Perspectives on the application of biotechnology to assist the genetic enhancement of plantain and banana (Musa spp.)

Perspectives on the application of biotechnology to assist the genetic enhancement of plantain and banana (Musa spp.)

Jonathan H. Crouch
Elsoms Seeds Ltd., Spalding, Lincolnshire, PE 11 1 QG, England. United Kingdom.
E-mail: crouch@globalnet.co.uk

Dirk Vuylsteke
International Institute of Tropical Agriculture (IITA), Eastern and Southern Africa Regional Centre (ESARC), P.O. Box 7878,
Kampala, Uganda.
E-mail: iita@imul.com

Rodomiro Ortiz*
International Crops Research Institute for the Semi-Arid Tropics
Patancheru 502 324, Andhra Pradesh, India
E-mail: R.Ortiz@cgiar.org

International Institute of Tropical Agriculture, PMB 5320, Oyo Road, Ibadan, Nigeria

ABSTRACT - Bananas and plantains (Musa spp.) are the most important tropical fruit crops. They form an integral component of the farming systems in the tropical humid forest regions of Africa, Central and South America, and Asia. Increases in Musa productivity have traditionally relied on improvements in crop husbandry. More recently, tissue culture-based techniques have facilitated intensive breeding of these crops. A broad array of biotechnologies is increasingly used to facilitate and enhance the handling and improvement of plantain and banana germplasm. Tissue culture is used for germplasm exchange, conservation and rapid multiplication, and in vitro seed germination for generating hybrid plants. DNA marker systems have been developed in Musa to assist germplasm management, breeding and disease diagnosis. In addition, gene cloning and genetic transformation of Musa crops has been routinely achieved and shows potential for the genetic betterment of the crop. This article discusses the current and future applications of biotechnology for the genetic enhancement of banana and plantain.

Plantains and bananas (Musa spp.) are both a staple food for rural and urban consumers in the humid tropics and an important source of rural income, particularly where smallholders produce them in compound or home gardens. Annual world production of these crops is around 85.5 million tonnes (FAO 1998), of which bananas cultivated for the export trade account for only 10%. In common with other horticultural crops, plantains and bananas provide high value produce. As such, the gross annual value of production in Africa, for example, exceeds that of many other crops such as cassava, maize and rice. Hence, fruit harvested from bananas and plantains are important components of food security in the tropical world, and provide income to the farming community through local and international trade.

Many pests and diseases have threatened the productivity of plantain and banana crops. In particular, Panama disease and black Sigatoka have spread dramatically and devastatingly during the past 20 years. For this reason, there is now considerable interest in the genetic improvement of Musa crops. Although some breeding programs have addressed improvement of export bananas, generally the goal has been to improve cultivars for local consumption in the tropical world (Ortiz and Vuylsteke 1996).

Musa breeders face many problems intrinsic to this crop, which slow genetic improvement. Plantains and bananas are generally triploid, and as such are virtually or completely sterile. In addition, these crops have a long life cycle (almost two years from seed to seed), and require large areas for field testing (6 m² per plant). Initial successes in Musa breeding relied upon a combination of conventional methods and tissue culture-based techniques. More recently, breeding programs are adopting emerging biotechniques such as genetic engineering and molecular marker-assisted selection to enhance the effectiveness of their operations.

Tissue culture-based techniques

Micropropagation and in vitro germination of hybrid seed have played a key role in plantain and banana improvement programs worldwide (Rowe and Rosales 1996; Vuylsteke et al. 1997). In vitro propagation has many advantages, such as higher rates of multiplying clean (pest and disease-free) planting material and the small amount of space required to multiply several thousands or millions of plants per year. Under optimum crop husbandry, micropropagated plants may establish faster, grow more vigorously, are taller, have a shorter and more uniform production cycle, and yield higher than conventional propagules (Vuylsteke 1998).

