Scientists are now using a new biotechnological tool called Virus Induced Gene Silencing (VIGS) to determine genetic function in the soybean.
What will this checkoff-funded research mean to soybean growers? Learning more about the roles specific genes play in resisting pathogens will greatly speed the process by which researchers can develop varieties with improved disease resistance, and this, they have learned, is key to improving yield.
Through previous DNA studies, scientists have successfully identified the genetic makeup of the soybean plant. “We already have a good understanding of that genome,” says John Hill, the ISU professor who is leading the study. “The next natural step is to ask about the function of each gene. That is where VIGS enters the picture as a valuable tool.”
VIGS is called reverse genetics because it utilizes a process the opposite of the traditional approach, called forward genetics. With forward genetics, scientists would induce mutations in a variety of genes, inoculate a plant with a pathogen, and look to see if mutation of a particular gene affects resistance or susceptibility.
Reverse genetics involve the disruption of genes, then observing what happens when they no longer function in their roles. Thus, VIGS is a two-step process, Hill explains.
First, a virus is used to carry the specific gene being studied into the plant. Since Hill’s team is working on disease resistance, they select soybean genes which they believe might help create resistance to a disease, based on comparisons to similar model plant systems in which genes have been identified. When the plant is thus inoculated, it defends itself against the invading virus as well as the inserted gene. In the process, the soybean’s own corresponding gene is degraded or “turned off,” meaning it will no longer function as it otherwise would.
Scientists then inoculate the plant a second time with a disease pathogen to see whether the plant’s resistance to that disease has been eliminated. If it has, they know the gene they inoculated into the plant corresponded to a gene that was part of the plant’s resistance to that pathogen.
VIGS has actually been used since the late 1990s, and has been implemented for numerous crops including tobacco, tomatoes, potatoes and, more recently, barley and wheat.
VIGS is just now being applied to soybeans, says Steve Whitham, an ISU associate professor and close colleague of Hill’s working on the project. Scientists must have a suitable virus with which to inoculate each type of plant, and it took until recently to find the virus that works with soybeans.
The process is not as simple as finding one gene and knowing it is solely responsible for resistance to a particular pathogen, Hill emphasizes. A particular gene does not function in isolation.
“As we test a particular gene we may identify that gene as being one of several in a metabolic pathway that creates resistance,” he says. “This is the counterpart of molecular mapping. Through screening against a variety of pathogens, like Sudden Death Syndrome and white mold, it may be determined that one gene may, in combination with others, help to make a plant resistant to several diseases. By identifying those that are most involved with a particular disease, we won’t waste time on others. We can then develop markers for breeding purposes to incorporate those genes into plants. This equates to marker assisted selection.”
Scientists have already used this method to better understand iron deficiency chlorosis, says Craig Grau, University of Wisconsin professor of plant pathology.
Using VIGS, researchers are also able to measure changes in plants that cannot be measured in other ways. Through VIGS, for instance, a gene associated with low lignin can be identified and a variety can be selected specifically for low lignin.
Why is this process important? Essentially, this research dramatically speeds up the process by which scientists will be able to tell if disease resistance genes are present in a particular plant. In past efforts to test for resistance in a soybean line, scientists would have to wait until plants had reached the reproductive stage. Then the environment had to be right for the disease to invade the plants. Often it wasn’t until the soybeans had reached maturity that susceptibility was evident. This process took months.
“With VIGS it is possible to inoculate the plant just weeks after planting, when it has only three leaves,” says David Wright, ISA’s director of contract research and strategic initiatives. A couple weeks later the pathogen can be introduced. In traditional research, it would take a full generation to do this work.
Thus, months are taken off a cycle of research. This will greatly speed up the process of selecting resistant varieties. “You get the same result but have speeded up the process and can actually test more accurately for a particular trait,” says Grau. “Though we won’t completely get rid of field research, the number one advantage of VIGS is we can greatly speed up greenhouse tests for a particular defense trait.”
VIGS will have many more applications besides testing for disease resistance. “VIGS is a tool to see how genes work together to make a viable plant,” Whitham says. “If you understand how the genes work together, you can manipulate them to develop improved germplasm. In our particular work, we are using VIGS for pathogen studies, but in addition, one of our goals is to develop VIGS techniques and make them available to the entire soybean research community so they can be applied to other traits as well. Other scientists will be able to use this tool to study soybean development, yield, how a plant reacts to nitrogen and so on. As researchers, our ultimate goal is to contribute to the long-term improvement of the germplasm.”
