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Bioscience in brief

Crop Improvement & Management

Bananas

Did you know?
Bananas do not grow on trees. The banana plant is a tall perennial herb, and the banana fruit is technically considered a berry. The correct name for a bunch of bananas is a hand of bananas, and a single banana is a finger.

Bananas and plantains

  • Bananas and plantains (from the Musa plant genus) are the world’s fourth most important food crop after rice, wheat, and maize. Bananas and plantains are very nutritious, and a good source of dietary fibre, Vitamin C, Potassium, Manganese and Vitamin B6.
  • Cooking bananas represent a major food and cash crop for smallholder farmers in Africa: 70 million people in 15 countries in Sub-Saharan Africa depend on bananas and plantains for their livelihood and food supply.
  • The Great Lakes Region covering parts of Uganda, Rwanda, Burundi, Tanzania, Kenya and Democratic Republic of Congo is the largest producer and consumer of bananas in Africa where per person consumption has been estimated at more than 200 kg: the highest in the world.
  • Banana plants fruit all year round; if healthy can live up to 100 years; and are suitable for intercropping.
  • Cultivated bananas are seedless and hence sterile.  The principal method of banana propagation by small-holder farmers is by division of suckers or pups which arise from the base of the main stem or from the underground root structure.

THREATS AND CHALLENGES
Banana production is decreasing in East Africa due to a variety of pests and diseases, compounded by the degradation of soils. In Central Uganda and West Uganda production is estimated at 6 and 17 tons/ha, compared with 60 tons/ha attainable on research stations. The life span of banana plantations has fallen from about 50 years to only 5-10 years in some areas.

While lack of seeds is a desirable agronomic characteristic in fruits, low fertility makes genetic improvement using traditional plant breeding techniques a prolonged and difficult process. This is further complicated by the long generation times and the large space requirement for field-testing banana plants.

COMBATING THE FOES
Tissue Culture Banana Since bananas are sterile and propagated by cuttings, any diseases affecting the mother plant will also be present in the derived plants. It is therefore essential to plant healthy material. Tissue Culture (TC) is the cultivation of healthy plant cells and tissues on specially formulated nutrient media in sterile conditions to regenerate large numbers of whole plants. TC is a relatively cheap technology and it requires only modest investments to set up. Plants produced are of high quality, disease-free and mature early and uniformly, so ideal for farmers with small farm sizes since it allows them to harvest and sell fairly large quantities of fruit at one time.

Tissue culture technology in Africa has increased banana productivity from 20 to 45 tons per hectare. More than 500,000 resource-poor farmers in Kenya have so far benefited from tissue culture banana technology transfer.

 
Gene Technology
Since improving bananas with conventional methods is very difficult because they are sterile, genetic transformation is a promising strategy to develop banana varieties that are resistant to pests and diseases. The sterility and the lack of pollen of any new GM banana varieties mean there is no risk of transferring new genes to other plants, so there are no biosafety risks. Furthermore, since bananas are propagated by cuttings farmers will be able to share planting material at no extra cost.

Green pepper to the rescue in the fight to Banana Xanthomonas Wilt
Banana Xanthomonas Wilt is emerging as a serious threat to banana and plantain production in Sub-Saharan Africa. It attacks all banana and plantains varieties and specifically destroys the fruit. Banana Xanthomonas Wilt is responsible for more than US$ 500 million yearly loses across East and Central Africa. There is no source of resistance to this disease in bananas.

Scientists from the International Institute of Tropical Agriculture (IITA) and the National Agricultural Research Organization (NARO) of Uganda have successfully transferred two genes from green peppers to bananas that enable the crop to resist the Banana Xanthomonas Wilt disease.

The genes work by activating a natural plant defence called Hypersensitivity Response: cells that are infected by the disease-causing bacteria die. In this way the bacteria are prevented from spreading to the rest of the plant.

The plants will be tested in confined trials in Uganda this year.

Starving and repelling nematodes off banana plants
Nematodes (microscopic worms) are responsible for yearly losses estimated at US$ 125 billion in Sub-Sahara Africa. Nematodes compromise the plant’s ability to absorb water and nutrients from the soil. Heavily infested plants eventually topple as the root system rots. An effective nematode control in Uganda would provide an estimated benefit of more than US $250 million over 30 years, with equal benefits to small scale farmers and consumers. Nematodes can be controlled by the application of pesticides, but these are expensive and very toxic.

Researchers in the UK and Africa are using genetic modification (GM) technologies to make banana and plantain crops resistant to attack from nematode worms. Two genes have been introduced into banana plants: 1) a maize gene that prevents nematodes from digesting proteins (and starving them to death); and 2) a synthetic nematode-repelling protein that affects the worms’ ability to find and colonise banana plants. In the study, scientists armed the plantain with either one or both of the genes for single or dual nematode defence.

Fighting fungus and insect pests
Black Sigatoka, a leaf spot disease caused by a fungus, is the most important banana disease worldwide. In East Africa it can reduce fruit production by up to half, and the most popular banana varieties are extremely susceptible to this disease. Black Sigatoka can be treated with fungicides, but these are expensive, toxic and hard to apply. In addition, the fungus is developing resistance to the chemicals used. Fusarium Wilt (also known as Panama disease) – another devastating fungal disease – is found in every banana/plantain producing area of the world. The fungus survives in the soil so it is impossible to grow susceptible banana varieties in contaminated locations for up to several decades. There is no method of control available for Fusarium Wilt.

Researchers from the United States and Uganda have joined forces to endow banana plants with the ability to resist Black Sigatoka and Fusarium Wilt. The approach chosen is based on a natural mechanism to turn off genes in cells called RNA interference (RNAi). The genes introduced in banana plants will turn off a specific gene only in the cells of the attacking fungus and in this way control the spread of disease.

The same strategy is being used for developing banana varieties resistant to banana weevils, the most important insect pest of bananas. East African Highland plantains are highly susceptible to weevils.

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