Tissue culture
Sweet potato has a very high genetic variability and thousands of varieties of sweet potato exist in germplasm collections. But as sweet potato is vegetatively propagated, the maintenance of its germplasm at gene banks can be a very laborious task. The International Potato Center (CIP) in Peru, which has the global mandate for sweet potato research, has nearly 4,000 accessions in its collection. The Asian Vegetable Research and Development Center (AVRDC) in Taiwan and the Agricultural Research Service of the United States Department of Agriculture (ARS/USDA) also hold extensive collections, albeit smaller. At all three locations, sweet potato germplasm is stored in vitro. Tissue culture assists the storage of disease­free collections and facilitates easier maintenance and distribution of germplasm.
While sweet potato is relatively easy to micropropagate (i.e. multiplying of small sterile shoots in test tubes to obtain a large number of genetically identical plants), it was recalcitrant to regenerate (i.e. obtaining adventitious plants from non­meristematic tissue such as leaf). An efficient method to regenerate sweet potato in tissue culture is essential to the production of transgenic plants. Several research groups have produced plants of sweet potato in tissue culture using diverse approaches, but most techniques result in low frequencies of shoots and employ cumbersome and time­consuming procedures. With funding assistance from United States Agency for International Development (USAID), USDA and National Aeronautics and Space Administration (NASA), scientists at Tuskegee University (USA) have embarked on an ambitious programme to understand and manipulate the genome of sweet potato. The NASA has chosen sweet potato as one of eight crops to be grown for long­term space missions. Recently, the collaborating institutes developed a tissue culture system that enables them to quickly produce large numbers of adventitious sweet potato plants. 
Somatic embryogenesis has also been achieved in sweet potato tissue culture, and scientists at the University of Florida have developed a system to produce artificial seeds of the sweet potato cultivar White Star encapsulated in gels. There are also reports from Japan and France of regeneration of sweet potato plants from protoplasts.
Transgenic sweet potato plants expressing marker genes have been developed by using the soil bacterium Agrobacterium tumefaciens as a vector for transformation. Foreign genes have also been introduced and expressed in sweet potato cells using the particle gun technique. However, lack of suitable tissue culture techniques earlier have hindered large­scale production of transgenic plants.

Sweet potato fact file Sweet potato is an ancient crop, originating in South America. Archaeological evidence from Peru shows that domestication of sweet potato dates back to 6000 BC. China is the largest producer of sweet potato with about 80 per cent of world production, followed by Vietnam. Sweet potato is in many ways an ideal crop for farmers, as it grows on low­nitrogen soils, tolerates droughts well, crowds out weeds and suffers from relatively few pests. Sweet potato provides nutritionally significant quantities of ascorbic acid, riboflavin, iron, calcium and protein. In addition, the orange fleshed sweet potatoes are rich in ß­carotene, a nutrient which may be effective in preventing certain types of cancer. Among the food crops, sweet potato has the highest recorded net protein utilization (based on percentage of food nitrogen retained in the body).

Improved disease resistance
There are several fungal, bacterial and viral diseases which infect the sweet potato crop. As sweet potato is grown primarily as a subsistence crop in most developing countries, chemical control of these diseases is not widely practised. Frequent replanting with virus­free stock is also no enduring solution as warm climates lead to a high re­infection rate.
Development of cultivars resistant to diseases is a viable option that makes both environmental and economic sense. The transfer of a cecropin gene from the giant silk moth has already been achieved in tobacco and potato and the resulting transgenic plants have reportedly demonstrated measurable resistance to bacteria and fungi. Synthetic versions of this gene, with improved stability and activity, are being introduced into elite cultivars of sweet potato at Tuskegee University. It remains to be seen whether transgenic sweet potato expressing this gene can tolerate the attack of pathogenic bacteria and fungi.
Sweet potato feathery mottle virus is a major problem that causes 'russet crack' disease and affects sweet potato production, particularly in Africa. Efforts are under way to develop resistance to the feathery mottle virus using the coat protein gene and anti­sense RNA genes. Currently, research at the US agrochemical company Monsanto and Tuskegee University aims to develop transgenic sweet potato plants using these genes. As a part of this project scientists from Africa receive training in the genetic engineering of sweet potato and cassava. USAID and Monsanto have both invested around US$ 150,000 for this project. Also Cuban and Chinese scientists have cloned the coat protein gene for sweet potato feathery mottle virus and are attempting to develop virus­resistant plants.

Sweet potato weevil
Sweet potato weevil is by far the greatest enemy of sweet potato especially in the tropics. Production losses due to this insect attack reaches 60 to 100 per cent in certain areas. The sweet potato weevil feeds on stored roots, thereby reducing their quality and yield; secondary compounds produced by roots in response to weevil attack make even slightly damaged roots unedible.
Unfortunately, very little resistance to weevil can be found in sweet potato germplasm. Therefore, the British Overseas Development Agency (ODA) has awarded a research grant to Agricultural Genetics Company to genetically engineer sweet potato with cowpea trypsin inhibitor genes (see box). This gene was found initially in a variety of cowpea that was highly resistant to bruchid insect infestation. Scientists at the University of Birmingham (UK) subsequently isolated, characterized and transferred this gene to tobacco which became resistant to many pests. It is still not clear whether trypsin inhibitor can protect sweet potato against the weevil, especially considering that trypsin inhibitors are already present in many sweet potato varieties. A parallel strategy may be to search for strains of Bacillus thuringiensis that attack sweet potato weevil and insert the found bacterial endotoxin gene into sweet potato.

