Breeding superior cooking bananas

jlorenzen Tuesday, 03 July 2012
Photo by L. Kumar
Photo by L. Kumar

Banana breeders are familiar with the routine of climbing a ladder in the early morning to collect male flowers and carrying them, and the ladder, over to the intended female plants to hand-pollinate female flowers. Since flowers open sequentially each day, each floral bunch is pollinated daily for a week. You could say that banana breeders serve as surrogates to natural pollinators, in this case bats.

While many of these pollinations will produce no seeds, some will produce a few and only a very few crosses will produce many seeds. Unfortunately, most of the seeds derived from edible banana cultivars 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. No wonder there are only about seven significant banana breeding programs 80 years after scientists started to use modern breeding concepts to breed bananas.

The first successes

The International Institute of Tropical Agriculture (IITA) for which I work, initiated its Plantain breeding program at the Onne High Rainfall research station in southeast Nigeria in the 1980s, following the arrival in the region of a new epidemic disease, black leaf streak. The program made relatively rapid progress in identifying fertile Plantain varieties to cross to wild sources of resistance, and in optimizing and implementing embryo rescue as a means of boosting germination (from <1% to 5?30%). By the early 1990s, it had produced its first resistant high-yielding hybrids. In 1994, realizing that the East African highlands, Africa’s main banana production area, was threatened by black leaf streak, IITA initiated a banana breeding program in Uganda in collaboration with the National Agricultural Research Organization (NARO). Working together, scientists identified fertile East African highland banana varieties, produced resistant high-yielding tetraploid hybrids to serve as parents, and initiated a program to produce resistant high-yielding triploid hybrids.

The original Plantain hybrids, as well as the 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 Côte 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 to officially release a banana variety bred in Africa. Kabana6 (nicknamed Kiwangaazi, for long-living) 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 local traditional type better than they do the “matooke” of Uganda. A couple of promising hybrids are finding acceptability in Burundi and eastern DR 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 that should make it easier to evaluate for adaptations to different environmental conditions, including drought.

Challenges and opportunities

Despite progress, banana breeding remains 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.

To support the breeding program, we are developing populations for molecular mapping studies, mapping genes, manipulating ploidy to increase the fertility of “sterile” lines, developing molecular tools to make breeding more efficient, and investigating gene expression in response to drought. IITA also 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 CIRAD and 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.

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 for breeding 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, especially if more wild relatives are introduced to collections and made available to breeders.

This post is based on a piece in IITA’s R4D Review.