Science - September 10, 2009

Potato bug genome is well-armed

The genome map for Phytophthora infestans has been charted. The potato disease pathogen has much in common with the malaria parasite.

Electronmicroscope-photograph of a potato-leaf that is infected with spores of Phytphtora infestans. The purple pathogen grows through the pores of the leaf, the spheres at the top will come loose and spread the infection.
The Phytophthora infestans genome, which causes potato blight, is much bigger than those of other Phytophthora types or of moulds. The oomyceet has a lot of so-called junk-DNA and an entire army of proteins which force their way into the potato plant and make it sick. This is revealed in an online publication this week in Nature by an international research team. One of its authors is Wageningen professor Dr. Francine Govers of the Laboratory of Phytopathology. She cautions plant breeders who want to produce potatoes resistant to Phytophthora:
'Phytophthora has a very flexible genome that can get around resistance.'
This pathogen causes huge damages worldwide on potato farms. Farmers bring herbicides to the rescue, but the spraying takes a heavy toll on the environment. The search for a resistant potato variety has therefore been going on for years.
An international group of researchers has now mapped the genome of the oomyceet. This genome is two and a half times as big as that of other Phytophthora types and six times as big as the genome of moulds. About three quarters of the P. infestans genome consists of so-called repetitive DNA: sequences with more of the same DNA which can quickly mutate. This repetitive DNA contains genes which produce effector proteins to infiltrate into the potato plant. As the genome is so dynamic, it always has effector proteins at its disposal which can break through the line of defence of the plant.
The outcome makes one wonder if incorporation of resistant genes into the potato genome is of any use, Govers says. 'The approach so far has been to look for resistant genes in wild potato varieties and to incorporate these into cultivars. Before using such a gene, it is very important to find out if P. infestans can get around the resistance and how long it takes to do so.' The pathogen has hitherto been compared to moulds, but it is more similar to the Malaria parasite. 'Like the Malaria parasite, P. infestans has an ingenious mechanism for transporting proteins to the inside of the host cell and suppressing any resistance there.