Scientists have discovered vital clues as to how the pathogen responsible for the Irish potato famine adapted to spread between different plant species.
Researchers at Oxford University and the Sainsbury Laboratory in Norwich, England, looked in unprecedented detail at how Phytophthora infestans, a pathogen that continues to blight potatoes and tomatoes today, evolved to target other plants.
The study, published today in the journal Science, is the first to show how pathogens switch from targeting one species to another through changes at the molecular level.
Researchers examined the biochemical differences between Phytophthora infestans and sister species Phytophthora mirabilis, a pathogen that split from P. infestans around 1,300 years ago to target the Mirabilis jalapa lant, commonly known as the four o’clock flower. They found that each pathogen species secretes specialized substances to shut down the defenses of their target hosts.
“Plants have these enzymes called proteases that play a key role in their defense systems,” said study co-author Dr. Renier van der Hoorn of Oxford University’s Department of Plant Sciences.
“When a plant becomes infected, proteases help plants to attack the invading pathogens and trigger immune responses. P. infestans secretes substances called effectors that disable proteases in potatoes and tomatoes. These are highly specialized to block specific proteases in the host plant, fitting like a key into a lock.”
The effectors secreted by P. infestans are less effective against proteases in other plants such as the four o’clock, as they do not fit well into the “locks.” The researchers found that P. mirabilis evolved effectors that disable the defenses of the four o’clock plant but are no longer effective against potatoes or tomatoes.
“For the first time, we have found a direct molecular mechanism underpinning the change in host specialization,” said van der Hoorn. “We looked at specialization in the blight pathogens’ secret weapon, a key family of effectors called EPIC that can pass through plants’ defenses undetected to disable the proteases. The EPIC effectors secreted by P. infestans have evolved to fit the structure of potato proteases just as P. mirabilis has evolved effectors that fit four o’clock proteases.
“If we could breed plants with proteases that can detect these stealthy EPIC effectors,” van der Hoorn continued, “we could prevent them from sneaking in and thus make more resistant plants. Within the next decade, we plan to exploit the specialized nature of these effectors to develop proteases that are resistant to their action or can even trap them and destroy the pathogen. Potato and tomato plants with such proteases would be resistant to the blight pathogens, and combined with other resistant traits could provide another wall of defense against the pathogens.”
The study, titled “Effector Specialization in a Lineage of the Irish Potato Famine Pathogen,” was funded by the Gatsby Charitable Foundation, the UK Biotechnology and Biological Sciences Research Council, Ohio State University and the U.S. Department of Agriculture. It can be found at www.sciencemag.org.
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