The banana apocalypse is nigh, but biologists may have found the key to their survival

The bananas you find in your supermarket and eat for breakfast are threatened with functional extinction due to banana fusarium wilt (FWB) disease, caused by a fungal pathogen called Fusarium oxysporum f.sp. cubense (Foc) tropical race 4 (TR4).

However, thanks to recent research by an international team of scientists led by the University of Massachusetts Amherst, we now know that Foc TR4 did not evolve from the strain that wiped out commercial banana crops in the 1950s, and that the virulence of this new strain appears to be caused by some accessory genes associated with nitric oxide production.

The research, published in Microbiology of natureopens the door to treatments and strategies that can slow or even control the still uncontrolled spread of Foc TR4.

“The type of banana we eat today is not the same as the one our grandparents ate. Those old bananas, the Gros Michel bananas, are functionally extinct, victims of the first Fusarium outbreak in the 1950s,” said Li-Jun Ma, professor of biochemistry and molecular biology at UMass Amherst and lead author of the study.

Today, the most popular banana variety on the market is the Cavendish variety, which was bred to be disease resistant following the extinction of the Gros Michel variety. For about 40 years, the Cavendish banana thrived around the world in the vast monoculture plantations that provide the majority of the world’s commercial banana crop.

But in the 1990s, the Cavendish banana’s boom began to end. “There was another outbreak of banana wilt,” says lead author Yong Zhang, who earned his PhD in UMass Amherst’s program in organismal and evolutionary biology under Ma. “The disease spread like wildfire from Southeast Asia to Africa and Central America.”

“We’ve spent the last decade studying this new banana wilt epidemic,” says Ma, an expert on Fusarium oxysporum, which is not a single species but a “species complex” of hundreds of different varieties that specialize in attacking different plant hosts. These varieties are determined by the acquisition of strain-specific accessory genes in addition to a common core genome.

“We now know that the TR4 pathogen that destroys Cavendish bananas did not evolve from the race that decimated Gros Michel bananas. The TR4 genome contains accessory genes related to nitric oxide production, which appears to be the key factor in TR4’s virulence.”

To reach this conclusion, Yong, Ma and their co-authors from China and South Africa as well as universities in the United States sequenced and compared 36 different strains of Foc collected from around the world, including those that attack Gros Michel bananas. Then, with help from the UMass Amherst Institute for Applied Life Sciences, the team discovered that Foc TR4, responsible for the current banana wilt epidemic, uses certain accessory genes for the production and detoxification of fungal nitric oxide to invade the host.

Although the team does not yet know exactly how these activities contribute to the Cavendish banana infestation, they were able to determine that the virulence of Foc TR4 was significantly reduced when two genes controlling nitric oxide production were knocked out.

“The identification of these accessory genetic sequences opens up many strategic avenues to mitigate, or even control, the spread of Foc TR4,” explains Yong.

Yet Ma is quick to point out that the ultimate problem facing one of our favorite breakfast foods is the practice of monoculture.

“When there is no diversity in a large commercial crop, it becomes an easy target for pathogens,” she says. “The next time you buy bananas, try other varieties that might be available at your local specialty food store.”