Breaking the genetic law

AN UNUSUAL DISCOVERY ABOUT SOYBEAN ROOT ROT

soybean root rot is known as one of the most destructive diseases facing Ontario growers. Despite decades of research projects focused on studying the disease, the root rot pathogen continues to overcome plant resistance and survive. 

Now, researchers at Agriculture and Agri-Food Canada (AAFC) have unveiled a significant discovery that will change how soybean research is conducted and could ultimately lead to improved control and management of root rot in the field.

Mark Gijzen, research scientist at AAFC’s Southern Crop Protection and Food Research Centre in London, Ontario, has found that Phytophthora sojae, the pathogen that causes root rot of soybeans, breaks Mendel’s Laws of Inheritance.

“Normally, traits are inherited using Mendel’s research but there are some exceptions known to this, such as what we’ve found occurring in Phytophthora sojae,” says Gijzen.

Mendel’s Laws of Inheritance
Gregor Mendel (1822-1884) is the namesake of the genetic laws that govern how characteristics are passed on through inheritance. He conducted his famous research through cross breeding common pea plants and observing the inheritance of traits from one set to the next, says Gijzen.

Mendel concluded that traits are inherited from one generation to the next through parent organisms passing on their traits and offspring obtaining those traits. 

new discovery
In the case of Phytophthora sojae, Gijzen and his team have made an unusual discovery in that virulence is passed on but not through the normal means. Contrary to Mendel’s studies, the pathogen uses transgenerational gene silencing to pass on the traits that enable it to infect and kill soybeans.

According to an AAFC news release, transgenerational gene silencing is an epigenetic phenomenon meaning the unit of inheritance is not the DNA sequence of the gene. Gijzen says there is instead some other self-propagating factor such as small RNA molecules.

He explains that this discovery has significant implications and will impact the evolution of this pathogen. That’s because researchers were previously testing strains using traditional Mendelian interactions; but now, they can adapt to more precise testing with the knowledge of the gene expression change.

focus on breeding
While the most immediate impact of the discovery for Gijzen will be how diagnostic tests are conducted, he says the findings will also impact how plant breeders study Phytophthora sojae right away.

“Breeders work to develop resistant strains and when root rot defeats the breed, it’s back to the drawing board,” says Gijzen. With knowledge of the unique inheritance method, breeders will now alter what they do to create better targeted methods of control and continue towards the goal of managing the disease through breeding resistant cultivars.

impact in the field
While nematodes rank first as the most widespread and damaging problem in Ontario soybean fields, Gijzen says root rot takes second place and is commonly referred to as an epidemic problem.

“Severity of the disease varies depending on environmental factors,” he explains. “Root rot is known to be more severe in heavy clay, wet soils, compacted soils and where no-till practices are used.”

According to AAFC, the damage caused by soybean root rot translates to production losses of between $40 and $50 million in Canada and between $1 and $2 billion globally every year.

collaboration between researchers
Given the significance of the disease, Gijzen finds value in working with plant researchers from Oregon State University and Nanjing Agricultural University in China. Because the root rot pathogen battled by Ontario growers is similar to what American and Chinese farmers face, the researchers take a team approach by comparing projects and sharing findings. 

Gijzen also collaborates with scientists focused on other crops such as potatoes. “Soybean root rot and potato late blight are closely related so we can learn a lot from each other in terms of what factors enable the organisms to defeat resistance,” he says.

next steps
While this unusual discovery marks exciting progress in the research of soybean root rot, a large component of how the pathogen passes plant defenses is still unknown. “We are now trying to understand how the pathogen is able to beat resistance at a genetic level,” says Gijzen.

He assures breeders and growers alike that the in-depth study of soybean root rot is not yet over. With a major discovery under his belt and new knowledge in mind, Gijzen’s eventual goal is to provide a toolbox of  research findings to breeders which in turn will enable effective management of the disease. •