Potato blight's gene weaponry revealed
Jumping genes may hold key to defeating mould that caused Irish famine.
The blight that caused the infamous Irish potato famine of the 1840s has yielded its genetic secrets. An international team has sequenced the DNA of the microorganism that was to blame.
Phytophthora infestans, the water mould that causes late blight in potatoes, consumes and rots the leaves and tubers of the plant. The mould still afflicts potatoes, tomatoes and related plants, and costs farmers around the world an estimated $6.7 billion a year1.
Now Chad Nusbaum, co-director of the Broad Institute's genome sequencing and analysis programme in Cambridge, Massachusetts, and co-workers from numerous labs in the United States and Europe have sequenced the pest, using techniques honed on other organisms, plus a few tweaks2.
In doing so, they've identified a number of genes that might be responsible for the blight's destructive powers — and keys to its undoing.
When comparing P. infestans with similar organisms in the same genus, stretches of the genome stood out as being highly variable, unusually large and full of transposons — sequences that make copies of themselves and jump around in the genome. The researchers believe that the transposons, which make up about 74% of this unusually unwieldy genome, code for the blight's 'weapons' against potatoes.
"That is an insane number. For microbes, 25% is a lot," says Nusbaum.
The variability of these regions suggest that they are quickly evolving, perhaps as the blight and its targets evolve countermeasures against one another.
"The pathogen has learned to live with these transposable elements, which could be a problem [for it] because it is expensive to copy all this DNA," Nusbaum says. The mould could be using the transposons to maintain the diversity of its weapons arsenal, he suggests.
The sequence, Nusbaum adds, will provide "a comprehensive list of these weapons genes. Researchers can now give them the individual treatment that they deserve to figure out what they are doing."
The paper is published online today before its publication in print in Nature2.
In the arms race between plant and pathogen, potatoes have long had an ally: human plant breeders, who have struggled to develop blight-resistant spuds. The breeders have been working in the genetic dark, however, not knowing exactly what genes they are promoting or what genetic changes keep the blight nimbly adapting to their new varieties.
"At the moment, the breeding strategy has been based on screening the wild relatives from the highlands of Mexico and parts of the Andes such as Bolivia that have resistance," says potato breeder John Bradshaw of the Scottish Crop Research Institute in Dundee, UK. "What has happened is after taking 15 years to incorporate this resistance in a cultivar, it would take Phytophthora infestans only a couple of years to defeat it."
But now that the sequence is complete, he says, plant pathologists will be working flat out on new strategies for breeders based on how the blight operates and its potential weaknesses. "With all this knowledge about how the pathogen attacks the host on the biochemical level, I would hope that some clever plant pathologist would be able to genetically engineer resistance."
- Haverkort, A. J. et al. Potato Res. 51, 47-57 (2008).
- Hass, B. et al. Nature advance online publication, doi:10.1038/nature08358 (2009).