With the growing spread of a new strain of Panama disease, which is devastating banana crops, there is an urgent need to boost their resilience - and potentially apply the same approach to protect other vulnerable crops.
Around 5 billion bananas are eaten each year in the UK, according to the Fairtrade Foundation. Almost every single one of those is a Cavendish.
The Cavendish variety dominates the western world and accounts for around half of all bananas grown globally. As these plants are all genetic clones, the Cavendish is incredibly vulnerable - unable to adapt to change or the emergence of new threats.
Panama disease has already devastated one banana variety and now it has the Cavendish in its sights. To save them, scientists are searching for protective genes hidden in strange and beautiful relatives.
Consumers in the UK could be forgiven for thinking all bananas are yellow, finger-like fruits. That’s because around half of all bananas grown worldwide are of the Cavendish variety, accounting for the overwhelming majority shipped to the western world.
But bananas can also be round or straight, spotted or striped, and wear a technicolour coat of dazzling variety - from red or green to blue, silvery, and even pink. These different bananas have unique tasting notes, with some more savoury and others more acidic.
Up until the 1950s, the banana of choice was the Gros Michel. It had a sweet and creamy flavour, with a thick skin that made it easy to transport.
Things looked good for ‘Big Mike’. But, a decade later, the biggest growers had all switched to the Cavendish cultivar.
To understand the dramatic shift, we first have to appreciate the importance of sexual reproduction in plants - and learn that banana plants aren’t quite as they first appear.
“Banana plants might look like trees but in fact they are herbs, related to ginger,” explains Dr Jose de Vega, Group Leader at the Earlham Institute. “They die back after fruiting.
“Most banana cultivars are sterile. The small black dots inside the banana are reduced remnants of seeds and are not viable.”
He explains that, once a commercial banana plant has produced fruit and died back, it leaves underground rhizomes, which later produce suckers.
Critically, the suckers are genetically identical clones of the parent plant.
“There is no breeding and no crossing,” says De Vega. “Cultivated bananas cannot be bred. Any genetic susceptibility in one plant will be present in all plants of the same species.”
This monoculture has provided an ideal environment for the fungus Fusarium oxysporum f. sp. cubense, a pathogen with an affinity for bananas. Fusarium race 1 - known as Panama disease - began infecting Gros Michel plantations worldwide during the 50s.
Cultivated bananas cannot be bred. Any genetic susceptibility in one plant will be present in all plants of the same species.
Dr Jose De Vega, Group Leader at the Earlham Institute
It wiped out hundreds of thousands of hectares of farmland globally, causing worldwide shortages and complete collapse of income for millions of people dependent on the industry. As the fungus can persist in the soil for years, these farmers couldn’t even reuse their land.
The race 1 strain was so destructive that growers had to look to other banana varieties. The Cavendish emerged as their best bet - they had a similar taste, were suitable for large-scale cultivation, and were unaffected by race 1.
Gros Michel is still grown in areas kept carefully disease free, but its susceptibility to race 1 means it will never again be cultivated on a global scale.
“You will sometimes hear older people saying that bananas taste different from the bananas they remember eating as a child,” recalls de Vega. “That’s true because the bananas they ate as children were Gros Michels.”
Now history is repeating itself. A new strain of Panama disease - TR4 - is infecting and destroying Cavendish plantations globally.
“This time there is no cultivar similar enough to Cavendish that is resistant,” explains de Vega, whose research is using genomics and data science to find sources of resistance in other banana cultivars.
The cultivation of bananas started in the tropics of Asia thousands of years ago. Here, wild banana plants still grow, and thousands of varieties are cultivated.
It is in any one of the Cavendish’s thousands of lesser-known cousins that a solution to help it fight TR4 might be found. De Vega and his Group are looking for genes that offer resistance so they can be introduced to the Cavendish plants.
This is one of the development areas at Tropic, a biotech company also based at Norwich Research Park. They are developing non-transgenic gene-edited resistant elite banana varieties.
“At the Earlham Institute, we’re investigating a range of banana variants to see what their resistance levels are to TR4,” explains de Vega. “So far, we have infected 10 of 20 different varieties.
“Some plants - like the Cavendish - died quickly, and some took far longer to die. And some showed no signs of infection at all.
“These plants - the resistant plants - are the ones we’re most excited about.”
PhD Researcher Carolina Olave-Achury working with infected banana plants in the research lab.
After the group has identified plants which have resistance to TR4, they will be examining the genome of these plants to find candidate resistance genes which could be edited into the Cavendish.
Carolina Olave Achury, a PhD student working on TR4 resistance in the De Vega Group and collaborating with Tropic, says the hunt for a resistant gene is a complicated game of DNA hide-and-seek. Carolina uses in vitro growing and infection protocols to maintain and test the plants in controlled conditions.
“The presence of a gene for resistance in a resistant plant but not in a sensitive one is a good clue, but regulation and expression are also important. This requires other genomic approaches in my PhD, like expression analysis,” she explains.
“There are certain things we can look for about its appearance - the immune system of a banana plant works in a similar way to our own immune systems, so a gene related to immunity is recognisable, based on the identification of motif patterns in the protein sequence.
“Looking for resistance genes in wild plants has been proven to be a success. It also brings back diversity to a crop not only mono cultured but also sterile.”
Looking at the genomes of related species isn’t exclusively applicable to bananas. Across a range of food crops, there is a heavy dependence on certain cultivars due to their commercially attractive traits. This leaves them equally vulnerable to the emergence of new plant pathogens.
The approaches taken by de Vega’s Group are also being applied by others at the Earlham Institute in a range of organisms, improving the resilience of anything from wheat to farmed fish.
Conserving and enhancing biodiversity will be critical if we are to overcome the urgent global challenges facing living organisms and ecosystems in the future.
