Plant Pathology Exam 2

+ssRNA genome arrangement

+ssRNA acts like mRNA and is immediately translated

• Tobacco mosaic virus (TMV)

• Sharka Disease, caused by Plum pox virus (PPV)

• Papaya ringspot virus (PRSV)

• Zucchini yellow mosaic virus (ZYMV)

• Cucumber mosaic virus (CMV)

-ssRNA

The –ssRNA needs to be transcribed into mRNA first before
translation

• Tomato spotted wilt virus (TSWV

• 
  

ssDNA

ssDNA viruses act like plasmids.

• African cassava mosaic virus (ACMV)

dsDNA

dsDNA viruses are like retroviruses in that they are replicated
via an RNA intermediate.

• Cauliflower mosaic virus (CaMV)

• Banana streak virus (BSV)

dsRNA

• Rice dwarf virus (RDV)

Replication strategies of +ssRNA (Positive-Sense Single-Stranded RNA) Viruses

• Positive-sense single-stranded RNA can be translated immediately into viral proteins because it acts as mRNA.

• They rely on an RNA-dependent RNA polymerase (RdRP) that is immediately translated inside the host cell.

Replication strategies of -ssRNA (Negative-Sense Single-Stranded RNA) Viruses

• -ssRNA cannot be translated directly and needs to be transcribed into mRNA first before translation.

• They package a viral RdRP directly within the virion to initiate this transcription upon entry.

Monopartite genome

• A monopartite genome consists of a single continuous segment of nucleic acid.

Multipartite genome

• A multipartite genome is divided into multiple distinct genomic regions or segments.

• These separate segments are packaged into multiple individual virus particles rather than a single capsid.

Core plant virus proteins (indicate which proteins are only expressed and found in infected plants)

Systemic movement of viruses within plant hosts

• Step 1: Local Replication and Cell-to-Cell Movement: After entering the plant, the virus replicates in the inoculated leaf and moves from cell to cell (through epidermal and mesophyll cells) via microscopic cytoplasmic channels called plasmodesmata. This local movement is facilitated by viral movement proteins (MPs).

• Step 2: Entry into the Phloem Vasculature: The virus and its movement complexes enter the plant's vascular parenchyma and companion cells, allowing them to access the sieve elements of the phloem for long-distance movement.

• Step 3: Long-Distance Transport: Once inside the phloem, the virus moves systemically throughout the plant via bulk flow and translocation, traveling along with sugars and nutrients from source tissues (such as mature leaves) to sink tissues (such as roots and developing leaves).

• Step 4: Exit and New Infection Sites: The virus exits the phloem vasculature into surrounding tissues to establish new infection sites in upper parts of the plant.

• Meristem Exclusion: Despite spreading systemically through the rest of the plant, viruses are usually excluded from the root and shoot meristems (undifferentiated, stem-like cells).

• Tracking Systemic Spread: Researchers can track the progression of systemic infection in real time using fluorescent proteins

Modes of viral transmission

• Insect vectors

• Mechanical damage and Contact

• Seeds and Pollen

• Propagative Material and Grafts

• Soil and Water

Insect vectors

• Non-persistent: Viruses are acquired and transmitted in seconds to minutes, remaining retained on the stylet tips for a short time (minutes to hours) without circulating in the insect's body or replicating.

• Semi-persistent: Acquired in minutes to hours and retained in the foregut for hours to days

• Persistent-circulative: Viruses circulate through the insect's hemolymph and can be passed to subsequent developmental stages, taking hours to weeks before transmission

• Persistent-propagative (circulative-propagative): Viruses both circulate and actively replicate within the insect vector often passing to progeny

Mechanical Damage and Contact

Can be Easily transmitted mechanically through physical contact, rubbing between leaves, handling, contaminated clothing, or tools like cutting and pruning implements and watering hose nozzles.

Seeds and Pollen

Many plant viruses can be transmitted through infected seed coats, infected seeds, or through pollen from a virus-infected blooming tree into a healthy tree.

