Banana genome sequencing gives a boost to pest-plagued fruit
Breeding a stronger, better banana is not for the weak-willed.
The plants are so sterile that scientists must mush up several acres’ worth of fruit to get just a couple of hundred seeds to work with — and those seeds are so feeble they must be coaxed in the laboratory to form baby banana plants.
That’s why tropical-fruit researchers were breaking out the banana daiquiris Wednesday to celebrate the sequencing of the banana genome by scientists in France.
If ever a plant needed biotechnology to help it, it’s the banana, these scientists said.
The DNA data — of a key banana species called Musa acuminata — will help researchers in their efforts to protect the fruit, which is under attack from all sides by a raft of noxious pests.
The biological blueprint published by the journal Nature will help conventional breeders and genetic engineers alike create varieties that are better suited to fighting disease and tolerating drought, not to mention being more nutritious.
More is at stake than the future of the Western fruit bowl. The $6-billion annual banana export market represents just 15% of banana production worldwide. The other 85% is food that hundreds of millions of people grow and eat to survive. In the developing world, bananas are the fourth most important crop after rice, wheat and maize.
“The people of the world — small farmers and multinationals alike — are growing varieties made by Mother Nature thousands of years ago,” said Rony Swennen, a banana researcher since 1978 and head of the Laboratory of Tropical Crop Improvement at KU Leuven in Belgium, where the world’s collection of 1,400 banana varieties is housed. “There’s no improved plants available for any of those growers.”
The problem stems from the fact that banana plants –- domesticated maybe 7,000 years ago and propagated since then through growth of new shoots — were selected precisely because they are sterile. Nobody wants to peel a banana and find a mess of seeds.
In edible bananas, seeds never mature. All we see of them are those little black dots. This produces a head-scratcher for those scientists who want to make the banana better.
“You need sterility to produce fruits that can be eaten, but you need fertility to improve the cultivar,” said Angelique D’Hont, lead author of the Nature paper and a genome scientist at CIRAD, a research center in Montpellier, France.
Improvements are sorely needed. The stacks of plump yellow fruit on supermarket shelves belie the mortal threat bananas of all types are facing.
Fungal spores borne by the wind blight the plants with black leaf streak disease, causing 50% crop losses if plantations aren’t sprayed weekly with fungicides.
Making matters worse, Fusarium mold in the soil is drying plants to death from the inside in a scourge known as Panama disease. There is no treatment.
A new strain of Panama disease — Race 4 — is poised to wipe out the Cavendish dessert banana we know so well, just as Panama Race 1 deep-sixed its commercial predecessor, Gros Michel, in the 1950s and 1960s.
Around the world, worms are nibbling at the banana’s roots and bacteria are withering its greenery. Banana bunchy top virus and weevils and other pests also feast on the plant.
Varieties resistant to some of these diseases do exist. But they generally lack other qualities people desire in a food they eat raw, steamed, boiled and fried, or drink as juice or beer.
And the export market is hyper-finicky. Every logistical detail — down to the shape of boxes used for shipping and the precise point at which green bananas are gassed with ethylene so they’ll turn ripe in time for the supermarket — has been standardized just for the Cavendish.
In the 1990s, researchers in Honduras unveiled a new banana christened “Goldfinger” that was resistant to black leaf streak disease and Panama Race 4. Their 24-year breeding effort involved analysis of 10,000 hybrids planted in the field.
But Goldfinger doesn’t withstand shipping as easily as Cavendish and does not taste as sweet.
As breeding efforts continue, scientists are working on other ways to get the qualities they want into bananas.
Over the centuries, spontaneous mutations in some banana lines have created variants with altered characteristics, such as the Cavendish banana’s resistance to Panama Race 1. Perhaps, some have noted, the process could be sped up.
Labs round the world, including ones at the International Atomic Energy Agency in Austria, have bombarded banana cells in test tubes with radioactive gamma rays. That has produced bananas with earlier flowering times, larger bunches, bigger fingers and some that appear more resistant to black leaf streak disease and Fusarium-induced wilt.
Some scientists, like Swennen, believe genetic engineering is the way ahead for the banana.
“I know there are a lot of arguments” about genetically modified foods, Swennen said from a banana meeting in Indonesia. “But if you are dealing with sterile plants and perennials, then GM is the way to go.”
At KU Leuven where he works, scientists have perfected the art of inserting genes into bananas and have a toolbox of DNA pieces that can dictate whether those genes are activated in leaves or roots. His team has engineered bananas with a disease-resistant gene from rice that, in field tests in Uganda, showed some limited ability to withstand black leaf streak disease. Ugandan scientists are working on refining the technology.
In another effort, genes from sweet peppers were spliced into bananas by a research team headed by Leena Tripathi of the International Institute of Tropical Agriculture in Nigeria. The team found 12 lines that were totally resistant to wilt caused by a bacterium called Xanthomonas, and they’re being evaluated in Uganda field trials now, Tripathi said.
Better nutrition and disease-resistance are the goals of James Dale of Queensland University of Technology in Brisbane, Australia. His team has engineered bananas that produce up to 25 times more beta carotene, a vitamin A precursor, than regular bananas. They’re being tested in Uganda, he said.
Another of Dale’s goals: bananas with more iron, which could be especially useful for people in India, whose vegetarian diets leave them prone to iron-deficiency.
Having the banana genome in hand will make all this work much easier, banana researchers say.
The Pahang wild species chosen for sequencing — which is resistant to Panama Race 4 and black leaf streak disease — is not good to eat. But it will serve as a guide to track down useful genes and improve the ones that are.
Already, scientists have detected certain genes among the 36,542 in the genome that leap into action when this variety is subjected to attack by black leaf streak disease, suggesting that they may be key to fighting off the fungus.
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