Features Agronomy Soybeans
Latest research on aphids aims to curb appetites

Key rests in gene in salivary glands

Research across North America is attempting to lessen the effects of pests and diseases on soybean yields.

Scientists at Kansas State University may have found a way to curb a pest’s appetite for crops like soybeans, corn and wheat. The researchers discovered that by silencing a gene in the salivary glands of pea aphids, the insect could not live for more than a few days on its host plant.  The findings could lead to new ways of controlling crop pests and reduce the need for chemical sprays. “It would not just increase the resistance level of the crop, it would turn that crop into a non-host for that species of aphid,” says Dr. John Reese, Kansas State professor of entomology.

The relationship between the aphids’ saliva and plants plays a key role in the research. Reese says they were so interested in salivary secretions because many aphids elicit striking responses in plants. Unlike chewing insects that crush cells, aphids have small mouth parts they use to pierce and suck the juices from the plant. Although it may be a more delicate feeding method, Reese notes that it causes many reactions in plants. For example, greenbugs on sorghum leave behind spots of reddish discoloration, while Russian wheat aphids cause streaking on wheat, as well as causing the crop to droop. In a resistant plant, Reese says aphid feeding may induce many factors that contribute to the resistance. In a susceptible plant, the aphids may be overcoming the plant’s defences. “So we wanted to get at these salivary secretions and see what was going on,” says Reese.

In their study, the researchers injected siRNA, a small strand of ribonucleic acid used to silence gene function, into adult pea aphids. In about six days, 100 per cent of the aphids that were injected were dead on the plant. While expecting that silencing the gene would have an impact, Reese says the injected aphids also exhibited some peculiar behaviour. “It’s not just that it takes them longer to get down to the sieve element in the plant where the nutrients are; it’s not just that it can’t stay in the sieve element, it’s affecting all sorts of things in their behaviour.”

Although silencing genes has been done in other insects, the work at Kansas State University is the first time anybody has silenced an aphid gene. Reese says it could pave the way for others, through biotechnology, to develop plants that would deliver the gene silencer. He readily admits that the prospect of such a development will be a challenge, adding that there are several hurdles to overcome. “With Bt, it wasn’t too difficult to put Bt transcript into plants. Then chewing insects ingest it and it kills them off,” explains Reese. “But with this situation, to figure out a way to do something to the plant to enable it to block the gene in a piercing and sucking insect like an aphid – that’s a pretty good trick.” 

Still, Reese believes the implications of the theory could be enormous, especially for controlling serious aphid pests that tend to have a narrow range of hosts. “Different species of aphids live on certain species of plants. Greenbugs can’t damage soybean plants, soybean aphids are never able to reproduce on a wheat plant or sorghum plant, and so forth,” says Reese. “But for that specific crop, you could theoretically turn it into a non-host.”

Research at Kansas State University is centred on making soybeans unappetizing to aphids, to the point where soybeans would be a non-host. Photo courtesy of Greg Zolnerowich, Kansas State University

Exactly when that might occur appears to be many years away. Reese says it is definitely a long-term venture. For the short term, Reese notes that the efforts to develop several resistant genes are becoming increasingly important. This assessment is particularly true for controlling the soybean aphid, which is starting to show signs of adapting to one of the stronger resistant genes. Reese says the major gene for soybean aphid resistance is Rag 1, discovered by researchers at the University of Illinois at Urbana-Champaign. Reese recalls that this gene appeared to offer powerful resistance against the soybean aphid. “We found aphids couldn’t just spend any time at all in the sieve element getting nutrients, so it was no wonder they weren’t reproducing and weren’t living very long. That was huge.”

However, a soybean aphid biotype, known as the Ohio biotype, has emerged and is breaking down the resistance offered by crops containing the Rag 1 gene. “Here, the soybean aphid has been in the US  since 2000, and in 2008, there’s already a biotype that can overcome this one major gene for resistance.”

It is not known how far this biotype extends, but Reese expects that it will gradually spread across the soybean growing region. As a result, the development will likely spark new interest in breeding programs in North America. Kansas State University plant breeders have been working with materials that do not contain Rag 1.  Reese describes these as having an intermediate level of resistance. “At first glance these materials didn’t look as exciting, but I thought this intermediate level is probably a different mechanism of resistance than the really strong ones.”

While perhaps less spectacular, Reese hopes the Kansas materials may carry resistance to the new biotype, as well as provide a more durable form of resistance, at least until the next biotype shows up. 

“It’s never say never with insects,” says Reese, referring to insects’ amazing ability to adapt. “It’s a constant battle, back and forth.”