Breeding superior banana/plantain hybrids

Jim Lorenzen (j.lorenzen@cgiar.org)
Banana Breeder, IITA, Tanzania

Banana (the term includes plantain in this article, Musa species), is a major staple crop in Africa. Although it originated in Asia and was introduced to Africa long ago, it has become more important as a food security crop in its new home in Africa than in its region of origin. From its early domestication in Southeast Asia and the islands extending toward Australia, banana spread to Africa before recorded history. Archaeological evidence suggests that it reached Central Africa several millennia ago.

Pollination of banana flowers. Photo by L. Kumar.
Pollination of banana flowers. Photo by L. Kumar.
The main types of cooking banana in Africa include plantain (AAB genome), East African Highland Banana (EAHB, AAA genome), and a wide range of other types including sweet dessert banana (AAA or AAB genome), starchy but sweet roasting or brewing banana (ABB genome), and a number of other types. The “genome” refers to the portion of the chromosomes that come from one of the progenitor species of banana, Musa acuminata (A genome) or Musa balbisiana (B genome). However, most banana production in sub-Saharan Africa (SSA) consists of the East Africa Highland type or plantains, two sets of varieties with very limited genetic diversity in either. This lack of genetic diversity is a serious concern. About 60% of African production occurs in Uganda and its immediate neighbor countries (Tanzania, Rwanda, Kenya, D.R. Congo; also including Burundi).

Since banana production is year-round, it serves as a buffering bridge crop to provide food in times of scarcity between cereal harvests. As a long-lived clonal crop, it (like cassava) also can serve as a famine-avoidance crop because it is less susceptible than annual crops to catastrophic failure in the event of unseasonable drought and can act as a survival crop during cereal crop failure. Banana also provides important ecological functions for sustainable agriculture by reducing erosion in sloping highland agriculture, and recycling nutrients through the crop residue returned to the soil in each production cycle. In some locations banana leaves and cut stems are an important fodder component in the livestock sector, providing some fodder even during the dry season.

Production constraints
While precolonial banana production may have been relatively stable, pests and diseases introduced into Africa in the last century have destabilized production in some areas. Some important introduced diseases and pests include black leaf streak (also known as Black Sigatoka), Banana bunchy top virus (BBTV), burrowing nematode, banana weevil, and Fusarium wilt. More recently, banana Xanthomonas wilt (BXW) has emerged as an important bacterial disease that apparently originated in Ethiopia and caused a major disease epidemic in much of East Africa in the last decade. Breeding for resistance to these diseases and pests provided the initial motivation for IITA and partners to initiate breeding in Africa.

Manual pollination of banana flowers. Photo by IITA.
Manual pollination of banana flowers. Photo by IITA.
Banana breeding history
Although early efforts to breed banana using modern breeding concepts were initiated by British scientists in the Caribbean about 80 years ago, even today the world has only about seven significant banana breeding programs. IITA initiated a plantain breeding program at the Onne High Rainfall research station in southeast Nigeria in the 1980s as a new epidemic disease, Black leaf streak, arrived in the region. This program made relatively rapid progress, identified fertile plantain varieties to cross to wild sources of resistance, optimized and implemented embryo rescue as a means of boosting germination from <1% to 5─30%, and produced resistant high-yielding hybrids by the early 1990s. Realizing that the bigger portion of African banana production was in highland East Africa and also threatened by black leaf streak, in 1995, IITA initiated a banana breeding program in Uganda in collaboration with the National Agricultural Research Organization (NARO). Working together, scientists identified fertile EAHB varieties, produced resistant high-yielding tetraploid hybrids to serve as parents, and initiated a program to produce resistant high-yielding triploid hybrids that were more likely to remain seedless.

Banana breeding process
Although most of the world eats banana, few realize that wild banana are full of hard seeds and domestication resulted in the seedless fruits that we now eat. Most varieties are triploids (have 3 sets of each chromosome), which are both more productive and more likely to remain sterile and seedless. However, some edible varieties retain a bit of residual fertility and will set a few seeds if pollinated with a strong source of viable pollen. Banana breeders serve as surrogates to natural pollinators (bats), climb ladders in the early morning to collect male flowers, and carry them and the ladders over to the intended female plants to hand-pollinate female flowers. The flowers open sequentially each day, so each floral bunch is pollinated daily for a week. While many pollinations produce no seeds, some produce a few and a very few produce many seeds. Unfortunately, due to the complex background of domesticated banana, most seeds will not germinate on their own. Therefore breeding programs extract embryos from surface-sterilized seeds and germinate them in test tubes in nutritious media, from which they can later be transplanted to sterile soil, hardened, and eventually planted in the field. Triploid hybrids are evaluated as potential new varieties, while diploid (2 sets of chromosomes) and tetraploid (4 sets) hybrids are evaluated as potential improved parents.

