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Wheat Blast Genomic Analyses Highlighted at PAG

SAN DIEGO (GenomeWeb) – With a new de novo fungal genome assembly, population sequence data, and other approaches, researchers from the US, France, and elsewhere are digging into an emerging plant pathogen that could have devastating food security and economic effects: a version of the rice blast fungus, Magnaporthe oryzae, adapted to infect wheat. 

Kansas State University plant pathology researcher Barbara Valent outlined some of these efforts to fight back against the blast during a plenary lecture at the Plant and Animal Genomes conference this week.

Wheat blast disease first turned up in Brazil in 1985, she explained, subsequently spreading to other countries in South America. University of Kentucky soil researchers found a single diseased head of wheat in a wheat trial plot in that state in 2011 — a pathogen they characterized in the journal Plant Disease in 2017.

The disease has since been documented in Bangladesh, where infected crops are burned to prevent wheat blast fungus spread. Such control efforts have not stopped the fungus from moving within the country, Valent noted, leaving it lurking on the doorstep of other wheat-producing regions in South Asia.

In a paper published in PLOS One last May, researchers from Mexico and Bangladesh forecast that a 5 to 10 percent production loss due to wheat blast in India, Pakistan, and Bangladesh would wipe out an estimated $132 to $264 million in wheat production in those countries each year.

Extensive research has been done on rice blast, including work aimed at uncovering rice features that confer resistance to M. oryzae and other pathogens. Such rice research efforts have informed resistance gene searches in wheat, Valent explained at PAG.

In particular, investigators are working to identify resistance genes in host plants, though far fewer of these "R" genes have been identified in wheat compared to rice, and wheat resistance genes typically confer only partial resistance to blast-causing infections.

And research is underway to characterize the fungal culprit and its diversity as well. Prior research suggests wheat blast is caused by the "Triticum" pathotype of M. oryzae, a soil-borne fungus species that appears to be highly evolved for pathogenizing grasses.

The fungi potently infect these grass plants with the help of specialized invasive hyphae and functionally redundant effector proteins that are expressed specifically in host plants, Valent explained. She presented data on some of these infectious fungal features at PAG, particularly insights into the nature of effector genes in M. oryzae Triticum, which has a compact, 40-megabase genome.

Some of that data came from a preprint paper appearing in BioRxiv last June, where she and her colleagues described a near-finished reference genome for an aggressive Bolivian isolate of M. oryzae Triticum called B71 that was assembled from Pacific Biosciences long reads and Illumina short reads.

Along with seven core chromosomes in the fungal genome, the fungal sequences fell into a dispensable mini-chromosome that contained not only repetitive sequences, but also effector gene sequences nabbed from the ends of fungal chromosomes. Together with re-sequencing data for eight more fungal isolates, their results hint that the mini-chromosomes contribute to the evolution of the wheat blast pathogen's effector repertoire.

"Our results, collectively, indicate that dispensable mini-chromosomes and non-dispensable core chromosomes undergo divergent evolutionary trajectories," Valent and her co-authors wrote in BioRxiv, "and mini-chromosomes and core chromosome ends are coupled as a mobile, fast-evolving effector compartment in the wheat pathogen genome."

Valent was also part of a team reporting in mBio in 2018 that used whole-genome sequences for 76 M. oryzae isolates to explore the genetic features in lineages of the blast fungus that infect plants from a dozen grass or cereal genera, including wheat and rice.

Findings of that study "provide greater understanding of the eco-evolutionary factors that underlie the diversification of M. oryzae and highlight the practicality of genomic data for epidemiological surveillance in this important multi-host pathogen," authors of that study wrote.

Additional presentations and posters at PAG attempted to answer still other questions about the nature of wheat blast infections and potential strategies for preventing it. For example, Valent and collaborators from the US, Uruguay, and Bolivia presented a wheat breeding-focused poster at PAG that included a search for candidate wheat blast resistance factors in wild wheat relatives such as Aegilops tauschii strangulata.

"Resistant lines will be selected and, using whole-genome sequencing data, we expect to characterize new [wheat blast] resistance genes," the authors explained in an online abstract accompanying the poster.