Transgenic banana for Africa

Leena Tripathi,

Banana (Musa spp.) are one of the most important food crops after maize, rice, wheat, and cassava. Annual production in the world is estimated at 130 million t, nearly one-third of it grown in sub-Saharan Africa, where the crop provides more than 25% of the food energy requirements for over 100 million people. East Africa is the region that produces and consumes the most banana in Africa. Uganda is the world’s second largest producer after India, with a total of about 10 million t.

Banana plantation damaged by Xanthomonas wilt. Photo by IITA.
Banana plantation damaged by Xanthomonas wilt. Photo by IITA.

The banana Xanthomonas wilt (BXW) disease caused by the bacterium Xanthomonas campestris pv. musacearum (Xcm) was first reported about 40 years ago in Ethiopia on Ensete spp., a close relative of banana. Outside Ethiopia, BXW was first identified in Uganda in 2001, subsequently in the DR Congo, Rwanda, Kenya, Tanzania, and Burundi. The disease is highly contagious and is spread plant-to-plant through the use of contaminated agricultural implements. It is also carried by insects that feed on male buds, and is present on plant material, including infected debris. The rapid spread of the disease has endangered the livelihoods of millions of farmers who rely on banana for staple food and cash.

Infection by Xcm results in the yellowing and wilting of leaves, uneven and premature ripening of fruits, and yellowish and dark brown scars in the pulp. Infected plants eventually wither and die. The pathogen infects all varieties, including East African Highland Banana (EAHB) and exotic types, resulting in annual losses of over US$500 million across East and Central Africa.

Options for BXW control using chemicals, biocontrol agents, or resistant cultivars are not available. Although BXW can be managed by following phytosaniary practices, including cutting and burying infected plants, restricting the movement of banana materials from BXW-affected areas, decapitating male buds, and using “clean” tools, the adoption of such practices has been inconsistent. They are labor-intensive and farmers believe that debudding affects the fruit quality.

The use of disease-resistant cultivars has been an effective and economically viable strategy for managing plant diseases. However, resistance to BXW has not been found in any banana cultivar. Even if resistant germplasm is identified, conventional banana breeding to transfer resistance to farmer-preferred cultivars is a difficult and lengthy process because of the sterility of most cultivars and also the long generation times.

Transgenic technologies that facilitate the transfer of useful genes across species have been shown to offer numerous advantages to avoid the natural delays and problems in breeding banana. They provide a cost-effective method to develop varieties resistant to BXW. Transgenic plants expressing the Hypersensitive Response Assisting Protein (Hrap) or Plant Ferredoxin Like Protein (Pflp) gene originating from sweet pepper (Capsicum annuum) has been shown to offer effective resistance to related Xanthomonas strains.

Plants established in confined field trial 5 months after planting. Source: L. Tripathi, IITA.
Plants established in confined field trial 5 months after planting. Source: L. Tripathi, IITA.

IITA, in partnership with the National Agricultural Research Organization (NARO)-Uganda and the African Agriculture Technology Foundation (AATF), has developed transgenic banana expressing the Hrap or Pflp gene using embryogenic cell suspensions or meristematic tissues of four banana cultivars, Sukali Ndiizi, Mpologoma, Nakinyika, and Pisang Awak. More than 300 putatively transformed plants were regenerated and validated via PCR assay and Southern blot. Of these, 65 transgenic plants have exhibited strong resistance to BXW in the laboratory and screenhouse tests. The plants did not exhibit any differences from their nontransformed controls, suggesting that the constitutive expression of these genes has no effect on plant physiology or other agronomic traits.

The 65 resistant lines were planted in a confined field trial in October 2010 at the National Agriculture Research Laboratories (NARL), Kawanda, Uganda, after approval was obtained from the National Biosafety Committee. These transgenic lines are under evaluation for disease resistance and agronomic performance in field conditions. The transgenic lines are slated for environmental and food safety assessment in compliance with Uganda’s biosafety regulations, and procedures for risk assessment and management, and seed registration and release. After completing the necessary biosafety validation and receiving approval from the Biosafety Committee, the Xcm-resistant cultivars are expected to be deregulated for cultivation in farmers’ fields in Uganda.

