WOOSTER, Ohio -- A soybean plant introduction (PI) has been found to show resistance to soybean aphid in Ohio, paving the way to control the insect through new resistant cultivars.
The soybean PI, labeled PI 243540, contains a single dominant gene that allows the plant to resist soybean aphid feeding and aphid colonization. These findings have been published in the September-October issue of Crop Science.
The soybean aphid, first discovered in Ohio in 2001, is a sapsucker whose voracious appetite can greatly damage untreated soybean fields. It also has been known to transmit a host of viruses, including soybean mosaic virus, soybean dwarf virus, and alfalfa mosaic virus not only in soybean but also in a number of vegetable crops.
Rouf Mian, a USDA-ARS molecular geneticist and adjunct assistant professor with Ohio State University's Ohio Agricultural Research and Development Center, said that the plant line was identified after a known soybean aphid-resistant plant, developed by University of Illinois, failed to produce the desired results in Ohio.
"We transferred the soybean aphid resistant gene, known as Rag1, from a resistant plant developed at the University of Illinois, to an Ohio cultivar and found that the hybrid was susceptible to soybean aphid, when it was expected to show resistance," said Mian, principal investigator. "It turns out the soybean aphid biotype in Ohio is different than the one in Illinois and this biotype can overcome resistance to the Rag1 gene."
A biotype is a subspecies of organism morphologically similar to but physiologically different from other members of the species. Some insect species are capable of developing new biotypes very rapidly and over a short period of time. Ron Hammond, an Ohio State University Extension entomologist, said that this is especially true for aphids.
"Soybean aphids, when they are feeding on soybeans, are all females and can produce a large number of offspring that develop very rapidly. One female can produce hundreds of offspring in a few days," said Hammond, who also holds a partial OARDC appointment. "Any individual with genetic resistance can pass the gene into the population, and this lends itself to new biotypes very quickly."
Researchers don't know how many soybean aphid biotypes are currently present in North America, but for now their focus is developing a soybean cultivar that the Ohio biotype won't colonize. Mian and his colleagues found that when given a choice between PI 243540 and a susceptible soybean variety, the soybean aphid chose the susceptible plant. After four weeks of the choice test, PI 243540 showed no feeding damage and only a few soybean aphids were found on the plant.
"That is significant given that aphid populations can reach into the thousands after only two weeks time. By four weeks, the impact from soybean aphid feeding and disease symptoms become evident on the plant," said Mian. "PI 243540 turned out to be a healthy plant."
In studies where soybean aphids were only given the choice to feed on PI 243540, researchers found that the aphids would "walk away" from the plant or populations would die off.
"We don't know what it is about PI 243540 and its gene, named Rag2, but soybean aphids don't like it," said Mian. "We are looking into the mechanism behind the resistance, but we think it might be a chemical compound being produced by the plant."
Mian is currently backcrossing PI 243540 with two OARDC-developed food-grade soybean cultivars: Wyandot and FG5. The work is designed to transfer the Rag2 gene into susceptible soybean varieties and test their resistance to the aphid in both the greenhouse and in the field. Researchers are seeing promising results and hope to have breeding lines available for release by the end of 2010 if agronomic characteristics, such as yield and resistance to major diseases, hold up.
"We chose to cross PI 243540 with food-grade soybean cultivars because of the importance of food-grade soybeans in the industry, especially when it comes to seed cosmetics and the potential economic impacts," said Mian. "This also opens up alternatives for producers looking to grow organic soybean varieties."
Soybean cultivars containing the Rag1 gene should be commercially available beginning next year. However, Hammond doesn't recommend growing cultivars with the Rag1 gene in Ohio.
"Given what we know about the biotype variations and current genetic work, we just don't think cultivars with the Rag1 gene will hold up here in Ohio," said Hammond. "Whether producers use those new cultivars or not, for now their control option remains spraying insecticides when aphid populations reach threshold."
In addition to PI 243540, Mian has identified other soybean plant introductions that show resistance to soybean aphid, albeit on a moderate level. Mian hopes to use those findings to develop partial-resistant soybean cultivars, whereby multiple genes rather than a single, dominant gene define resistance.
"The good thing about single genes is that they are easy to transfer into other plant cultivars. The drawback is that their resistance doesn't last long, maybe 10 years," said Mian. "Partial-resistant varieties hold up much longer because it's more difficult to overcome multiple resistant genes rather than just one."
Over the past few years OARDC has developed into a leading institution for soybean aphid research with experts forging efforts in molecular genetics, population genetics, genomics, protenomics, virus transmission, landscape ecology, and aphid origins and movement studies. OARDC's work is the latest in a series of national research efforts to more effectively manage soybean aphid. University of Illinois, Michigan State University and Kansas State University have all identified their own soybean plant lines that show resistance to soybean aphid.