Soybean root rot pathogen breaks the rules

For every rule there’s an exception.

Agriculture and Agri-Food Canada researchers have discovered a devastating soybean pathogen has developed a means of breaking Mendel’s laws, which sets out the rules on how genetic characteristics are passed on via inheritance.

The discovery could lead to better control of soybean root rot disease, a major problem worldwide, says researcher Mark Gijzen.

“There’s some other factor that is controlling virulence on soybeans,” said Gijzen, noting the pathogen — called Phytophthora sojae — has an “extremely unusual” ability to work around plant defences.

Virulence is the measure of the pathogen’s ability to attack soybeans, and Gijzen and his team have found that it wasn’t passed on by normal means of inheritance from parent organisms to their offspring described by Mendel’s laws. Instead, it uses something called transgenerational gene silencing to pass on traits that enable it to infect and kill soybean plants.

“A good analogy would be a person who had inherited the genes for brown eyes yet they had blue eyes,” said Gijzen.

“Transgenerational gene silencing is an epigenetic phenomenon, meaning the unit of inheritance is not the DNA sequence of the gene but rather some other self-propagating factor. In this case we believe it to be small RNA molecules. This has big implications that will affect the evolution of this pathogen and how we control it.”

Soybean root rot causes an estimated $40 million to $50 million in crop damage each year in Canada, and another $1 billion to $2 billion worldwide.

Canadian researchers have been seeking ways to combat it since it first showed up in southern Ontario in the 1950s. But lately scientists have been rethinking how genetics work, and whether genes are solely responsible for passing on traits. The relatively new field of epigenetics has found evidence for mysterious and hitherto unknown factors in plant and animal evolution such as environmental stress.

The discovery could help seed companies develop new types of resistant varieties or make more effective use of fungicides.

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