Chapter 8. Microbiology
Microbes are extremely small organisms or quasi-living particles traditionally studied by botanists, but now treated within the more specialized field of Microbiology. As a specialized field, Microbiology has its own methodologies and terminologies often very different from those used by botanists. Nonetheless, there is value for beginning botany students to learn something of these organisms—to gain understanding of their presumably more primitive nature and significance as the causes of plant diseases.
A bacterium (plural: bacteria) is a single celled organism belonging to the Domain Bacteria, in the three domain scheme. It can also be a type of organism belonging to one of the three major branches of life. Traditionally classified as one of the five kingdoms, bacteria are microscopic and relatively simple cells, called prokaryotes. These cells lack the nucleus and organelles of the more complex cells called eukaryotes that plants are constructed of. However, like the cells of plants, most bacteria possess a carbohydrate-based cell wall. In common speech, "bacteria" still refers also to archaeabacteria, although the latter recently have been classified as an independent branch or "domain" of life.
True bacteria are the oldest organisms on Earth, with the possible exception of the Archaea, and they are also the most abundant. Bacteria exist in soil, water, and as parasites of other organisms. Species and strains of bacteria cause many if not most non-hereditary diseases. They are the target of the drugs known as antibiotics.
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A virus is an obligate cellular parasite that is completely dependent on the host cell for its replication. The genome of the virus may consist of single stranded or double stranded DNA or RNA. The size of viral genomes varies widely and may encode between one and 250 genes. Of most interest within the study of Botany are viruses that are plant pathogens. The majority of plant viruses have single-stranded, messenger-sense RNA genomes (Class IV) and encode only between one and 12 proteins. These proteins function in virus transmission, in replication, cell-to-cell and systemic movement, in the structure of the virus, and in the suppression of plant host defense mechanisms. In many cases, virus replication takes place in distinct virus-induced regions of the cell, the so-called viroplasms, and induces the synthesis of a pool of virus components followed by assembly of many virus particles from this pool.
Viruses can usually be horizontally transmitted between hosts. In many cases the transmission is dependent on insects, nematodes, fungi or other vectors. However, some other viruses, for example Tobacco mosaic virus (TMV), are transmitted mechanically by physical contact between plant tissue and virus-contaminated surfaces. Once the virus is transmitted and has successfully entered into the plant cell, it moves locally from cell to cell until it enters the phloem, which allows the virus to enter distant tissue to cause a systemic infection.
Even smaller than viruses are viroids . Viroids are infectious agents that consist of single-stranded RNA, which does not encode any proteins.
The first virus to be identified was TMV. A. E. Mayer was a professor at the Agricultural College in Wageningen, the Netherlands that was established in 1876. Soon after he was asked by farmers to investigate a highly contagious disorder in tobacco, which he called mosaic disease. In 1898, M. W. Beijerinck in the Netherlands concluded that the tobacco mosaic disease-causing agent was neither a bacterium nor any corpuscular body, but rather a contagium vivum fluidum, an infectious fluid. The next big step was made in 1935, when TMV was chemically purified by Nobel prizewinner Wendell M. Stanley in the United States. This was followed soon after in 1939 by the first visual observation of the rod shaped TMV particles by electron microscopy by Kausche, Pfankuch and Ruska in Germany. For a long time electron microscopy remained a main tool in virology, and many viruses were isolated and visualized. The study of the mechanisms of viral infection cycles became more important after molecular biology tools became available.
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Chapter 7 - Plant Systematics ~ :Chapter 8 - Microbiology ~ Chapter 9 - Algae