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Corn+Soybean Digest

Transgenic Disease Fighter

If everything holds, the first genetically engineered disease resistance in soybeans will become reality - and the odds look super.

The research breakthrough came at Iowa State University (ISU). And it means that soybean mosaic virus, an increasing disease threat in the northern Soybean Belt, may have met its match.

Even more important, it could pave the way for transgenic protection against a whole squadron of viral diseases, including bean pod mottle virus, which is rapidly becoming a bigger problem.

That would be no small accomplishment since, as in humans, viruses are tough culprits to conquer. And, in fact, it's estimated that the complex of viruses affecting soybeans stole 9.2 million bushels from U.S. growers in 1998 - up nearly 7 million from the two previous years.

"This is, we believe, the first genetically engineered disease resistance in soybean that seems to be successful," says John Hill, who leads the ISU research team focusing on the soybean mosaic virus challenge. Scientists elsewhere are working on transgenic resistance to diseases like sclerotinia stem rot (white mold), but aren't as far along as the Iowa project, which is partially funded by grower checkoff funds.

What makes this apparent breakthrough especially exciting and satisfying for the scientists working on it is that they succeeded despite the low gene transfer efficiency achieved so far with soybeans, compared to, say, fruits and vegetables. Soybeans have proved to be a real challenge.

So how did Hill and colleagues get this transgenic resistance?

"There is a concept called pathogen-derived resistance," Hill explains. "You take a gene from the pathogen and, through biotechnology techniques, put it into the plant (soybean). That gene then, if you used the right one and are lucky, makes the plant resistant. You have to get the gene integrated into the plant's chromosome, which at this state of the technology is a random event we have no control over, so some luck is involved."

In the lab, the transgenic resistance looked like a home run.

"We challenged the resistance with virus levels that are way higher - in fact totally unreal - compared to what would be encountered in the field, and it holds," says Hill. "Based on some molecular biology work we've done, we think there is good reason to believe it's going to be fairly durable."

The all-important field tests that reflect conditions in a farmer's field were conducted last summer.

"On a preliminary basis, it looks like the disease spread where we hoped it would in the control plots but not so in the transgenic plots," says Hill. "That gives us a fairly good end-of-the season snapshot, and it looks like it was working the way it was supposed to. Of course, one year's data isn't conclusive.

"At this point, I can't imagine what would go wrong. However, that's always a dangerous thing to say. But it should work."

If you haven't encountered or even heard much about soybean mosaic virus in soybeans, don't feel out of touch. For one thing, it's hard to spot in the field in summer. Also, although the disease has likely been present for years, it has only been recognized as an increasingly important problem the past few years.

"It has been a bigger problem in the South in the past," Hill explains. "But I think that is changing, and I suspect it is somehow linked to our milder winters in general."

Symptoms are subtle, Hill cautions, and you could easily walk by infected plants without knowing it. The color may be a little off, more yellow, and plants may be shorter. Leaves may be malformed and blistered.

"Then when a farmer harvests, he notices yield is knocked down a bit but then looks at seed quality and says, 'What happened?' The elevator will likely confirm the bad news with dockage for black or brown mottled seed, the color seemingly bleeding from the hilum."

The virus is seed-borne, and then aphids spread it in a secondary fashion. These aphids, which apparently don't overwinter in the North, ride low-level jet streams and fall out in downdrafts and thundershowers in spring. Since the aphids don't colonize in spots in the field, they're hard for farmers to spot.

An aphid can make a 5- to 10-second probe of an infected soybean plant, then is able to transmit the virus to other plants for up to about 24 hours.

Spraying insecticides is futile for this problem, Hill says, because by the time the insecticide could work, the aphids have done their dirty work of transmitting the virus.

The hope is that the biotech model used by the ISU scientists will be a pathway they can utilize to develop a transgenically produced resistance to bean pod mottle virus - and possibly other diseases, too.

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