We plan to stack the Pflp and Hrap genes in the same cultivars to enhance the durability of resistance against Xcm. We have developed more than 500 transgenic lines with the double genes construct (pBI-HRAP-PFLP) which are being evaluated for disease resistance under contained screenhouse conditions.

This technology may also provide effective control of other bacterial diseases such as moko or blood disease, of banana occurring in other parts of the world. The elicitor-induced resistance could be a very useful strategy for developing broad-spectrum resistance. The elicitor is a protein secreted by pathogens that induce resistance. The transgenic banana carrying these genes may also display resistance to fungal diseases such as black sigatoka and Fusarium wilt. Experiments on this are being conducted in our lab in Uganda.

Confined field trial of banana plants. Source: L. Tripathi, IITA.
Confined field trial of banana plants. Source: L. Tripathi, IITA.

We are also planning to stack genes for resistance to Xcm and nematodes into one line to produce cultivars with dual resistance that would tackle two of the most important production constraints in Eastern Africa.

The development of Xcm-resistant banana using the transgenic approach is a significant technological advance that will increase the available arsenal of weapons to fight the BXW epidemic and save livelihoods in Africa. It can become a high-value product for farmers.

This research is supported by the Gatsby Charitable Foundation, AATF, and USAID.

Note: The Pflp and Hrap genes are owned by Taiwan’s Academia Sinica, the patent holder. IITA has negotiated a royalty-free license through the AATF for access to these genes for use in the commercial production of BXW-resistant banana varieties in sub-Saharan Africa.

Bacterial wilt-resistant banana

Crop scientists have successfully transferred genes from green pepper to banana that enable the crop to resist the Banana Xanthomonas Wilt (BXW). BXW or bacterial wilt is one of the most devastating diseases of banana in the Great Lakes region of Africa. It causes about half a billion dollars worth of damage yearly.

The transformed banana, infused with plant ferredoxin-like amphipathic protein (Pflp) or hypersensitive response-assisting protein (Hrap) from green pepper, have exhibited strong resistance to BXW in the laboratory and screenhouses.

The Hrap and Pflp are novel plant proteins that give crops enhanced resistance against deadly pathogens. They work by rapidly killing the cells that come into contact with the disease-spreading bacteria, preventing them from spreading any further. They can also provide effective control against other BXW-like bacterial diseases in other parts of the world such as “Moko”, Blood, and “Bugtok”. The genes used in this research were acquired under an agreement from the Academia Sinica in Taiwan.

The mechanism is known as hypersensitivity response and activates the defense of surrounding and even distant uninfected banana plants leading to a systemic acquired resistance.

Scientists from IITA and the National Agricultural Research Organization of Uganda, in partnership with African Agricultural Technology Foundation, would soon be evaluating these promising resistant lines under confined field trials after the Ugandan National Biosafety Committee recently approved the conduct of the tests.

Presently, there are no commercial chemicals, biocontrol agents, or resistant varieties that could control the spread of BXW. Developing a truly resistant banana through conventional breeding would be extremely difficult and would take years, given the sterile nature and long gestation period of the crop.

Save African bananas!

Banana bunch. Photo by IITA
Banana bunch. Photo by IITA

A high-level international effort is needed to save African banana and plantain, currently under threat from two deadly diseases— banana Xanthomonas wilt (BXW) and banana bunchy top (BBTD).

Xanthomonas wilt started in Uganda and is spreading to neighboring countries in eastern Africa. Meanwhile, the banana bunchy top disease is creeping across southern and central Africa. Currently, all bananas are susceptible to the two diseases.

If these diseases are not checked, they will affect the food security and livelihoods of 30 million farmers in 15 countries in sub-Saharan Africa (SSA), who cultivate banana and plantain in 4.5 million ha. They can spread to neighboring countries and further worsen the regional situation.

Looming crisis
BBTD is characterized by stunted growth and leaves in tight whorls sprouting from the top of the plant. It is present in Angola, Burundi, Cameroon, Central African Republic, Congo, Democratic Republic of Congo (DRC), Equatorial Guinea, Gabon, Malawi, Rwanda, and Zambia. The disease is transmitted by aphids, and once established, it spreads rapidly and is almost impossible to eradicate.

Banana bacterial wilt disease was first identified in Ethiopia and is now ravaging production in the Great Lakes Region of DRC, Kenya, Rwanda, Tanzania, and Uganda.

