Saving the banana
Scientists race against time to fight diseases
threatening the survival of the world’s most
popular fruit
ALISON PEARCE STEVENS
AUG 28, 2014 — 8:53 AM EST
These berries — known across the world as bananas — are the world’s most popular fruit.
They’re also being threatened by a host of diseases that could one day make them all but
disappear from store shelves.
ANTB/ ISTOCKPHOTO
Meet the world’s most popular fruit. Snack-size, portable and each with
its own wrapper — it’s the banana! Diners consume billions each year
throughout the world. Americans eat more bananas than apples and
oranges combined. And in banana-producing countries, more than 400
million people rely on bananas in order to survive.
The big, bright-yellow banana most commonly found in American and
European grocery stores is known as the Cavendish. Known as a
dessert banana, it is eaten only when it is ripe and sweet. But dessert
bananas make up only 40 percent of the world’s 400 edible varieties of
this fruit. Bananas vary greatly in their size, shape, flavor and texture.
Those that aren’t sweet, such as the plantain, must be cooked or fried
before they are eaten.
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.
AYAITA/WIKIMEDIA COMMONS (CC BY-SA)
Despite their broad popularity, many bananas — including that popular
Cavendish — could disappear from store shelves in the not too distant
future. A variety of diseases threaten their survival. 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. Another
fungus also causes Panama disease. Called Race 1, back in the 1950s
it nearly wiped out one popular banana variety — the Gros Michel, or
Big Mike. That’s when farmers switched to growing the Cavendish we
eat today.
But now TR4 threatens not only the Cavendish, but also many other
varieties of bananas. This aggressive fungus has moved from Asia to
the Middle East, Australia and, most recently, Africa.
Losing out on a banana split wouldn’t be great. If these diseases
succeed at wiping out popular types of this fruit, however, many people
who live in banana-growing regions might starve. That’s why scientists
are working with farmers to stop banana threats such as TR4 in their
tracks. They are developing plants that resist the fungus. But it’s a race
against time as this newest menace hops, skips and jumps around the
globe.
Berry-liscious
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.”
KALAI/WIKIMEDIA COMMONS CC BY-SA
Bananas don’t grow on trees. Rather, they are the world’s biggest
berries growing on the world’s tallest herb. Reaching a height of nearly
3.5 meters (11.5 feet), banana plants lack woody trunks. Instead, layers
of leaf stalks support the towering structure. Broad leaves unfurl from
the top. That’s also where an unusual cluster of flowers begins growing
into “fingers” of fruit.
Banana plants grow fast. It takes most only about nine months to reach
full height and begin flowering. Once the plant finishes producing fruit,
the above-ground portion dies. A sucker (a kind of shoot) then begins to
grow from the bottom of the plant. That sucker eventually develops into
a new plant that produces more fruit.
Suckers play an important role in banana farming. The most common
types of banana don’t produce any seeds. 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. Both
processes result in clones: bananas that are essentially identical twins
of their parent. These clones are being planted over and over
throughout the world.
And that risks trouble. 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. And those seeds may hold a key to protecting
bananas from the diseases that threaten them.
Lurking threat
Last year, banana growers in Mozambique noticed some unhealthy
plants. Their leaves had wilted and yellowed. They broke away from the
stem, forming a skirt of dead leaves on the ground. Fearing the worst,
farmers had their plants tested. The diagnosis: TR4 had arrived.
First line of defense
One approach to stopping the fungus: finding plants that are resistant to
it. Such disease-resistant plants wouldn’t become infected.
Unfortunately, it takes years to create bananas that not only are
resistant, but also keep the texture and flavor people enjoy. In the
meantime, banana growers must slow TR4’s spread.
That means farmers need to be on high alert for the invader, detecting it
before it can move on.
Fighting back
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. It’s at the International Transit Centre of Biodiversity
International, or ITC. It’s in Leuven, Belgium, and houses all 1,400 of
the world’s edible and inedible varieties. These bananas come from
across the globe. And this center makes them freely available to people
who need them. So far, more than 90,000 plants have been given to
farmers in more than 100 countries.
Most bananas at ITC are tropical. Some, however, come from places in
China where it snows. Others prefer dry sites. Such broad varieties with
so many different traits mean that lots of genetic material exists for
breeders like Swennen to work with.
Some varieties hold genes that give a banana its flavor, climate
hardiness or vigor. Some genes also may provide resistance to Panama
disease. 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 crossbreed African bananas. These members of the International Institute of
Tropical Agriculture (IITA) start their days early in the morning. They
climb ladders to get to the tops of banana plants. They use brushes to
remove pollen from the male flowers of one plant. They brush that
pollen onto the newly opened female flowers of another. And then they
wait. After three to four months, the flowers turn into fruits. From those
fruits, the workers must collect seeds and then grow those seeds into
new plants.
Seeds, though, are in short supply. Swennen estimates that he gets
only 5 to 20 seeds per bunch of bananas (a bunch can contain
anywhere from one to 200 fruits). Another complication: Those seeds
can’t simply be planted in soil. They won’t grow. Instead, Swennen must
cut open the seed, releasing the tiny embryo into a nutrient bath. 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. The
process is painfully slow: Those 27 varieties took 20 years of crossbreeding.
