BananApocalypse – How concerned should we be?

Albert Eckhout: Still-life of citrus fruit and bananas

The alarm about the impending extinction of bananas has been raised over and over in the media over the past decade. How worried do we need to be? And what are plant geneticists doing to ensure long life for this fruit loved by consumers around the world and celebrated by so many artists?

From Paul Gauguin’s oil-on-canvas to Maurizio Cattelan’s perishable sculpture, from Joséphine Baker’s skirt to the famous “banana album” by Andy Warhol and the Velvet Underground, this elongated berry has become an icon of our culture. But the stakes are much more than symbolic, because the banana industry is worth $8 billion a year. And there are about 150 tropical and sub-tropical countries where this fruit represents both a cash crop and a staple crop. That is, an export crop that supports smallholder farmers’ incomes and also a staple food, which is eaten both cooked and raw and provides carbohydrates, vitamins, and minerals to millions of people.

International attention has focused, in particular, on the fates of the variety we find in every store – Cavendish – and its biological enemy number one: Fusarium oxysporum. An old strain of this fungus (TR1) decimated the Gros Michel variety in vogue until the 1960s favoring the advent of Cavendish, which was resistant. Another strain of Fusarium called TR4 now threatens the latter, but this time we have no commercially attractive resistant varieties ready to replace it.

Global production has held up for now, but the infection travels inexorably, carried by soil that sticks to shoes, wheels, tools. From Asia, the disease has reached Latin America and even Mozambique, raising concern for all of Africa, as this continent produces one-third of the world’s bananas. It is estimated that in the Great Lakes region, where Burundi, Kenya, parts of Congo, Rwanda, Tanzania and Uganda are located, annual per capita consumption is 220-460 kilograms, fifteen times the global average. In these countries, bananas provide 30-60% of daily caloric intake.

“TR4 is a global problem and attacks Cavendish, so a solution needs to be developed fast. We recently started working on fungal pathogens with small funding and we are optimistic. We will be getting some results by the end of 2023,” says Leena Tripathi of the International Institute of Tropical Agriculture in Nairobi, Kenya. The IITA group is part of the nonprofit CGIAR network and is testing in the greenhouse banana plants in which a susceptibility gene has been turned down through genome editing.

Good news also comes from Australia, where James Dale first succeeded in creating TR4-resistant transgenic bananas by transferring a gene (RGA2) from a wild variety (Musa acuminata malaccensis). Then the Queensland University of Technology geneticist achieved the same result with genome editing by switching on the corresponding endogenous gene, which was present in a dormant state.

Besides the queen of commercial varieties, there are more than a thousand varieties that lack the productivity and palatability to which we are accustomed, but which we might begin to appreciate by expanding our food choices. Their fruits are mostly marketed locally and used in diverse food preparations from desserts to beer, and it is not known how many of these cultivars are vulnerable to TR4.

There are many pests and pathogens that threaten banana trees besides Fusarium, including microorganisms, nematodes, and insects. From the African point of view, at the top of the priority list is the bacterium Xanthomonas campestris. “At my institute we have been working on bacterial and viral diseases for a long time,” explains Tripathi, who recently published a review in the International Journal of Molecular Sciences.

Developing resistant banana plants using traditional breeding is challenging because of low genetic variability available in Musa germplasm, lengthy production cycle, polyploidy, and sterility of most of the cultivars. Instead, “the availability of a robust genetic transformation protocol and the whole-genome sequence makes the banana an excellent candidate for gene editing.”

The achievement Tripathi says she is most proud of is “the CRISPR-edited banana resistant to bacterial disease,” although she points out that this is a proof of principle. “Varieties are not ready for the market yet. We have made good progress but still need a few more years before the product is ready for commercialization.” Among the genes targeted at IITA against Xanthomonas are “susceptibility genes, negative regulators of plant defense, or nutrient transporters.”

To identify potentially interesting targets, researchers compare the expression profiles of resistant and susceptible varieties exposed to infection (comparative transcriptomic analysis), or rely on the scientific literature on other species in which similar molecular mechanisms may be present. An example of the first type has been identified in the only known resistant variety (Musa balbisiana), an example of the second type is the banana orthologue of the downy mildew resistance gene isolated in the model plant Arabidopsis (MusaDMR6).

Scientifically speaking, two strategies are both effective: knock out a susceptibility gene or activate a protective gene. But to avoid problems from a regulatory point of view, it is preferable that the final products be free of foreign DNA, because the international trend is for lighter regulation of plants that do not contain transgenes. “Kenya, Nigeria and Malawi have already approved editing guidelines. Ethiopia and Ghana are also in the final stages of the process,” says Tripathi.

New insights for future experiments could come from genetic analysis of a growing number of wild and cultivated varieties. A recent study has unraveled a still obscure part of the banana’s history, revealing three previously unknown and possibly still extant ancestors. The data suggest that they originated in New Guinea, Thailand and the area between Borneo and the Philippines, respectively.

Most likely we would not like to eat their fruit, because before domestication, bananas were poor in pulp and full of large seeds. But by unearthing and studying the distant relatives, experts hope to find a treasure trove of genes useful for keeping modern varieties healthy.

[Translated from an article written by Anna Meldolesi for AgriScienza]

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