Progress
The original plantain hybrids, as well as superior hybrids developed later, are currently being tested for agronomic performance, yield, and consumer acceptability in a number of countries in West and Central Africa, including Nigeria, Cameroon, Ghana, and Coté d’Ivoire. In the meantime, IITA’s original East African partner in banana breeding, NARO, has grown to be one of the largest banana research programs in the world, with internationally recognized capacity in several disciplines.

Fittingly, in 2010 NARO became the first national program in Africa to officially release a banana variety bred in Africa. Kabana6 (nicknamed Kiwangaazi) is a high-yielding variety with resistance to black leaf streak and partial resistance to nematodes and weevils. More encouragingly, newer selections likely to be more acceptable to Ugandan consumers are “in the pipeline,” and procedures are now in place to move some jointly developed NARO-IITA hybrids to countries where their cooked texture and appearance fit the traditional variety “type” better than they do the “matooke” variety type of Uganda. A couple of promising hybrids are finding acceptability in Burundi and eastern D.R. Congo, and hopefully will also be released as varieties. IITA recently opened a second East African breeding site near Arusha, Tanzania, a country with a broader range of environments and irrigation opportunities, potentially better to breed widely adapted varieties and providing the opportunity to screen more systematically for drought tolerance.

Physical measurements of banana fruits. Photo by IITA.
Physical measurements of banana fruits. Photo by IITA.
Other aspects
To support the breeding program, other genetics studies are being conducted, including development of populations for molecular mapping studies, mapping genes controlling important traits, manipulating ploidy to try to create fertility in “sterile” lines, developing molecular “tools” to make breeding more efficient, and investigating gene expression in response to drought. IITA has excellent capacity for screening for resistance to pests and diseases.

The entire banana improvement program depends on collaborative relationships, both within IITA and from a range of partners within Africa and in other continents. The pending release of the reference genome sequence from La recherche agronomique pour le développement (CIRAD)/Genoscope in France should greatly accelerate genetics research on banana and its relatives. In light of the challenges of breeding and the lack of good sources of resistance for two important pathogens (BXW and BBTV), IITA is also investing in biotechnology approaches to banana improvement, with promising signs of resistance in early laboratory, screenhouse, and confined field trials (companion article by Tripathi).

Challenges
While encouraging progress is being made, banana breeding is challenging, slow, and expensive. Low fertility, poor seed set, and low germination rates mean that it is difficult to produce large numbers of progeny to evaluate. Banana plants are large, so evaluation plots are likewise large and expensive, and plants require up to 3 years to progress through two fruiting cycles. Much of the background genetics underlying key traits have yet to be properly investigated, so the list of research opportunities to make breeding more efficient and productive is long.

Musa is one of the major crops in the world for which wild relatives have yet to be systematically collected, so access to wild species for breeding for more resistant or more nutritious hybrids is problematic. Unfortunately, the global gene pool with the resistance and quality genes for future breeders remains at risk. Hopefully arrangements can be made for collection expeditions in the center of origin (Southeast Asia) in the near future while wild Musa still remain.

Future
Although banana has been a neglected crop in terms of research investment and scientists’ effort in many countries, key decision makers are beginning to realize the essential role of banana/plantain in food security, enhanced livelihoods, and resilient agricultural systems for Africa. The potential to breed superior hybrids has been demonstrated, and there are numerous opportunities for improving both the process and the product, and for realizing impact from already developed hybrids. The future for banana crop improvement looks promising.

Transgenic banana for Africa

Leena Tripathi, l.tripathi@cgiar.org

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.

A hot bath for the suckers!

An effective treatment against nematode and weevil pests of banana and plantain

Plantain plant with three sword suckers, field trial on IITA campus, Ibadan, Nigeria. Photo by A. zumFelde, IITA.
Plantain plant with three sword suckers, field trial on IITA campus, Ibadan, Nigeria. Photo by A. zumFelde, IITA.

