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A New Solution

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Posted Sep. 21, 2012

Noble Research Institute researcher pioneers new method for building disease resistance in crops
Kiran Mysore
Kiran Mysore, Ph.D., examines inoculated plants as part of his laboratory's research into nonhost resistance.
A switchgrass plant is covered in rust, a fungal pathogen that impacts many key crops worldwide.

As far as nightmare scenarios go, wheat rust remains near the top of the apocalyptic list. For generations, this fungal pathogen has devastated wheat crops, causing massive grain losses and famine in the early 1900s and 1950s.

Wheat, like rice, is essential to life on Earth. This hardy grass constitutes roughly 20 percent of the world's food calories. The Norman Borlaug-led Green Revolution of the 1960s saw plant breeders and researchers race to build genetic safeguards into wheat to provide necessary disease resistance. And it worked until 1999 when a new variety emerged in Uganda (Ug99) that has consistently overcome the previously resistant wheat varieties. Ug99 continues to devastate wheat crops throughout the Middle East and Asia. The new rust which actually includes half a dozen mutations has virility comparable to antibiotic-resistant superbugs for humans. It devastates without prejudice, leaving fields of black stems and no harvestable grain.

In an op-ed piece written for The New York Times in 2008, Borlaug said Ug99 was more dangerous than the original variety he defeated half a century ago. That version destroyed 20 percent of the American wheat crop. The Nobel Prize winner estimated that 80-90 percent of today's wheat varieties are susceptible to Ug99.

And since the fungal pathogen's spores are carried by the wind, it literally goes wherever the wind takes it. "Wheat and rice are staples," said Noble Research Institute Professor Kiran Mysore, Ph.D. "If wheat is gone, we're in trouble."

Plant breeding 101
Plant scientists like Borlaug and Mysore have continually sought ways to improve crops, focusing on increasing yields and endowing disease resistance.

Today, plant breeders have several ways to accomplish their tasks. The most common involves finding a plant within the same species that already exhibits the desired trait. Plants like humans inevitably have family members who are less susceptible to a particular disease. While one alfalfa plant may be destroyed by a pathogen, the next one may be completely unharmed. Plant breeders seek out these naturally occurring variations and then breed for the trait. It is a costly, time-consuming and difficult process.

Advanced genomic technologies have aided the process in the last few decades, reducing breeding time and cost. Researchers can even pinpoint the gene responsible for these defenses and breed specifically for it.

Still, for every technological leap, there is a pathogen that evolves to overcome the latest genetic defense, keeping plant breeders on a constant quest to find new solutions.

Mysore is among a handful of researchers around the world currently pioneering an advanced method for incorporating disease resistance into key agricultural crops.

The new process may show that plant breeders have been looking for their source traits in the wrong place. Or at least the wrong plant.

A new solution
The lasting answer to disease resistance may be found, not in the plant affected by a pathogen, but in a completely different species. This "nonhost resistance" is based on a fundamental principle: there is specificity between a plant and its pathogen.

Simply put, what makes one plant sick may have absolutely no impact on another species. Instead of breeding in a resistance gene from the plant's own family, researchers search nature for another species that has already created the blueprint for blocking the disease. For example, a pathogen that causes blight in tomatoes may have no ramifications in soybeans, because soybean has evolved a natural defense mechanism.

"There are thousands of fungal and bacterial pathogens in nature, and any given plant is susceptible to a minute fraction of these pathogens," Mysore said. "All we have to do is figure out which plant resists our target pathogen and apply it's mechanism to the host plant."

Of course, that's much easier said than done.

Mysore uses two methods to identify disease resistance genes and move them into a target crop.

The first involves silencing a gene, which is a method where researchers alter a gene's "expression" level in a model plant. Through this process, the gene's impact on the plant's physical traits is reduced like turning down the volume on a stereo.

Once particular genes have been turned down, Mysore's team challenges the plant with a type of bacterium which normally does not affect the species. If the plant gets sick, the gene in question has a role in plant defense.

In the second approach, Mysore's team uses Noble's extensive collection of mutants in a model legume. Mysore's team infects a whole population with a fungal pathogen that normally does not infect the model legume due to nonhost resistance. Mysore's team will then identify mutants that exhibit disease and research further to find the underlying genetic controls. Those genes are then added to a list of candidate genes that may be moved into another crop species.

The question of durability
The ultimate goal of Mysore's research is to find longer lasting disease resistance. Nonhost resistance seems to be a viable solution. Instead of focusing on one gene, Mysore envisions layering several of the candidate genes together into one new variety, providing multiple levels of defense.

"Nonhost resistance is clearly more durable than resistance based on a single gene," Mysore said. "Because it is intrinsic to the plant, and will usually last much longer."

Asian soybean rust, as well as various grass rusts (e.g., wheat and switchgrass), remain the target of Mysore's nonhost resistance work. However, the concept is being applied across the spectrum of plant diseases and crops. Mysore's international counterparts are attempting to breed newly discovered genes into crops like rice.

Though Mysore estimated it would take another decade to fully realize the potential of nonhost resistance, his research has already identified multiple candidate genes, including one promising fungal defender.

"This is a huge opportunity for agriculture," Mysore said. "We have several steps to go, but giving crops broad spectrum resistance once was a dream. Now, it's becoming a reality."