Both diseases can rapidly kill banana plants and eliminate production, causing losses worth hundreds of millions of US dollars each year. At the moment, no disease-resistant varieties are available, and pesticides for controlling insect vectors are ineffective against disease spread.

BBTV (orange)- and BXW (green)-affected countries in Africa. Yellow indicates countries with both diseases
BBTV (orange)- and BXW (green)-affected countries in Africa. Yellow indicates countries with both diseases

For both diseases, the only way to prevent further spread in the field is to uproot, chop, and sun-dry infected plants, and to replace them with clean planting material.

Experts respond to crisis
Due to increasing concerns about the two diseases, the international community came together in a meeting held in Arusha, Tanzania, in August this year. Experts from 15 banana and plantain-growing countries in SSA (Burundi, Cameroon, Central African Republic, Congo, DRC, Kenya, Malawi, Mozambique, Nigeria, Rwanda, South Africa, Tanzania, Uganda, Zambia, and Zimbabwe) and also from Australia, India, and United Kingdom evaluated the current status and scientific knowledge on BBTD and BXW.

The meeting was convened by the Southern Africa Development Cooperation (SADC), with support from the UN Food and Agriculture Organization (FAO), IITA, and Bioversity International.

Participants recognized the need to plan strategies to manage the diseases in countries where they are already established and to prevent spread to those that are not yet affected. A recently issued joint statement said that “urgent action by national research organizations and governments in SSA, international research organizations, and development partners is necessary to avert or reduce the impact of the looming crisis.”

Framework for disease management
Building on ongoing regional efforts, FAO, IITA, and Bioversity International will work with relevant regional organizations, such as SADC and the Association for Strengthening Agricultural Research in East and Southern Africa, to develop a banana disease management framework for SSA.

The framework aims to support effective networking and coordination for information gathering and sharing, and to identify national needs and gaps in funding. It will support efforts to increase awareness and raise funds so that disease eradication and management activities contribute to a common goal.

The participants further called on relevant research and development organizations, partners, and national authorities to affirm their commitment to collaborate on regional responses to banana diseases, ensure coordination of activities and optimal use of resources to provide technical advice, develop sustainable new technologies, and deploy eradication and management practices across SSA.

A concerted effort among various stakeholders would ensure the success of measures to eradicate or prevent these diseases from further ravaging the livelihoods of banana and plantain farmers in Africa.

A high-level meeting will be convened in early 2010 to endorse the framework and take steps towards putting it into effect.

Banana and plantain are important staples in East Africa.
Banana and plantain are important staples in East Africa.

Participants’ recommendations
• Urgently support active management and control programs in countries where production systems are already affected to mitigate the impact of the two diseases.
• Make arrangements to provide safe and sufficient replanting material to encourage farmers to destroy diseased plants so as to safeguard and sustain production.
• To prevent infection and further spread of these diseases, unaffected countries need to increase vigilance to take preemptive action to destroy infected plants as soon as they appear. Monitoring strategies and the capacity to recognize the diseases and to deploy rapid eradication responses are vital.
• Educate and advise farmers on the importance of these diseases and mobilize them to report and combat them at the first sign of appearance. Large-scale, community-level action will be needed, supported by local and national governments and subregional organizations.
• Allocate funds for the following measures in countries with banana and plantain production—whether at commercial or subsistence level:

– large-scale campaigns for disease awareness, surveillance, training of sufficient staff in sampling, field disease recognition, and laboratory diagnosis, and the production and distribution of clean planting material to respond to any disease outbreaks.
– community-level measures to support actions in response to the diseases, helping to ensure local adherence to recommended practices.
– reporting and communication systems to ensure an early and appropriate reaction once the diseases have been recognized at field level, and to monitor the success of eradication and management practices.
– improved arrangements for production of clean planting materials through training and facilities to improve the quality of materials, and to ensure that enough are made available to farmers.
– national contingency plans developed with the involvement of all concerned with banana disease management, including the creation or strengthening of national task forces to ensure country preparedness to implement disease eradication campaigns and monitor the impact of these efforts.
– other longer term programs to improve field practices for disease eradication (for countries where invasion is recent) and management (where diseases are already established); and also to include review, information sharing, and update of policy frameworks, such as plant health/quarantine legislation.

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