Banana and plantain (Musa spp.) are important food crops for millions of people all over the world. The banana is the most popular fruit in the world and number one in international trade. The FAO estimates that over 100 million t of banana and plantain were produced worldwide in 2007. In sub-Saharan Africa (SSA), over 70 million smallholder farmers depend on the two crops for their food and income.

Banana and plantain production is greatly constrained by pests and diseases that lead to annual losses of millions of US dollars. The most important pests are nematodes (several species) and weevils (Cosmopolites sordidus) that are found in the soil and roots.

Nematodes attack the roots, hampering the uptake of nutrients from the soil and drastically reducing yield. In severe cases, they topple the whole plant. Weevils, on the other hand, attack the plant’s underground corm, weakening the plant and causing stem breakage. Average production losses from nematodes are estimated at 30% of the harvests of highland banana in East Africa and can exceed 60% for plantain in West Africa.

These two pests are spread from one farm to another through the planting of infested suckers. Farmers can avoid infesting their farms by ensuring that they plant disease- and pest-free suckers, such as those derived from tissue culture. These are, however, out of reach for the millions of small-scale farmers in sub-Saharan Africa.

Farmers dipping peeled suckers in boiling water. Source: D. Coyne, IITA.
Farmers dipping peeled suckers in boiling water. Source: D. Coyne, IITA.

Research has shown that peeling and treating the suckers in hot water, at 50 °C, can effectively remove both nematodes and weevils and their eggs. This method has worked successfully for commercial farms and organized cooperatives but not for small-scale farmers. This is because a thermometer must be used to ensure precision and the right temperature and this is not readily accessible to the farmers in SSA.

IITA’s scientists Danny Coyne and Stefan Hauser have developed an easier method that is just as effective by simply immersing the peeled or unpeeled suckers in boiling water for 20–30 seconds.

The counting
The duration of 20–30 seconds can be achieved by simply counting from 1 to 30. Farmers can also use small objects, such as pebbles, to mark the time: picking the pebbles one by one and placing them in a small container. The counting takes about 1 second/item but farmers can check the time for more accuracy.

This technique has proven to be friendly to small-scale farmers and is better than the hot water treatment at 50 °C as the time taken to treat a sucker is reduced and the measurement of the temperature and timing is simplified. It effectively disinfects suckers of various sizes without affecting their germination

Plantain field planted with suckers treated in boiling water. Photo by A. zumFelde, IITA.
Plantain field planted with suckers treated in boiling water. Photo by A. zumFelde, IITA.

The method is radical and requires skill and care when it is promoted to farmers who may be sceptical at first. The scientists recommend the use of a demonstration plot to introduce the technology and convince farmers to adopt it. They must keep within 30 seconds as otherwise they risk damaging the suckers, especially those that are small-sized.

Although the technology requires a fuel/energy source and the process has to be followed precisely, it is definitely a much easier method to use than the hot water treatment.

Using boiling water to treat the suckers has the potential to improve banana and plantain productivity by eliminating the two pests.

Sustainable production and distribution of clean banana

Bi Irie Vroh, b.vroh@cgiar.org

Banana (Musa spp.) including the plantain type are among Africa’s most important staple food and cash crops. Nearly 30 million t of banana are produced yearly in Africa, mostly by smallholders and consumed locally.

The major edible types are parthenocarpic (produces fruit without fertilization) and seedless. They are propagated traditionally by planting corms and suckers (daughter plants that grow from the rhizomes at the base of mother plants).

However, propagation material derived from the infected mother stocks results in perpetuation of diseases (e.g., viruses such as banana bunchy top, banana streak) and pests (e.g., nematodes and weevils) leading to low yields and poor quality fruits.
Due to the unavailability of disease- and pest-free or clean planting materials, farmers in sub-Saharan Africa traditionally plant suckers derived from their own plantations, most of which are affected with pests and diseases.

IITA has been using three approaches to generate clean planting material of farmer-favored banana cultivars:

Boiling water treatment of suckers: Suckers are submerged in boiling water for 30 seconds to kill nematodes and weevils. This method is efficient and easy for farmers, but it has low output and is laborious.

