Science - March 31, 2005

Phytophthora like fungus after all

Plant pathologists have fought a bitter battle for years to grant Phytophthora infestans a separate status. They have asserted repeatedly that the cause of the dreaded potato blight is not a fungus but a type of oomycete, or water mould, more closely related to algae. A large-scale genetic analysis has now shown however that there are a number of remarkable similarities between Phytophthora and fungi.

‘It’s a bit like bats and birds,’ explains Dr Francine Govers of the Phytopathology group. ‘They can both fly and both have wings. They resemble each other, but are not closely related. That Phytophthora has pathogenic characteristics that are similar to those of many fungi is a clear example of convergent evolution. They have evolved independently, but use the same range of weapons to infect plants.’ Govers is co-author of the article in the journal Molecular Plant-Microbe Interactions, to which ten research groups from all over the world contributed. The researchers trawled a collection of over 75,000 effective genes (ESTs) that were isolated in a big project sponsored by Syngenta, and which was recently made publicly available on the internet. ‘ESTs, or Expressed Sequence Tags are pieces of cloned DNA, derived from RNA,’ continues Govers. ‘We have done things in reverse, using messenger RNA to reconstruct DNA. This way you get DNA from genes that are actually expressed.’

Analyses of the ESTs showed that the genes of the Phytophthora that make potato plants sick, strongly resemble those of true fungi. ‘In one way it’s not really that surprising,’ says Govers. ‘After all it’s about the genes that are involved in breaking down cell walls.’ What is more remarkable is that Phytophthora also has a gene that enables it to make chitin. ‘We tell students that fungi always have chitin in their cell walls and oomycetes do not, but it’s not as simple as that. They are capable of making chitin, but do not always do so, or only in very small quantities.’/ GvM