A genome-level study of two closely-related infectious fungi, that can affect maize crops, is offering scientists new ways of understanding how to breed more disease-resistant crop plants.

An international research team that includes Dr Britta Winterberg of the ANU Research School of Biology has been looking at the genes of two very similar pathogenic fungi. Both fungi - Ustilago maydis and Sporisorium reilianum - infect maize plants. However, they lead to very distinctive infection symptoms on their host plant. While U. maydis forms large tumors on leaves and flowers in which it develops millions of spores, an infection with S. reilianum is symptomless until the plant flowers. By looking at the diseases at a genetic level, the researchers have been able to identify differences betweens the genomes and manipulate the fungi to be both less, and more, virulent.

“Both fungi are so called biotrophic, meaning that they feed on living plant material. They secrete proteins into the host plant to escape the immune system of the plant and to manipulate the host,” said Dr Winterberg. “During evolution the plants develop new ‘weapons’ against the fungi. This evolution leads to ‘armament’; the plant develops new proteins to detect and kill the pathogen, and the fungus mutates to escape this recognition.”

By comparing the fungi, the researchers were able to see where the differences were at a genetic level.

“Regions that are different – or divergent - between the two fungi are probably under high evolutionary pressure, because they need to evolve to protect the fungus from the plant immune system. The identification of divergent gene clusters gives insight into the proteins that are essential for infection,” she said.

“From those identified regions we randomly chose six and deleted them from the genetic code of U. maydis. Three of the deletions lead to a reduced virulence, one increased virulence and two had no effect.

“While neither of these two plant diseases accounts for dramatic crop losses, they are model organisms that are easy to handle in the lab. And by understanding the mechanisms of virulence in these fungi we can transfer that knowledege to fungi with a higher economic impact, which could lead to more resistant crop plants,” said Dr Winterberg.

Not everyone would be delighted with the researchers’ genetic tinkering with U. maydis, however. In Mexico, the tumors it produces on maize crops are regarded as a delicacy and served with rice.

The results of their work are published in the latest issue of Science.