IITA’s Emmanuel Njukwe, Paula Bramel, and Bi Irie Vroh visit the Fritz Jakob Foundation. Source: B. Vroh, IITA.
IITA’s Emmanuel Njukwe, Paula Bramel, and Bi Irie Vroh visit the Fritz Jakob Foundation. Source: B. Vroh, IITA.

Macropropagation using the PIF technique: Through the technique known as PIF (plantes Issues de Fragments de tige) tens of good quality plantlets are produced within two months at relatively low costs. In this approach, the primary buds of entire suckers or fragments of corms are destroyed and axillary buds are exposed to high humidity to induce sprouts which are then harvested, hardened, and distributed.

This approach can be implemented in remote rural areas near farmers’ fields or by NGOs in direct contact with farmers for training and the distribution of good planting materials. This procedure is simple to replicate using locally made humidity chambers.

Micropropagation: Also known as in vitro production of tissue culture (TC) material this is the most efficient approach to the production of clean planting material in terms of throughput and germplasm exchanges across international borders. In vitro plantlets are micropropagated in the TC laboratory of IITA in Ibadan, Nigeria, and hardened first in the acclimatizing rooms, then in screenhouses before being distributed to farmers. Planting materials from preferred landraces and improved hybrids are propagated through TC, and hardened for use or maintained in a conservation cold room where each genotype is replicated several times from the initial meristem for future use.

IITA’s Delphine Amah holding racks of TC plants in a growth room. Source: B. Vroh, IITA.
IITA’s Delphine Amah holding racks of TC plants in a growth room. Source: B. Vroh, IITA.

Combining the TC pipeline with the macropropagation through PIF, IITA regularly distributes thousands of seedlings to NARS, NGOs, and farmers in West and Central Africa. Besides the preferred local varieties, the most distributed improved materials include the plantain hybrids PITA 14, PITA 21, and PITA 23 and the cooking banana hybrid BITA 3. These hybrids express a higher level of tolerance for black Sigatoka diseases compared with local varieties.

IITA trains farmers in applying boiling water treatment of suckers and macropropagation by PIF to produce clean planting material. However, IITA primarily uses micropropagation as the method of choice for conservation, propagation, and distribution of germplasm, and also to support its breeding programs. IITA also provides training programs on TC operations for NARS. For IITA’s projects in West Africa, clean planting materials are produced by TC or by PIF, hardened and raised in screenhouses, and then transferred to specific project sites.

Hardening of clean planting materials produced by TC and PIF methods. Source: B. Vroh, IITA.
Hardening of clean planting materials produced by TC and PIF methods. Source: B. Vroh, IITA.

In rural communities, IITA emphasizes training for farmers and rural entrepreneurs so they can produce clean planting materials in their own communities. These various efforts enhance the farmers’ access to clean planting materials and also encourage involvement of commercial operators in distribution of planting materials. The improvement of the capacity of NARS and the involvement of the private sector are needed to scale up the technologies for the sustainable production of clean planting materials of banana and plantain.

Banana and Plantain Systems

Bananas drawg
Grown by smallholder farmers, bananas and plantains are major food staples and two of the leading cash crops, both in the East African Great Lakes zone and the West African humid lowlands. Diverse banana cultivars are grown for a number of uses, including brewing (juice bananas), cooking and roasting, as well as sweet dessert bananas. Banana starch, flour, and chips are processed banana products whose markets are yet to be fully developed. Because of its cash crop status, farmers are more likely to adopt high-level management technologies in order to intensify production and yet sustain the natural resource base in the systems. Clearly, much progress can be realized with more profitable and sustainable banana systems.

Since the 1970s, reports of low and declining banana yields have been widespread. Some have attributed this phenomenon to constraints such as soil degradation, pests, poor crop husbandry, and drought. In this project, the hypothesis is that these constraints are, to a large extent, interlinked. Although each constraint can, on its own, potentially cause serious yield decline, the complex interactions between a number of them compound yield losses.

The overall purpose of the banana and plantain project is to enhance the performance of banana and plantain systems within smallholder farms in sub-Saharan Africa.

The main project objectives are:

  • To increase knowledge on ecosystems, social systems, and commodity chains related to banana and plantain production in
  • To research ways to improve profitability of banana and plantain systems in Africa
  • To improve the quality of banana- and plantain-based food products