uncoating: A few coat protein (CP) subunits are removed, which exposes the 5' end of the genome. Co-translational disassembly: Translation, the genomic RNA begins as the virions are being disassembled
• The first ORF codes for an RNA-dependent RNA polymerase (RdRp) that has helicase and methyltransferase activity
• The first ORF is made up of 2 proteins of 126 kd or 183 kd and 10% of the time, the 126 kd protein is produced because the ribosome stops translation at the UAG
• 2nd ORF codes for a 30 kd movement protein
• 3rd ORF codes for 17.5 kd coat protein
• The TMV genome RNA acts like to be translated by host machinery, After RdRP proteins are translated, the resulting viral replication complexes (VRCs) initiate RNA replication in cytoplasm of plant cell
• It is synthesis of the full length -ssRNA, which acts as template for RNA synthesis of (i)progeny full length +ssRNA and (ii)2 shorter +ss mRNAs
• TMV and other viruses spread into the next cell through plasmodesmata (PD), breaking the cell wall and use specialized movement proteins to modify/open PD encouraging to cell virus movement
• PD are membrane lined channels that traverse the cell wall and connect neighboring cells: red circles represent soluble molecules capable of moving through the desmotubule of PD
• Spread is enter through wounds than continues locally
• Systemic virus spread occurs: long distance movement via phloem, gets to phloem→ base→ pumps up & trickles down, fast

TMV Control Methods

• To create infectious transcripts in vitro start transcription in-vitro with T7 Phage RNA polymerase
• The steps are Viral RNA→ cDNA [use PCR to make]→ dsDNA copy of viral genome inserted plasmid vector of E. coli, a method using scientist's growth and amplify large amounts of viral DNA
• Genetic changes can be introduced in DNA copy of genome
• It is nearly impossible to prevent TMV infection as it occurs wherever tobacco is grown
• On average, TMV reduces crop yields by 30-35%, reducing the market value and stable in dead plant matter in the soil and on contaminated seeds
• Control the spread of TMV by: Use only uncontaminated soils for seedling production, Prohibition of smoking, wash hands, remove all infected plants, do not plant new plants in the infected place, spray skim milk to limit rips, rotate the crops

Analysis and Resistance

• Analysis of disease resistance against TMV in wildtype (WT) and 35S/PtDrl01 transgenic tobacco plants has been tested
• PtDrl01 is full-length cDNA, Populus tomentosa

Citrus Tristeza Virus

• Citrus Tristeza Virus: occurs wherever citrus is grown, infects nearly all citrus species, especially sweet oranges, mandarins, lemons, limes and grapefruit which means sadness in Portuguese and Spanish
• In the 1930s farmers found devastating epidemics in Brazil and Argentina
• The symptomps are: Dieback, defoliation and small poor quality
• 3 viral suppressors of RNAI

Citrus Tristeza Genome

• Naked, helical flexuous rods, 2000 x 11 nm
• The genome has the same expression and stratigies as human Coronaviruses
• Consists of 12 ORFS, and the VSR stops the virus frominfecting

Virus Transmissions and Control

• Occurs by aphdis and grafting
• Transgenic virus-resistant stock roots: not susceptible that creates resistant varienties

Issues Caused by Viroid Complexes

• Viriods: the smallest known pathogens, are naked, circular, single-stranded RNA molecules that do not encode protein yet replicate autonomously when introduced into host plants; only infect plants
• Viroid disease action: produce small RNAs that act to silence host genes by complementary binding sites, triggers host defense response, and ribosome activity that can stall

Satellite Viruses, RIbozymes and Bioterrism

• encode their own protein coat but lack genes necessary for replication and are completely dependent on helper viruses for replication
• Cmv and sat-RNA infection killed tomatoes
• Targets a bioterrism

Ebola Discovery

• 1976: 2 simultaneous outbreaks of unidentified disease took place- 1 in South Sudan and 1 in Democratic Republic of Congo
• The first case was record in Zarie, and named after the Ebola river

Ebloa Mortality Symptoms

• Ebola's moratlity is 88%
• Common symptomps are: fever above 101, muscle pain, no appatite
• The worst effects includes: bleeding form nose, eyes mouth dirrea

Ebloa Transmission

• The trasmission happen by bodily fluids like blood, faeces, sweat and rare selavia
• 2019, the Ebola vaccines were made, before tht were some localised infectione outise of ebloa, causing globale concerns

Ebloa Viral Evation

• Evades with the help of the glocoprotein cleavage product is the SGP
• The virsls vP24 and VP35 block IFN's

Ebloa Precautions

• Use to be only supportive care
• As of 2019 the FDA approved many anti EBVOLA

Herepes Virus and Types

• Name orgined form the creep or crawl effect
• People stay impacted there whole life
• Herepes simplex 1, infects mouth
• Type 2 infects GENITALS
• Varicella ZOster
• Cytomegalo

Structure cycle of DNA

• Most are easily distrupted by heat, and only traansmited by directly contact of sex

EBOLA VIRUS- PENDING

• Proper handling of bushmeat
• Proper sanitation procedure
• Most the treatment infected patients with IV

Viral gene enconding

• Gag, Pol
• There encondes many genes In DNA it does the attment and goes striaght for the prot

HIV structure:

• Transmitred by the same routes
• CD4 cells are destroyed easily

Viral replication.

• Makes cd4 stronger and relasesd

Poliovirus

• Have vaccines to help it
• Comes in 4 types

Virus 17

• Can do vaccine modified for people's use
• Vaccia is the prototype