Week PPPodcast: 8
Welcome to the PPPodcast for week 8!
The Major Concept for this week is NEMATODES. Just what are nematodes, and how do they
differ from other plant pathogens?
First, nematodes are animals, like you and me - although not so very much like you and me. All
animals are eukaryotic, with genetic material contained within the nuclei of our cells, and all
animals are multicellular. Remember that plants and most fungi are also eukaryotic and
multicellular, while bacteria are prokaryotic and unicellular. Nematodes are also like us in that
they arecapable of locomotion, swimming with snake-like movements for very short distances
through the water in soil. Finally, nematodes are like us in that they feed by consuming other
organisms. Most nematodes feed on other small forms of life, such as bacteria, and on organic
matter. A few nematodes feed on, or parasitize, animals, causing diseases such as heartworm in
dogs and trichinosis in humans. And about 10% of all known nematodes parasitize plants. All of
the plant-infecting nematodes are obligate parasites, meaning they must have a living plant host
in order to survive.
But after these basic similarities, nematodes are quite different animals from human beings.
Nematodes are nonsegmented roundworms, most of them so slender that they are typically
invisible to the naked eye. Most of the plant pathogenic nematodes are between 300 and 1000
micrometers long - thats less than 4/100th of an inch! In some species, more rounded females
are produced, which can be seen without the aid of a microscope.
All plant pathogenic nematodes feed on their hosts with a stylet, a sharp, pointed feeding
structure that resembles a needle. They use their stylets to penetrate plant tissues, to secrete
enzymes into the plant, and to ingest the contents of the plant cells.
Some plant pathogenic nematodes spend their entire life outside of their host plants, just
inserting their stylets to feed. These nematodes are called ectoparasites. Most ectoparasites
are migratory, moving from cell to cell as they feed. Other species of plant pathogenic
nematodes are endoparasites, which enter the plant to feed from the inside. These nematodes
may be either migratory, moving through the plant tissue as they feed, or sedentary, inducing the
formation of specialized feeding cells from which they feed until they die.
Of course, before they die, nematodes need to reproduce. Plant pathogenic nematodes can
reproduce sexually, asexually, or sometimes in both of these ways. Their asexual reproduction
process is called parthenogenesis - a process in which fertile eggs are produced without mating.
Their sexual reproduction process requires that eggs the produced by a female be fertilized by
sperm produced by a male. Regardless of how the eggs were produced, nematodes develop into
first stage juveniles, or J1 nematodes, inside the eggs. These juveniles then shed their skin, or
molt, three more times to produce the next three juvenile stages called J2, J3 and J4. Lastly the
J4 juveniles molt into adults, the sexually mature males and females. A single female can
produce
hundreds of eggs to begin this simple life cycle again.
This weeks Point of Confusion is the terms genetic diversity and monoculture.
An area in which only one plant species is grown in close proximity, with few or no other types
of plants present, is called amonoculture. Many monocultures are created by people, such as
bountiful corn fields or fall flower beds full of chrysanthemums. We grow the corn in a
monoculture because it is easier to plant and harvest; we grow the chrysanthemums in a
monoculture because they look beautiful.
Plants within different monoculture systems can vary greatly in their level of genetic diversity.
For example, the potatoes grown by the Irish in the 1840s were not only all Solanum tuberosum,
they were all the same cultivar, Lumper. Since the Lumper potatoes were propagated asexually,
they were probably all nearly genetically identical. In this situation the potato monoculture had a
very low level of genetic diversity and when late blight struck, all of the potato plants were in
jeopardy.
In contrast, apple orchards are monocultures of the species Malus domestica, but in many apple
orchards growers plant several different apple cultivars. You might see Red Delicious, Empire,
Granny Smith and Gala apple trees all growing in the same orchard. This apple monoculture has
a higher level of genetic diversity than the Irish potato fields, and if a particular disease problem
arises, it is unlikely to kill all of the apple trees present. For example, Granny Smith apple trees
are very susceptible to fire blight, while Red Delicious apple trees are quite resistant and will
survive an epidemic of this bacterial disease.
Here are some Study Hints for week eight:
Question one: How are nematodes like and unlike other groups of plant pathogens that we
have studied?
Question two: What are some characteristics of all plant pathogenic nematodes, and what
are some characteristics that can be used to divide them into subgroups?
Question three: Why were quarantines not effective for managing soybean cyst in the
midwestern part of the United States?
Question four: How is the pine wilt nematode disseminated to new host plants?
And question five: How can a plant monoculture have a high level of genetic diversity?