The purposes of this lesson are to identify different characteristics of viruses, to develop a dichotomous key for plant virus classification, and to classify plant viruses using the key. This activity can be used to introduce the notions of taxonomy, dichotomous classification, and diversity of plant viruses.
Viruses are noncellular microscopic particles that need other organisms to reproduce, move, and survive. A virus particle consists of nucleic acid enclosed in a hollow protein cage called a “capsid.” In addition to being enclosed in a capsid, some viruses are wrapped by an outer membrane of lipids and proteins.
Viruses can contain different genetic material, can come in many different shapes and sizes, and, depending on the virus, can infect bacteria, fungi, plants, and/or animals. Plant virus genetic material is composed of strands of RNA or DNA and be configured in any of five ways:
- Single-stranded RNA with positive polarity (ssRNA+)
- Single-stranded RNA with negative polarity (ssRNA-)
- Double-stranded RNA (dsRNA)
- Single-stranded DNA (ssDNA)
- Double-stranded DNA (dsDNA)
Each genetic configuration is associated with a different method for reproducing the virus particles within the plant cell.
Plant viruses come in two basic shapes: helical and polyhedral. Helical virus particles have an elongated shape and can be rigid rods or flexible filaments. Polyhedral virus particles are roughly spherical. The genetic material of a plant virus may be enclosed in one particle or split between two or more particles. Here are some examples of plant viruses and their characteristics:
- Tobacco mosaic virus (TMV) infects the leaves of tobacco, tomato, bean, and pepper plants and is an example of a helical, rigid rod virus; each particle is about 300 nanometers (nm) in length and 18 nm in diameter.
- Papaya ringspot virus (PRSV) infects melon and papaya and is an example of a helical, flexible filament virus; each particle is about 800 nm in length and 12 nm in diameter.
- Barley yellow dwarf virus (BYDV) infects barley, oat, maize, rice, and wheat and is an example of a polyhedral virus (specifically, an icosahedral virus, which is a polyhedral shape with 20 faces); each particle is about 26 nm of diameter.
An image of each virus taken with an electron microscope is shown in Figure 1.
Figure 1. Virus particles come in different shapes and sizes.
(Note: In the Part B image, the particles were negatively stained.)
Because plant viruses have no legs, wings, tails, or cilia, they reach plants with the aid of carriers (also called “vectors”) and other mechanisms. Viruses can be transmitted through living organisms (e.g., insects, nematodes, mites, and soil microorganisms), through plant seeds, and, for only a few viruses, through direct contact between a healthy and an infected plant.
Symptoms of a viral infection can appear in the plant’s leaves, flowers, and/or fruit. These symptoms are very diverse and depend on the virus, the infected plant, and the environmental conditions (e.g., drought, humidity, and temperature). The most common leaf symptoms include mosaic patterns, streaking, yellowing, curling, vein banding, and lesions. The flowers of an infected plant can show changes in color and shape, and the fruit can show discoloration, deformation, ringspots, and mosaic patterns. Examples of symptoms of virus infections on plant leaves, flowers, and fruit are shown in Figure 2.
Figure 2. Symptoms of viral infection on plant leaves, flowers, and fruit.
Because of the vast diversity of viruses, scientists have classified them into groups, families, genera, and species. One classification criterion is the virus’s genetic material—for example, is the virus genome made of DNA or RNA? Within these two groups, viruses can be further classified based on genome properties:
- Single- or double-stranded
- Positive or negative polarity of strands
- Strategies to produce proteins
Additional virus classification criteria include the shape, the host plant (i.e., the plant a virus infects), the mode of virus transmission (i.e., through vectors, seeds, or direct contact), and the disease symptoms produced on the infected plant (e.g., leaves with a mosaic pattern, yellowing or curling of leaves, abnormal flowers and/or fruit).
Organisms with common characteristics can be organized into groups. Members of a group are expected to have similar traits, such that studying a few individuals of a group makes it possible to describe and predict the characteristics of all the group members. The use of a hierarchy of characteristics to describe levels of classification suggests a sequence of taxonomic ranks, as follows (from low to high): species, genus, family, order, class, phylum, kingdom, and domain. As the rank increases, the number of taxonomic subgroups increases and so does the biological diversity.
A common method of classification is the use of a dichotomous key. A dichotomous key is a tool 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.
In this lesson, students will develop a dichotomous key to classify plant viruses based on genome, shape, mode of transmission, host plant, and disease symptoms.