Potato photos, this page: Courtesy John Helgeson, Department of Plant Pathology, University of Wisconsin-Madison.
The individual cells made from protoplast fusion are starting to develop into tiny potato plants. These plants will be tested for resistance to late blight.
Chemicals may slow late blight, but wouldn't it be nice to find a spud that just ignored -- resisted -- p. infestans? While some potato varieties are less susceptible to late blight than others, nothing on the market -- so far -- is immune. After years of breeding, fungicides are still needed to get a good crop in damp seasons, even for varieties with limited resistance to late blight.
Resistance, conferred by the plant's genes, involves dimly understood defense mechanisms that still play a critical part in our ability to feed ourselves. Over the millennia since agriculture was invented, selection came mainly by default: Farmers and breeders raised the plants that survived any diseases or insects that were present. In fact, agriculture arose from simple evolution, artificially controlled.
Plants may look passive, but they have some tricks up their stalks. What can happen when a disease reaches a plant?
None of this jibes with our image of plants as passive victims of whatever nature throws at them, but it's critical to the problem of survival -- for plants and all the animals that eat them.
The task then becomes moving that trait into a strain with other desired traits -- say heavy yield, strong stalk and resistance to other diseases. The process takes years at best, but it's a key reason why agriculture can feed a booming population.
But related species don't always cross. And that was the problem facing Helgeson in 1994, when late blight struck a field of spuds he was testing for resistance to another disease. By the middle of August, he says, the field "was essentially dead stubble, with the exception of a few plants carrying genes from a wild potato relative from Mexico."
This plant couldn't care less about late blight -- it's equipped with a resistance gene derived from a wild relative of potato.
By no coincidence, the wild relative (Solanum bulbocastanum) came from an area west of Mexico City, in the shadow of the Volcano Toluca, that's considered the home of late blight -- Phytophthora infestans. "There are a tremendous variety of fungal isolates there," says Helgeson.
Because the two plants would not cross-breed, Helgeson turned to protoplast fusion. Here's how this "shotgun wedding" between related plants is done:
Eventually, after five to 20 or more plant generations, you may move the resistance gene -- without genetic engineering -- from the wild relative into a resistant crop. And that is exactly what Helgeson has done, as you can see from the photo above. The gene in question, he says, "Slows down the infection on the plant. It just doesn't allow the fungus to grow very fast, so it does not take over the plant and reduce it to nothing."
A better approach, she suggests, might involve "quantitative resistance," which depends on many genes rather than one. "If you have quantitative resistance, in many genes, each changes expression with changes in the environment." The advantage, she says, is that this type of resistance "can erode gradually, not go kaboom" like strong resistance from a single gene.
For his part, Helgeson says the new variety has survived fierce field tests. "We were very concerned, that was the reason to test them in Mexico," he says, where many strains of late blight are active. "Our idea initially was that what's resistant here in Wisconsin might not be resistant in a year or two. However, since the plant eventually becomes infected, these genes may not be as readily overcome as the 'strong ones' mentioned by Nelson," he says.
The potatoes, he says, have grown in Toluca, where the fungus is so virulent that farmers spray susceptible potatoes with fungicides 25 times a season. "The pressure is very intense," says Helgeson. "Day after day you get reinoculation [reinfection]. So far, it's looking very good."
Helgeson's variety hasn't reached the market, and may never help poor, third-world farmers. Can they fight blight?
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