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​Plant Virus Classification: Dichotomous Key Answers

Answers to Study Questions:

  1. Describe the types of genomes you observed in your plant virus sample (card set).

    There are five types of plant virus genomes: ssRNA+, ssRNA-, dsRNA, dsDNA, and ssDNA.

    a) Most plant viruses have positive-sense, single-stranded RNA (ssRNA+) genomes. The “sense” of a nucleic acid refers to the polarity of its molecules. Positive-sense viral RNA is the template for protein synthesis, which is a process called “translation.” Examples of ssRNA+ plant viruses are Wheat streak mosaic virus (WSMV) and Lettuce mosaic virus (LMV).

    b) Fewer viruses have negative-sense, single-stranded RNA (ssRNA-) genomes. ssRNA- must be converted to a positive-sense, single-stranded RNA (ssRNA+) by an enzyme through a process called “transcription.” Once the virus RNA is in the positive sense, it can serve as a template for the synthesis of proteins. Examples of ssRNA- plant viruses are Fig mosaic virus (FMV) and Raspberry leaf blotch virus (RLBV).

    c) In virus genomes with double-stranded RNA (dsRNA), one strand encodes proteins and the other strand encodes enzymes involved in RNA transcription. The presence of dsRNA in a plant cell can be associated with the replication process of an ssRNA virus or the genome of an endornavirus: a stable virus that can infect avocado and bell pepper. Viruses with this genome infect mostly fungi, but recently, viral sequences of dsRNA were isolated from plants. Examples of dsRNA plant viruses are Bell pepper endornavirus (BPEV), Persea americana endornavirus (PaEV), and Radish yellow edge virus (RYEV).

    d) Reverse-transcribing plant viruses have double-stranded DNA (dsDNA) genomes. The production of nucleic acid involves the synthesis of DNA from RNA by an enzyme called “reverse transcriptase.” Only one family of plant viruses in this group can infect cauliflower and turnip plants. Examples of dsDNA viruses are Banana streak virus (BSV) and Cauliflower mosaic virus (CaMV).

    e) Single-stranded DNA (ssDNA) plant viruses include families with small, circular genome components that contain positive or negative DNA strands. Examples of ssDNA viruses are Squash leaf curl virus (SLCV), which infects maize, tomato, spinach, and bean, and Tobacco leaf curl virus (TLCV), which infects beet, banana, and bean.

  2. Describe the plant virus particle shapes you observed in your plant virus sample (card set).

    Plant virus particles come in two basic shapes: helical and polyhedral. Helical viruses have an elongated shape and can be short, rigid rods or long, flexible filaments. Polyhedral viruses are roughly spherical. The genetic material of plant viruses may be enclosed in one particle or split between two or more particles.

  3. Describe the disease symptoms produced by the plant viruses you observed in your plant virus sample (card set).

    The most common leaf symptoms include mosaic patterns, streaks, yellowing, curling, vein banding, and lesions. The flowers of infected plants can show changes in color and shape, and the fruits can show discoloration, deformation, ringspots, and mosaic patterns.

  4. What is a dichotomous key?

    A dichotomous key is a set of characteristics of organisms that allows classifying them based on a set of hierarchal criteria. A dichotomous key is formed using a set of “yes/no” questions about the characteristics of a given set of objects. The objects are divided into groups based on the responses to the questions at the various steps of the classification process. Successive questions in the key allow the user to separate objects into smaller and smaller groups until only one object is left in a group.

  5. What is the purpose of a dichotomous key?

    The purpose of a dichotomous key is to classify organisms that present different or similar characteristics and then identify members of different groups.

  6. Why do you think scientists classify organisms?

    Scientists classify organisms because members of a group are expected to have similar traits and processes, such that studying a few individuals in a group makes it possible to describe and predict characteristics of all the group members.

  7. Suppose you receive in your lab several small tomatoes with ringspots. Based on your dichotomous key, which virus may be causing this symptom on tomato fruit? Provide details about this virus based on the card information.

    A possible cause is Tomato bushy stunt virus (TBSV), which has an ssRNA+ genome. The modes of transmission are through seeds, soil microorganisms, and mechanical transmission. This virus also infects pepper, potato, and eggplant.

  8. Suppose a farmer sends you a plant with an unknown plant virus. Using an electron microscope, you see that the virus particles have a helical shape. Based on your dichotomous key, what type or types of genomes (e.g., ssRNA+, dsDNA, etc.) could the virus have?

    The virus particles could be ssRNA+, ssRNA-, or dsDNA.

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