Quality characteristics
The most useful applications of genetic engineering in sweet potato may be in the improvement of nutritional and quality traits. As a result of the high yield per ha, sweet potato rates very high in protein production and has the highest recorded net protein utilization among major food crops. However, like other plant proteins, sweet potato protein is deficient in many essential amino acids. To address this problem, research at Tuskegee University is attempting to introduce a synthetic storage protein gene that codes for essential amino acids. Theoretically, the nutritional quality of the 'artificial storage protein' is similar to that of milk or egg protein. As the leaf tips of sweet potato are also consumed as a green vegetable, targeting of improved protein gene expression to young leaves would also be nutritionally beneficial. Genes that code for sulphur­containing amino acids such as those found in Brazil nut may also be useful.

Insect­resistant potato and sweet potato The British Agricultural Genetics Company (AGC) has signed a contract with the Plant Research Programme of the UK's Overseas Development Administration (ODA) to produce transgenic insect­resistant sweet potato and potato. Over 40 different insect species damage sweet potato in the field and in storage. The most destructive insect is the sweet potato weevil (SPW). The major damage comes from larval feeding in the storage roots, making these roots unfit for human and animal consumption. The most important insect pest of potato in developing countries is the potato tuber moth (PTM), which infects tubers in the field and in storage. PTM causes losses through (1) increased number of discards, (2) reduced prices for damaged potatoes, (3) increased handling costs, and (4) increased expenditure on pesticides. If PTM is present in their storage facilities, farmers may also be forced to sell their potato crop early, when prices are not at their maximum. Agronomists, entomologists and plant breeders have tried to develop agronomic practices and elite germplasm to overcome the destructive effect of SPW and PTM. However, this has not been effective enough to provide adequate insect control for the small­holder farmer in the developing world. This has lead ODA and AGC to conclude that these insect pests may be better controlled through the development of genetic resistance in high­yielding elite germplasm, using genetic engineering technology. For AGC, an internationally operating biotechnology company, the development of insect­resistant transgenic plants is a major strategic target. Since its foundation in 1983, AGC has isolated and developed, often in collaboration with the University of Durham (UK), fifteen different plant genes that are insecticidal. The ODA has contracted AGC to produce transgenic germplasm of both sweet potato and potato expressing a number of AGC's proprietary insect resistance genes. As transformation and regeneration methodologies have already been developed for potato, it is expected that the production of transgenic germplasm should be straightforward. However, this is not the case for sweet potato and considerable effort is needed to develop a transformation and regeneration system. The transgenic germplasm will be tested at the University of Durham and at the International Potato Centre (CIP), to select those lines that give the greatest level of resistance to the targeted pests. AGC has granted ODA a non­exclusive royalty­free licence to its proprietary technology, to allow ODA to distribute any transgenic germplasm resulting from the research programme to plant breeders in the developing world. AGC is carrying out the research programme within the financial guidelines laid down by ODA. ODA will be responsible for the co­ordination of field trials and the incorporation of the novel transgenic breeding lines in conventional breeding programmes. Source: P.D. Barfoot, "Plant molecular biology for developing countries: A project to develop insect­resistant potatoes and sweet potatoes". AgBiotech News and Information, 1993, vol. 5, no. 11, pp. 397N­402N.

DNA markers
While genetic engineering aims at the rapid development of improved cultivars, commercialization of transgenic sweet potato is still a medium­ to long­term goal considering many research and regulatory challenges that lie ahead. However, an immediate pay­off from molecular genetics to sweet potato improvement may be in the form of DNA markers. Restriction Fragment Length Polymorphism (RFLP) markers have been used to asses the relationship between cultivated sweet potato and wild Ipomoea species at ARS/USDA. In a collaborative effort between scientists at Tuskegee University, USDA and Auburn University (USA), a new approach has recently been employed to genetically fingerprint sweet potato varieties. This technique, DNA amplification fingerprinting, uses the polymerase chain reaction process to define polymorphic DNA markers. The method is fast and enables a single technician to screen 200 sweet potato varieties in one day. Analysis of preliminary results reveal considerable genetic variation in sweet potato germplasm collected from across the globe but little variation in US sweet potato varieties. DNA fingerprinting techniques can thus be employed to assess genetic variation and also to identify duplicates in the germplasm collection. It also facilitates improved germplasm collection activity by identifying those geographic areas with greatest genetic diversity. DNA fingerprints are valuable to breeders by enabling them to identify divergent parental lines for hybridization and to monitor somatic hybrids and somaclonal variation.

Research in developing countries
Sweet potato is also subject to biotechnological research in many developing countries. Scientists in India and Peru employ tissue culture to maintain the sweet potato germplasm. In India, methods have been developed to maintain sweet potato plants in tissue culture. Scientists at the International Institute for Tropical Agriculture (IITA) in Nigeria, have developed methods to produce virus­free sweet potato plants through meristem culture. Considerable research on sweet potato tissue culture is also being pursued in China, while Indonesia has linked up with the Michigan State University, USA, to genetically engineer sweet potato for resistance to sweet potato weevil. 
C.S. Prakash (Tuskegee University, USA)

Sources
W. A. Hill, C.K. Bonsi and P.A. Loretan (eds) (1992), Sweet Potato Technology for the 21st Century. Tuskegee, USA: Tuskegee University.
R.K. Jansson and K.V. Raman (eds) (1991), Sweet Potato Pest Management: A Global Perspective. Boulder, USA: Westview Press.

C.S. Prakash and U. Varadarajan (1992), "Genetic transformation of sweet potato by particle bombardment". Plant Cell Reports, Vol. 11, pp. 53­57.

J.A. Woolfe (1992), Sweet Potato, An Untapped Food Resource. New York/Cambridge, UK: Cambridge University Press.