Shoot-tip culture combined with virus indexing, facilitates safe exchange of Musa germplasm. This methodology reduces volume and weight compared to conventional propagules and improves the phytosanitary status of germplasm exchange. Virus testing of Musa germplasm is particularly important, as some viruses affecting these crops are not eliminated by tissue culture (Diekmann and Putter 1996).

Genetic variation resulting from in vitro culture (somaclonal variation) can occur at high frequency in some Musa genotypes and is, therefore, a risk associated with the application of in vitro culture techniques for germplasm handling and storage. To counteract this problem, DNA markers are being identified for a range of serious mutant genotypes to enable them to be removed from micropropagated populations at an early stage. Conversely, somaclonal variation can provide a source of novel and useful variability, although most somaclonal variants recovered to date have been found to exhibit naturally occurring variation or defective phenotypes (Vuylsteke 1998).

Routine transformation techniques have been successfully applied to a range of plantain and banana cultivars and synthetic hybrids (May et al. 1995; Sagi et al. 1995). Genes coding for antifungal proteins that show broad antifungal activity in vitro have been introduced in a plantain cultivar and the resulting transgenic plants await field-testing. However, it is likely that transformation techniques will initially have their greatest impact on the improvement of sterile triploid dessert banana cultivars.

DNA marker-based techniques

The limited genetic and cytogenetic knowledge for many tropical crops has delayed their genetic improvement. The application of molecular biology techniques has the potential to speed-up the breeding of these crops and may unleash the potential of exotic germplasm in breeding programs.
Microsatellite markers and AFLP analysis appear to be the most appropriate technologies for marker assisted breeding of Musa crops (Crouch et al. 1999). Microsatellite markers are extremely effective and appropriate tools for molecular breeding but are expensive and time consuming to develop. In contrast, AFLP markers can be generated rapidly and efficiently but are not highly appropriate to routine screening of large breeding populations. DNA markers have proven powerful tools for genetic analysis in Musa (Crouch et al. 1998) but may not provide an effective means of predicting progeny performance (Tenkouano et al. 1999).

Recent advances suggest that is will be possible to use AFLP technology in an advanced lab. for the rapid and efficient identification of markers for important agronomic characters. A specific marker may then be converted into a simple assay for easy routine large-scale marker-assisted selection within the breeding station.

Progress in cytogenetic understanding in Musa has been extremely slow due to the very small compact nature of the chromosomes. However, flow cytometric techniques have proven most effective for rapid routine ploidy determination in Musa (Dolezel et al. 1994). More recently, molecular cytogenetic techniques have provided powerful tools for more detailed analysis (Osuji et al. 1997).

The cloning of genes underlying important agronomic characters offers to revolutionize progress in plant research and breeding, particularly in the area of pest and disease resistance. In addition, map-based cloning of gene(s) responsible for parthenocarpy, dwarfism and albinism in plantains and bananas, would be of great value to both fundamental and applied researchers of many crops.

In Musa, we believe that the aim of genetic engineering should be to put unique, important genes into elite germplasm for subsequent use in conventional Musa breeding. Currently, virus resistance (to BSV and BBTV) seems to be the most appropriate target to investigate the usefulness of transgenic methods in Musa improvement.

Conclusions

The scope for further genetic improvement of Musa is huge in comparison to most of the cereal crops (de Vries et al. 1967). In many instances, DNA markers will provide a vital link in the development of knowledge-led breeding schemes. However, biotechnology techniques must be integrated along with other approaches already available in current Musa improvement programs. Adoption of recent developments in other disciplines such as biometrics and statistics must also be followed to facilitate the development and utilization of greater understanding of complex genetic factors. New scientific partnerships, strategic research alliances, and joint ventures between public and private breeders must be explored to successfully achieve these goals. Today, biotechnology provides to banana and plantain improvement programs clean and fast multiplication of genotypes via micropropagation, diagnostics to ensure virus-tested germplasm, and genetic markers for assisted selection and gene introgression.

References

FAO (1998). http://apps.fao.org/lim500 /nph-wrap.pl?Production.Crops.Primary&Domain=SUA