But it’s worth the effort, says Swennen. Disease resistance is
particularly important for farmers who don’t have the money to spray
their crops with pesticides.
Some varieties of bananas are filled with pea-sized seeds.
GERT KEMA, WAGENINGEN UNIVERSITY AND RESEARCH CENTRE
When small farmers in Asia and Africa plant the new varieties, they will
be able to grow enough bananas to sustain their families. And these
farmers won’t just eat the fruit. They’ll use all parts of the plant.
Broad leaves become the family’s plates. Stacked on the roof, those
leaves will keep out the rain. Fibers from the banana plant’s stem go
into making clothing and ropes. And once the banana fruits have come
and gone, farm families will eat the plant’s stem and the bulbous base
that grows underground, called the corm. Different types of bananas
also have different flavors and are prepared in different ways. That’s
why a farmer may grow between four and 20 different varieties.
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. That diversity helps protect the
plants: Most of the crops a disease-causing organism encounters will
not be susceptible. That will make it harder for a pathogen to find a
suitable host. And even if one type of banana gets infected and wiped
out, the grower will still have other varieties on which to rely.
Helping Mother Nature
Half a world away, in Latin America, TR4 remains a distant threat. It has
to cross an ocean before it can wreak havoc on the huge plantations
there. On this western side of the Atlantic, plantations specialize in
Cavendish bananas. Eight out of 10 Cavendish sold in other parts of the
world will have come from Latin America.
Every Cavendish plant is genetically identical to all others. So all of
these clones are equally susceptible to TR4. Scientists believe it is only
a matter of time before the fungus arrives. When it does, TR4 could
spell the end of the sweet dessert banana Westerners have come to
know and love.
But that won’t mean it’s the end of dessert bananas. At least not if
James Dale has his way. This biotechnologist at Queensland University
of Technology, in Australia, hopes to protect the varieties now on store
shelves.
Cross-breeding produces new varieties of bananas that can have
different tastes or textures than the ones we now eat, he explains. 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.
Dale doesn’t just want to protect Cavendish. He also hopes to bring
back Gros Michel. It’s much more flavorful than Cavendish, he says. 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. The
plant cells incorporate the genes into their own DNA. As they grow, they
are able to fight off both types of Panama disease.
Dale’s studies are still in the trial stage. But he hopes that soon,
resistant Cavendish and Gros Michel will be available to growers. If he
succeeds, it would buy more time for bananas. Despite these advances,
however, scientists will have to remain vigilant in the fight to protect the
world’s most popular fruit.
Power Words
bacterium (plural bacteria) A single-celled organism forming one of
the three domains of life. These dwell nearly everywhere on Earth, from
the bottom of the sea to inside animals.
biotechnologist
things.
A scientist who uses living cells to make useful
clone
An organism that has exactly the same genes as another, like
identical twins. 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.
cross-breeding Taking pollen from one plant variety and using it to
pollinate flowers of another variety. The cross-bred plants will exhibit
features of both parent plants.
DNA (short for deoxyribonucleic acid) A long, spiral-shaped molecule
inside most living cells that carries genetic instructions. In all living
things, from plants and animals to microbes, these instructions tell cells
which molecules to make.
fungus (plural: fungi) Any of a group of unicellular or multicellular,
spore-producing organisms that feed on organic matter, both living and
decaying. Molds, yeast and mushrooms are all types of fungi.
gene A segment of DNA that codes, or holds instructions, for producing
a protein. Offspring inherit genes from their parents. Genes influence
how an organism looks and behaves.
hypha (plural: hyphae) A tubular, threadlike structure that makes up
part of many fungi.
immune Able to ward off a particular infection or show no impacts
from a particular poison. More generally, the term may signal that
something cannot be hurt by a particular drug, disease or chemical.
monoculture
Large areas planted with a single type of crop.
nematode Type of roundworm, usually found in soil. It is very small,
with no eyes, ears or nose.
nutrients Vitamins, minerals, fats, carbohydrates and proteins
needed by organisms to live, and which are extracted through the diet.
pathogen An organism that causes disease.
pathologist
organisms.
Someone who studies disease and how it affects infected
photosynthesis (verb: photosynthize)The process by which green
plants and some other organisms use sunlight to produce foods from
carbon dioxide and water.
plantain A starchy, green banana that is fried before it is eaten.
polymerase chain reaction (PCR) A biochemical process that
repeatedly copies a particular sequence of DNA.
resistance (as in disease resistance) The ability of plants to fight off
disease.
spore The single-celled reproductive stage of a fungus (functioning
much like a seed) that is released and spread by wind or water. Most
are protected against drying out or heat and can remain viable for long
periods, until conditions are right for their growth.
sucker A shoot from the base of a plant.
tissue culture (as in plant tissue culture) The process of taking cells
from a plant, putting them in a nutrient bath and adding plant hormones
to stimulate the cells to develop into new plants. Tissue culture is
commonly used to create genetically identical plants.
Tropical Race 4 (TR4) The fourth identified strain of the fungus that
causes Panama disease. TR4 is the deadliest strain, currently
threatening nearly 85 percent of the world’s bananas.