Propagative Material and Grafts

Viruses can be spread through infected vegetative cuttings, rootstocks, and grafting procedures (such as budding and grafting in fruit trees).

Soil and Water

Viruses can also move or persist through soil and water environments.

Insect vector transmission (non-persistent, semi-transmission, persistent circulative, and persistent-propagative)

Co-infection of viruses and how they can result in synergistic viral diseases.

• Co-infection: When more than one microorganism is infecting a plant

• Synergistic effect: When a co-infection of microorganisms causes worse disease symptoms

ELISA

• used to detect viruses in plants

Virus detection methods

Viruses can be detected via visual inspection, immunoassays,
protein-binding assays, nucleic acid amplification, and sequencing

Sandwich ELISA

• Most used because it has high sensitivity and specificity

• Important points:

  • High sensitivity and specificity.

  • Limitation: antibody optimization can be difficult.

Anatomy of fungi

septate versus coenocytic hypae

septate: cross walls within hyphae

coenocytic: hyphae without septa

how fungi are dispersed from plant to plant

Fungi are dispersed by spores. Spores can germinate and form new hyphae, allowing the fungus to establish a new mycelium

• rain

• wind

• animals

• moving water

Anamorph vs teleomorph

• Anamorph = asexual stage of a fungus, where it produces asexual spores (conidia).

• Teleomorph = sexual stage of a fungus, where it produces sexual spores

Life cycle of ascomycetes

general fungal life cycle

• Germination

• Mitosis

• Plasmogamy

• Karyogamy

• Meiosis

Anastomosis

A process in which hyphal branches fuse, forming
networks that enable communication and material exchange

Lifecycle of basiomycetes

Ascomycetes

• produce a sack-like ascus (asci) that contains eight ascospores

• lifecycle includes both sexual and asexual cycles and the haploid (n) stage dominates

• Ascocarps are sexual reproductive structures of ascomycetes,
  including cleistothecia (chasmothecia), perithecia, and apothecia

• sexual spores are called ascospores

• make asexual spores

• most have septate hyphae

Basidiomycetes

• has a dominant dikaryotic stage and
  does not include an asexual cycle

• clamp connections are needed for each cell to
  be bi-nucleate as the hyphae elongate

• All vegetative hyphae and visible structures are dikaryotic
  (n+n) and sexual recombination occurs in the gills

• Basidiomycete fruiting bodies are diverse

• Basidiomycete fungi can cause pseudo-anthers and
  pseudoflower formation which produce spores

Cryptic species

• Cryptic species are species that are difficult to distinguish based on morphology but can be identified by their DNA sequences and sometimes hosts.

• cryptic species often make up a species complex

Species complex

a group of closely related organisms that are so similar in appearance that it is hard to distinguish them apart

Forma specialis

The term “Forma specialis” denotes taxonomic groups with a specialized host range within a fungal species

Definition of a fungal race

a subdivision within a forma specialis that differs in its ability to infect particular varieties of a host plant.

race

is defined as a population (within a species) of which all individuals carry the same combination of virulence genes

Alternate vs alternative host

Alternate host: The other plant host the pathogen needs to infect and cause disease to complete its life cycle

Alternative host: Another host a pathogen could infect and cause disease but is not necessary to complete its life cycle

Macrocyclic

Macrocyclic rusts produce all five spore types

Demicyclic

Demicyclic rusts produce four spore types because they lack urediniospores

Microcyclic

Microcyclic rusts produce only teliospores and basidiospores.

Endomycorrozae (AMF)

• Penetrate root cells and colonize within the cells.

• Also called Arbuscular Mycorrhizal Fungi (AMF).

• Form arbuscules (tree-like hyphal structures) inside plant cells.

• Found in 85%–90% of the world's plant species.

• The fungus grows inside root cells as a highly branched arbuscule

Ectomycorrhizae

• Colonize the root surface and the apoplast.

