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Genomic Studies Tackle Banana Plant Pathogens

NEW YORK (GenomeWeb) – A pair of studies by independent research teams reporting in PLOS Genetics provide a genomic framework for attempting to combat the fungal pathogen behind black Sigatoka, or black streak disease, in banana plants.

For one of the papers, an international team led by investigators at Wageningen University and the US Department of Agriculture's Agricultural Research Service used Sanger and Illumina sequencing to assess genomic DNA from two strains of Pseudocercospora fijiensis fungus (also known as Mycosphaerella fijiensis).

With the help of a genetic map for P. fijiensis, the researchers put together a 74-million-base genome assembly for the banana leaf-damaging ascomycete fungal species, uncovering 13,107 predicted protein-coding genes and sequence expansions related to LTR retrotransposon proliferation. By genotyping hundreds more P. fijiensis isolates that do or do not respond to fungicide treatment, they also narrowed in on potential markers for fungicide resistance.

Other information in fungal and banana genomes may eventually lead to banana improvements aimed at bolstering banana crop resistance and reducing the industry's reliance on fungicide. For example, the team identified interactions between P. fijiensis genes and apparent resistance mechanisms in the black streak disease-resistant Calcutta 4 banana cultivar that might benefit other banana cultivars.

"This provides us with leads for increasing the sustainability of banana cultivation, making it better for the environment, the local population and the economy," co-corresponding author Gert Kema, a tropical phytopathology researcher at Wageningen University, said in a statement.

The black sigatoka pathogen is causing concern in the banana industry since it was first reported in Fiji in the 1960s. The airborne disease causes an estimated $500 million or more in banana production losses each year by damaging leaves and leading to early fruit ripening, Kema and his colleagues noted. The most-commonly grown Cavendish banana-related clones that the banana trade relies on is particularly prone to the disease.

In the other PLOS Genetics report, researchers from the US and the Netherlands delved into the evolutionary history of the Sigatoka disease complex — a closely-related group of fungal species that includes P. fijiensis, the eumusae leaf spot-causing P. eumusae, and the yellow sigatoka-causing species P. musae.

Their comparative genomic analysis — which included new genome sequences for P. musae and P. eumusae — revealed metabolic adaptations in P. fijiensis and P. eumusae that seem to have bolstered the ability of these fungi to nab nutrients from banana plants during black Sigatoka and eumusae leaf spot diseases, respectively.

"We have discovered that the two more destructive pathogens share a pattern of parallel changes in their core metabolic pathways that enables them to exploit more efficiently the nutrient resources available in banana," that study's senior author Ioannis Stergiopoulos, a plant physiology researcher at the University of California at Davis, said in a statement. "Now, for the first time, we understand the genomic basis of the evolution of virulence in these fungal diseases, thus giving us an opportunity for intervention."

In a perspectives article appearing alongside the papers, Timothy Friesen from the USDA-ARS Cereal Crops Research Unit discussed findings from both teams within the broader context of banana cultivation and sustainability. 

He noted that the studies "have together laid a strong foundation for future work, providing information and testable hypotheses for generating solutions to this devastating banana  disease complex that hitherto was virtually untouched, despite the large economic value of the crop as a staple food for millions of people and as the global top fruit."