How the Soybean Cyst Nematode Enters and Changes a Soybean Plant


The nematode needs to eat in order to grow and it must get its food from the soybean. While some nematodes move through the plant, eating cells as they go, the soybean cyst nematode prefers to have its food delivered. Here is how it works. Like all plants, soybeans use photosynthesis to make nutrients in their leaves. These nutrients are delivered to all the cells in the plant, both above and below ground. Inside the root, the nematode selects a location to collect nutrients from the plant. Here we see a nematode inside of a soybean root. It has broken through a cell wall using its stylet, which is like a sharp straw, but it did not damage the cell membrane, just inside the wall. The nematode won’t eat this cell, instead, it uses the stylet to deliver signal molecules. Some of these molecules may cross the cell membrane, while others may bind to its surface. Some signal molecules have the ability to change what is happening in the cell’s nucleus, where most of the DNA, or genes, are found. The nematode signals will affect which genes are expressed, in other words, which genes are turned on, and, which genes are turned off. But before describing the effect of the nematode signal molecules, let’s first consider what might be happening in the nucleus before any of those signals arrive. The yellow molecule, moving along the DNA, represents a transcription factor. This molecule is produced by the soybean plant and can turn on one or more genes. It first binds to the DNA just before the start of a gene, and then a second molecule called RNA polymerase, joins and starts producing RNA from the DNA template. The transcription factor then falls off the DNA, but it may bind again later. As you can see in this model, one transcription factor can sometimes turn on multiple genes located in different parts of the DNA. Transcription factor molecules are very important in turning on genes because they help start transcription, or the making of RNA. Many genes cannot be transcribed into RNA unless their transcription factors are present and working properly. The RNA produced here in the nucleus can be used to make proteins that allow the cell to survive and complete all of its normal functions. Direct your attention to the top of the animation where another piece of DNA has just been added to the model. A teal colored transcription factor has entered the nucleus and turned on a gene here. This helps illustrate another aspect of genetic regulation. At any moment there may be many different transcription factors present. As a consequence, many genes can be turned on, or expressed, at the same time and it is the combination of all the genes that are being expressed which determines a cell’s function. Where do all of these transcription factors come from? Some are the products of regulatory genes. A regulatory gene is responsible for controlling the expression of other genes. For example, imagine that the yellow transcription factor turned on several genes, one of which made the teal transcription factor. Then, the teal transcription factor entered the nucleus and turned on the gene for the blue transcription factor. At the bottom of the screen you can see that the blue transcription factor is responsible for turning on yet another gene. So now consider what might happen if the yellow transcription factor were to be altered in some way by signals from the nematode. The small bright green molecule entering the animation represents a possible signal from the nematode and here it binds to the yellow transcription factor of the plant cell. This change to the transcription factor might cause it to recognize a different starting point for transcription. In this case, the original starting point was supposed to be the yellow section of DNA and the new starting point is represented by a green section of DNA. The result is an unexpected RNA molecule that will produce an unexpected protein. If this sort of change happens to a regulatory gene, then it is possible that multiple genes may be turned on or turned off by a single molecule from the nematode. It is also possible for signal molecules to block transcription factors from binding to the DNA. In this model we see the blue molecule from the nematode binding to the DNA. This gene is effectively silenced, or turned off as long as the signal molecule is present. This is another way for the parasite to change gene expression in the soybean cell. Zooming out we can see how these events in the nucleus are causing changes throughout the cell. We see changes in the size of the nucleus and the central vacuole. The blue color, spreading throughout the cell, represents the products from the new combination of genes that are now being expressed in the nucleus. The appearance and the function of the cell is altered. Notice that the cell wall even begins to break down. This cell, which was genetically reprogrammed by the nematode, now appears to induce the same changes in its neighbors by sending out its own chemical signals. The signals alter the gene expression of the neighboring cells and this genetic reprogramming moves, in a wave-like fashion, until hundreds of cells eventually fuse into one structure with many nuclei. This is called the syncitium and it serves to provide food for the growing nematode. The nematode has successfully altered these soybean root cells so that they now deliver the nutrients from the leaves, where photosynthesis takes place, directly to the nematode.

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