Research examines function of clubroot resistant genes

The battle to stay ahead of clubroot will be neither quick nor easy.

But a leading clubroot expert at Agriculture Canada says new research is revealing more about the disease and could result in more robust sources of genetic resistance in the next few years.

“Unfortunately, clubroot pathogen is very adaptable and current sources of genetic resistance are probably quite narrow,” said Gary Peng, a Saskatoon scientist who is among the country’s leading authorities of the topic.

“Although we have a limited number of clubroot pathotypes currently identified, in reality, the pathogen population is probably much more diverse than we realized before,” he added.

“For every resistant gene that we put out there, there is probably a clubroot pathotype that can overcome that resistance eventually … so it’s going to be a continuous battle.”

Over the past few years, Peng has dedicated much of his career to studying clubroot and devising new strategies aimed at minimizing its impact on canola, the most valuable cash crop in Western Canada.

Although several clubroot resistant canola varieties are already on the market, the durability of resistance in those varieties can be eroded quickly, especially if crop rotations are too short.

Peng’s work is aimed at enhancing resistance and extending the durability of the genetic resources currently known to researchers.

“Unfortunately, there is not a lot of diversity in our resistant sources right now…” he said.

“It’s hard to say definitively but it’s most likely that most of the clubroot resistant canola varieties carry only a single resistance gene.”

To learn more about resistant genes and how they work, Peng and his colleagues used the Canadian Light Source (CLS) synchrotron in Saskatoon to take a closer look at how a specific clubroot resistance gene functions.

That gene is known as Rcr1.

Until recently, it was still somewhat unclear how the Rcr1 gene conferred resistance against certain clubroot pathotypes.

But with the help of the CLS, Peng’s research has confirmed that the gene’s primary mode of action involves strengthening the plant’s cell walls and preventing the clubroot pathogen from entering or moving through the modified natural barrier of root cells.

“For this particular gene, the lignan accumulation in the cell wall, in response to the pathogen, is increased, meaning it will play a role in conferring resistance,” Peng explained.

The finding is significant because it could help researchers to identify genes that use different modes of resistance to fight clubroot.

If that can be accomplished, then the next step would be the development of new canola varieties that offer multiple or stacked gene resistance — or more specifically, resistance based on multiple genes that use more than one mode of action.

Canola breeders already know that the Rcr1 gene offers a level of resistance to some clubroot pathotypes.

But clubroot is a resilient disease that can adapt quickly to overcome single-gene sources of resistance.

The canola industry is working to develop new varieties that offer stacked gene resistance.

One stacked-gene cultivar is being tested in Alberta this year and others are likely to be entered into trials in 2019.

But knowledge of how the resistant genes work is limited.

Peng’s research, with help from the CLS, will increase the acuity of breeding work by identifying new genes or gene combinations with new modes of resistant action.

In theory, that could clear the way for the development of new canola varieties with improved stacked gene defences that offer protection against a broader spectrum of clubroot pathotypes.

It could also lead to additional varieties that offer protection against the 5x pathotype, a clubroot pathotype that has already overcome existing sources of single-gene resistance.

“We know that when you put the two genes together in a specific way, it becomes resistant to the new pathotype that we call 5x,” Peng said.

“It’s not completely immune but it does suppress the (pathotype) significantly.”

Employing the CLS to study gene functionality at a cellular level would allow breeders to meet their breeding goals more efficiently.

Peng’s research team has already assessed about 1,000 different brassica accessions from around the world.

Among those, a handful have been identified as clubroot resistant, warranting greater study.

“We really need to have a better handle on how … these genes function against different (clubroot) pathotypes,” Peng said.

“As long as we know the resistance genes have different modes of action or different responses to different clubroot pathotypes … we will be able to significantly enhance the durability of the resistance” using a multi-gene strategy.

Regardless of how many genetic advancements are made, rotation will remain a critically important factor, Peng warned.

“One of the tough things about this disease is the longevity of the pathogen itself, the durability of the resting spores,” he said.

“The literature says the spores can last for 15 to 17 years in the soil, but if you extend the rotation to more than two years… the level of infection … and the presence of inoculum is reduced significantly.”

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