Sequencing Project Targets Soybean Wilt

DNA sequence of two pathogens arm scientists and practitioners to assist in the development of new Phytophthora disease control measures.

Published on: Sep 6, 2006

New research identified the DNA sequence of two destructive group of pathogens called Phytophthora - the culprits behind an endemic pathogen of soybeans responsible for $1 billion to $2 billion in losses worldwide each year.

The two sequenced genomes belong to Phytophthora sojae and Phytophthora ramorum. P. ramorum, a newly discovered species, is associated with sudden oak death, which has devastated the coastal live oak ecosystems and the nursery industry in California, Oregon and Washington.

"These results, which identify the DNA sequence of the two pathogens, arm scientists and practitioners to assist in the development of new Phytophthora disease control measures," says Gale Buchanan, USDA under secretary for Research, Education and Economics. "Defeating these two pathogens could significantly reduce the billions of dollars lost to crop damage worldwide each year."

There are more than 80 species of Phytophthora, which attack a broad range of plants of agricultural, ornamental and ecological importance. Phytophthora affects nearly all broad-leafed (dicotyledonous) plants. Phytophthora pathogens are oomycetes, fungal-like organisms that are closely related to marine algae such as diatoms and kelp.

The researchers found that these oomycete pathogens have nearly twice as many genes as fungal pathogens, and that more than 40% of the genes in the two species are undergoing rapid change. Many of the rapidly evolving genes encode toxins and other proteins that may act to weaken the plant. In particular, each Phytophthora species hosts more than 350 genes related to oomycete avirulence genes. In a sense, the avirulence genes work against the pathogen because they allow the plant to detect the presence of the pathogen and mount a defense response. Because avirulence genes are often necessary for pathogen survival, there is pressure on the pathogen to modify the avirulence genes so the plant can no longer recognize them.

"We speculate that the rapidly changing genes are being driven to evolve by pressure from the defense systems of the pathogens' host plants," says Virginia Tech University’s Virginia Bioinformatics Institute Professor Brett Tyler, the project's principal investigator.

The genome sequences suggest many new avenues for attempting to improve plants' resistance to oomycete pathogens, according to Professor Jeffrey Boore of Department of Energy's Joint Genome Institute.
The Microbial Genome Sequencing Program has more recently funded the genome sequencing of three additional oomycetes, the potato late blight pathogen P. infestans, the broad host range pathogen P. capsici, and the downy mildew pathogen of mustards, Hyaloperonospora parasitica. These additional sequences will further accelerate understanding of these highly successful pathogens.