A blackleg fungal lesion on the stem of a canola plant.
Credit: Barbara Howlett, University of Melbourne
SYDNEY: The discovery of a unique genome organisation in the blackleg fungus, which infects and damages canola crops worldwide, will help farmers to better protect their harvest from the disease, researchers report.
Changes in the virulence of populations of blackleg can lead to disease epidemics for certain canola varieties. The sequenced genome was used to develop molecular markers in fungal populations to allow scientists to note when large changes in virulence occur, and identify the variety of canola that is less susceptible.
The 12,500 genes that constitute the genetic blue print for the fungus Leptosphaeria maculans have been identified and now can be mined to discover how this fungus causes the deadly disease,” said Barbara Howlett of the University of Melbourne.
Deadly patchwork genome discovered
Howlett led an Australian research team that, together with French scientists, cracked the genetic code of the blackleg fungus, providing them with a map for identifying the genes that cause a disease which produces blackened lesions at the base of the stem in canola plants.
It was discovered that the blackleg has a unique “patchwork” genetic structure in which the disease-associated genes are not evenly spaced in the genome, the researchers publishing their findings in Nature Communications.
“The location of disease genes means that they are easily gained, lost and changed in the blackleg fungus,” said Barbara Howlett of the University of Melbourne. “This fungus produces millions of spores that are genetically different. If just one of these spores contains a different disease gene that the canola plants don’t recognise, large crop losses can eventuate.”
Resistance only allows short-term immunity
The researchers found that the fungus’ disease-associated genes are embedded within discrete blocks of junk DNA separate from blocks of ‘housekeeping’ genes, which are genes necessary for the survival of the fungus but that do not cause disease.
Canola plants have natural resistance genes to help defend against blackleg infections, and selective breeding is used to produce highly-resistant varieties of canola. But immunity to the fungus is only short-term. Resistant crops grown in monoculture are vulnerable to any population of blackleg that can overcome the plants’ resistance genes.
An evolutionary arms race
A new resistant canola variety with almost complete immunity to blackleg fungus was released in 2000. By 2003, farmers who had grown this canola variety each year suffered yield losses of up to 90% in some parts of Australia.
“Blackleg fungus and canola are in an evolutionary arms race. Growing a single highly-resistant canola variety provides crop immunity over the short term, but in the long run we are putting the canola at a disadvantage,” said Howlett.
