Saving the banana | Science News for Students
Saving the banana | Science News for Students
These Cavendish bananas probably look familiar because they are the kind found in grocery stores across North America and Europe. But Cavendish bananas are just one of hundreds of varieties eaten around the world.
Among the biggest worries is a nasty soil-borne fungus known as Tropical Race 4, or TR4. It causes “Panama disease,” which makes plants wilt and die.
But now TR4 threatens not only the Cavendish, but also many other varieties of bananas. The stalk on the end of a banana plant has a big structure at the end that is the male flower. Female flowers ring the stalk. Each of those female flowers develops into a single banana. The bananas are called “fingers;” the rows of bananas are “hands.”
Bananas don’t grow on trees. Reaching a height of nearly 3.5 meters (11.5 feet), banana plants lack woody trunks.
Banana plants grow fast.
One way that farmers grow new plants from these types is by removing suckers from mature plants and transplanting them.
Large plantations, however, typically start new plants from tissue culture. That means they take cells from a plant, put them in a nutrient broth and allow those cell clumps to develop into new plants. These clones are being planted over and over throughout the world.
Because each plant now has the exact same set of genes, if one plant is susceptible to a disease, all its clones will be too.
Not all bananas are seedless, though. Some bananas contain dozens of pea-sized seeds.
Last year, banana growers in Mozambique noticed some unhealthy plants. Fearing the worst, farmers had their plants tested. As a bunch of bananas ripen on one stem, a sucker (on the left of the plant’s main stalk) begins to grow from the base of the plant. When the taller plant is finished fruiting, the sucker will grow to take its place.
This fungus — and the Panama disease it causes — is one of the world’s biggest threats to bananas. The name of the disease reflects the country where it was first discovered attacking Gros Michel bananas.
The 2013 Mozambique blight was the first time Panama disease had showed up in Africa, notes Altus Viljoen.
Even careful control measures can’t completely protect plants from the fungus. That’s because TR4 is patient. Once a new banana plant begins to grow next to it, the fungus can attack once more.
Panama disease is not the only threat to bananas. Bacteria infect banana plants, causing them to wilt. Tiny worm-like nematodes can burrow into the roots, making the plants fall over. Unable to photosynthesize in the damaged areas, infected plants produce fewer and smaller fruit.
Now Viljoen, Swennen and other scientists are racing against time to stop the further spread of TR4 — which threatens more than half of the world’s edible bananas.
One approach to stopping the fungus: finding plants that are resistant to it. Such disease-resistant plants wouldn’t become infected. In the meantime, banana growers must slow TR4’s spread.
The center plant displays yellowing leaves, one of the telltale symptoms of Panama disease.
He’s a plant pathologist at Wageningen University and Research Centre in the Netherlands. The new test would allow farmers to test suspicious plants, soil and water for those snippets of TR4’s DNA, even out in their banana fields.
If those measures are successful, farmers could stop the disease in its tracks, guarding healthy nearby plants.
Detection of the fungus is essential. Still, the ultimate goal is the development of disease-resistant plants.
Swennen, the banana breeder, oversees a massive collection of banana plants. These bananas come from across the globe. So far, more than 90,000 plants have been given to farmers in more than 100 countries.
Bananas aren’t always big and yellow. Some of the world’s 1,400 banana varieties, 400 of which are edible, are small and reddish.
Most bananas at ITC are tropical.
Breeding new varieties from one parent that is tasty and another that is disease-resistant (a technique called cross-breeding) might one day create dessert bananas immune to TR4.
Swennen works with teams in Uganda, Tanzania and Nigeria to cross-breed African bananas. They climb ladders to get to the tops of banana plants. From those fruits, the workers must collect seeds and then grow those seeds into new plants.
Another complication: Those seeds can’t simply be planted in soil. Only 30 percent of the released embryos will develop into plants.
Despite such challenges, IITA and Uganda's National Agricultural Research Organization have succeeded in breeding disease-resistant plants. So far, they have 27 varieties of East African cooking bananas that are resistant to both the Black Sigatoka fungus and the worm-like nematodes that have long been problems for banana growers. Some varieties of bananas are filled with pea-sized seeds.
When small farmers in Asia and Africa plant the new varieties, they will be able to grow enough bananas to sustain their families.
Fibers from the banana plant’s stem go into making clothing and ropes.
In some ways, small farmers in Asia and Africa have less to worry about than do those at big plantations, Swennen says. That’s because each small farmer plants a variety of crops.
On this western side of the Atlantic, plantations specialize in Cavendish bananas. Small farmers in Africa and Asia may grow several different varieties of bananas. They also grow many kinds of crops, such as maize, cassava, okra and melon, under and around their banana plants.
So Dale adds to existing varieties the genes that make a plant resistant to TR4. His method may keep the best features of our dessert banana (flavor, texture and its ability to be shipped long distances) while adding disease resistance.
Dale starts by taking genes from wild bananas. These bananas are full of big, hard seeds, so they aren’t good for eating. But the wild plants are resistant to both strains of Panama disease — TR4 and Race 1.
So he is inserting resistance genes into both types of plants.
To do so, Dale uses a common soil bacterium that naturally inserts pieces of DNA into plants. He takes a single cell from a Cavendish or Gros Michel banana plant. Then he uses the bacterium to insert the genes that would make the plant resist infection by TR4 or Race 1.
Often a clone, particularly among plants, has been created using the cell of an existing organism.
cooking banana Bananas eaten while still green. Grown in the East African highlands, cooking bananas are steamed or boiled into a porridge.
The cross-bred plants will exhibit features of both parent plants.
In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.
monoculture Large areas planted with a single type of crop.
resistance (as in disease resistance) The ability of plants to fight off disease.
Tissue culture is commonly used to create genetically identical plants.
TR4 is the deadliest strain, currently threatening nearly 85 percent of the world’s bananas.