LINDELL BEACH, B.C. – A notorious organism that was responsible for the Irish potato famine in the mid-1800s remains threatening today.
However, genetic scientists recently made a breakthrough in understanding how late blight functions and how it is capable of such destruction in potato and tomato crops.
The new information couldn’t come at a better time, considering the organism is poised to threaten tomato and potato crops in the United States this year.
“This pathogen has an exquisite ability to adapt and change and that’s what makes it so dangerous,” said Chad Nusbaum, co-director of the Broad Institute’s genome sequencing and analysis program.
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“We now have a comprehensive view of its genome, revealing the unusual properties that drive its remarkable adaptability.”
The potato is the world’s fourth largest food crop behind rice, wheat and corn.
It originated in the Andean region of southern Peru and a major subspecies flourished in the Chilean Archipelago, where it was cultivated as long as 10,000 years ago.
There are 5,000 potato varieties worldwide, 3,000 of which are found in the Andes.
Nusbaum, senior author of a major study published last fall in Nature magazine, said his international team discovered that the organism P. infestans has an unusually large genome size and an extraordinary structure that allows for the rapid evolution of genes, especially those involved in plant infection. Its unusual mechanism lets it outsmart the host plant.
“The notion is that the genome, as compared to other related organisms, is four to five times larger,” Nusbaum.
What surprised his team was not that the genome had more genes but that it contained a huge expansion in the amount of repetitive DNA, accounting for about 75 percent of the genome.
As the researchers unravelled the complex structure, they discovered that it is made up of regions rich in repeat DNA and other regions that are gene dense.
It is the repeat-rich regions that undergo rapid changes. They contain genes that are essentially knock-offs of each other that evolve, change and die rapidly. They play such crucial roles in plant infection that the host potato can’t keep up.
“These regions are chock-a-block with repeats,” Nusbaum said.
“We call them attack genes, like weapons. They bend the host plant to the will of the pathogen, which makes thousands of these attack genes. Many of them work, but if some fail there are many others to take their place. If the host plant has a resistant gene to the pathogen, the sheer volume of attack genes will overwhelm it.”
This rapid ability to change allows the organism to quickly adapt to new plant hosts, attacking genetically resistant potatoes.
The mystery over how it adapts so quickly to supposedly immune potatoes has puzzled scientists for years. However, armed with this study, researchers may have a lock on new techniques to mitigate the problem.
“One strategy is to develop a plant with more durable resistance,” said Brian Haas, the Broad Institute’s manager of genome annotation, outreach and analysis. “Now we can survey for these genes across other related pathogen species … and develop more robust plants.”
Story of a famine
The potato found its way to Europe with returning Spanish explorers and it spread as a staple food into England and Ireland.
However, while South American communities grew many potato varieties, Irish farmers grew only a small number, resulting in a lack of genetic diversity and leaving the tuber vulnerable to disease, notably late blight.
The disease was caused by Phytophthora infestans, a member of the oomycetes or water moulds family, which is closely related to the malaria parasite. It thrived in cool, wet weather and the blight could devastate entire fields in days.
In early 19th century Ireland, a meshing of political, social and economic pressures forced poor villagers and peasants to become entirely dependent on the potato, which they grew on marginal land after British gentry took over prime pasture to raise cattle.
As a result, the impoverished Irish were at ground zero for the Gorta Mor, “the great hunger.” From 1845-52, mass starvation and disease caused by the potato blight killed a million people. The famine was so devastating it entered folk memory and became the dividing line between pre- and post-famine periods.
