BLACKSBURG, Va., Jan. 13, 2004 - Plant pathogens of the genus Phytophthora are notoriously destructive, causing diseases such as potato blight and 'oak sudden death'. Now, US geneticists are sequencing Phytophthora genomes in an effort to identify genetic factors that might help plants resist infection.
"[Phytophthora] cause tens of billions of dollars of damage each year to a huge range of agriculturally and ornamentally important plants," said Brett Tyler, research professor at Virginia Bioinformatics Institute in Blacksburg, USA.
The genus includes the soybean pathogen (Phytophthora sojae), the organism responsible for potato blight (Phytophthora infestans), and the new 'oak sudden death' pathogen Phytophthora ramorum.
Although they are often confused with fungi, Phytophthora actually belong to a completely different kingdom, the stramenopiles. "Fungal pathogens of plants are actually more related to humans than they are to the stramenopiles," said Tyler.
"There are only a limited number of chemicals that are effective against these [pathogens], but resistance to the chemicals builds up very quickly," Tyler noted. He and his colleagues are analyzing the genome sequences of several species to identify genes involved in pathogenicity and targets that might be used to control infection.
The researchers used infected soybean tissues and cultures of the P. sojae pathogen at different life stages, such as the swimming spores, which are the major infective form, or from mycelia. From these they created libraries of so-called expressed sequence tags (ESTs), which are essentially the front portions of the gene expression products, converted back to DNA.
The useful part of this EST strategy, apart from the fact it helps to sequence the genes quickly and ignores any intervening DNA, is that the frequency with which a particular EST occurs in the libraries gives a rough picture of when the gene is upregulated or downregulated under different circumstances.
The investigators found that many of the P. sojae genes that are most highly expressed during infection and are most specific to infection, encoded surface proteins and cell-wall proteins. "There seems to be quite extensive remodelling of the surface of the pathogen during the infection process," concluded Tyler. "But we are still in the process of starting to put together a picture of which of these ESTs actually has a key functional role and which is perhaps more related to the nutrient acquisition."
The team have also begun looking at how plants respond to these infections. The researchers compared their library, which came from a soybean strain that is fairly susceptible to infection, with one developed from a more resistant soybean strain.
Although the vast majority of soybean genes that were upregulated in response to infection are the same in the two strains, there were some differences. "There are a number of interesting genes that are specific to the susceptible interaction," said Tyler, "and some which seem very specific to the resistant interaction, especially some genes which are involved in protecting soybean from oxidative stress." He wants to examine the expression patterns of these genes further using microarray analysis to find candidates that could be added to infection-susceptible soybeans to protect them from the P. sojae.
The results were presented at the Twelfth Plant and Animal Genomes Conference in San Diego, California.
January 12, 2004