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Solving the puzzle of a clever fungus

UWA Media Statement:

Scientists from the CSIRO and UWA's Institute of Agriculture (IOA), with financial aid from the Grains Research and Development Corporation, have sequenced the genome of a strain of the common fungus Rhizoctonia solani and found some surprising results.R. solani infects many plants including major crops. Currently there is no effective resistance to it, leading to serious losses. The AG8 strain of the fungus has a very broad host range and causes bare patch and root rot diseases of cereals, canola and legumes, costing Australian agriculture about $87 million annually.

It is vital to investigate R. solani in order to discover exactly how it induces disease and how a plant's immune system may be adapted to overcome infection.The genome sequencing process was complicated given that R. solani has an average of eight nuclie per cell, and that there are numerous sequence mutations between these nuclei. By contrast humans have one nucleus per cell in most cell types.

"The multiple nuclei and the sequence mutations between them made assembly of its genome challenging, and required novel techniques," said the CSIRO's Dr James Hane, who is the lead author of a recently published paper about R. solani subspecies AG8 in PLoS Genetics."Sequencing one isolate was like mixing the pieces of different, but similar, jigsaw puzzles and then trying to put them back together in the correct position" said Winthrop Professor Karam Singh from the IOA and CSIRO.

"Having multiple nuclei is not the norm for plant pathogens. The level of diversity is similar to entire populations of other fungal pathogens having a single nucleus per cell. The difference is that all this diversity is held within one isolate," explained Professor Singh.The researchers are investigating how this high level of diversity and potential for genome adaptability may relate to the pathogen's broad host range and why no resistant crop varieties were able to be bred in the past.From previous investigations the team knows that two very different resistance mechanisms exist in two model plants that AG8 can infect, Medicago trunculata and Arabidopsis species. This suggests that different approaches may be needed to develop resistance against AG8 in different crop plants.

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