• Never penetrate plant cell walls and often form symbioses with tree roots.

• Occur on the roots of about 5% of the world's plants.

• Nutrient exchange occurs through the cell membranes and cell walls of both the plant and fungus

Potyvirus replication (explain the special characteristics of this genus of viruses)

• The +ssRNA genome encodes a large polyprotein that is then
  cleaved into smaller proteins during infection

Main protein components of potyvirus

• Helper component proteinase (HC-Pro): helper component for aphid transmission and RNA silencing suppressor

• Coat protein (CP): necessary for virion protection, viral movement, and aphid transmission

• Nuclear inclusion protein b (Nib) replicase: necessary for replication (from +ssRNA/-ssRNA) (intermediate form that cannot be used directly to synthesize proteins)/progeny +ssRNA)

• Cylindrical inclusion (CI): part of replicase, necessary for pinwheel formations, and movement.

What is a diagnostic feature of potyvirus infection?

Pinwheel-shaped inclusion bodies are a diagnostic feature of potyvirus infection

Insect vector transmission and how that will impact acquisition time, latent period, and retention of virus particles inside of the insect.

As a virus and its insect vector becomes more persistent, acquisition time increases, a latent period appears, and retention of the virus inside the insect becomes longe

Example of when a plant virus was successfully eradicated or controlled and what occurred to achieve that outcome.

Papaya ringspot virus (PRSV) causes papaya ringspot disease

• Typical management practices were not effective at stopping
  the papaya ringspot disease epidemic in Hawaii.

• Scientists created transgenic plants by inserting the PRSV CP
  gene sequence into the papaya genome.

• The bombardment of nanoparticles covered with CP genes
  from a gene gun was used to transform papaya plant.

  • The gene is tagged with GFP and transformed cells will fluoresce green.

Example of when a fungal race caused a devastating epidemic and the circumstances that led to the epidemic.

• Example: Rice blast, caused by the fungus Magnaporhe oryzae (which has a broad host range in Poaceae, though individual isolates are host-limited). it is a Hemi-biotroph.

• Devastating Impact: It is a devastating disease of rice—a staple crop—resulting in an annual 30% global loss, which is equivalent to affecting 60 million people.

• Circumstances Leading to the Disease Cycle and Severity:

  • Inoculum and Spread: The fungus primarily survives on infected seed and residue from previous years, and its spores spread widely to current-year plants via air and wind.

  • Environmental Triggers: Favorable environmental conditions required for infection include high humidity and moderate temperatures (around 24 °C or 75 °F), and drought stress specifically increases the severity of the blast.

  • Specialized Infection Mechanism: The fungus uses conidia that attach to the plant using mucilage, germinate, and form a specialized structure called an appressorium. The appressorium develops a melanin layer that builds up immense hydrostatic pressure to drive a penetration peg through the plant cell wall.

  • Biotrophic to Necrotrophic Transition: During the infection cycle, the fungus first secretes molecules to inhibit plant detection and defense during the biotrophic stage, and later releases toxins during the necrotrophic stage.

Corn smut spore types

Teliospores and Basidiospores

Teliospores and Basidiospores. what are they and what disease are they found in?

• Teliospores

  • found in galls

  • overwinter in soil or plant debris

• Basidiospores

  • infect growing tissue

  • direct penetration

  • germinate, compatible, (+ and -) hyphae fuse to form dikaryotic mycelium

mycorrhizae symbiosis: how both plants and mycorrhizae benefit from each other

• How plants benefit from mycorrhizae:

  • Enhanced uptake of essential nutrients and minerals, such as phosphorus, nitrogen, potassium, and zinc.

  • Improved water access and absorption.

  • Greater overall growth and physical development.

• How mycorrhizae benefit from plants:

  • Acquisition of carbon, carbohydrates, and sugars produced by the plant via photosynthesis (with up to 10% of plant sugars going to the fungi)