Gunning for belter cassava

There is snow outside its windows half the year and a cold wind sweeps the campus, but the laboratory filled with cassava shoots at the University of Guelph (near Toronto), Canada, isn't trying to develop winter-hardy varieties of a staple tropical crop. The presence of cassava this far north is only another indication of the worldwide scope of an expanding research effort to enhance cassava's nutritional and economic value.

Increasingly, this effort is drawing upon the latest techniques in plant genetic engineering - including a device known as a particle gun designed to achieve genetic transformation of target plant species by literally bombarding selected cell tissue with DNA-coated particles. Since transformation and regeneration of transgenic cassava plants in large numbers has so far proven difficult, the initial goal of the Guelph project isn't to change specific genes, but only to develop viable transformation technology.

The helium-powered particle gun has been used in biotechnology laboratories for five or six years in the United States, most notably in private sector development of the world's first transgenic maize plants. "Cassava isn't like corn or coffee", points out Prof. Larry Erickson, a plant geneticist who heads the Guelph project. "No companies are making any money on it". Recruited recently from private industry, Erickson is carrying out the two-year project on a US$100 000 grant from the Rockefeller Foundation. He recognizes that any transformed cultivars developed from his or related projects will have to be channelled largely through the international research institute network and the national research and extension services of developing countries, in order to be available to the smallholders who have traditionally been cassava's principal producers.

Widely held misconceptions

Until recent years, funds allocated for cassava research and development have been scanty, owing, as Dr James H. Cock, former coordinator of cassava programs at the International Centre for Tropical Agriculture (CIAT) has remarked, "to widely held misconceptions about the crop". This despite the fact that an estimated 420 million people scattered through more than two dozen tropical countries depend on cassava for 50 per cent or more of their total dietary energy. While direct human consumption accounts for approximately two-thirds of the annual global production of about 130 million tons, livestock feed and industrial uses also offer significant markets to producers.

There are admittedly problems. On the nutritional side, the low protein content of the cassava tuber - normally less than one per cent - has adversely affected its rating as a desirable food staple. So too has the presence in raw cassava of a pair of glycoside compounds which convert to toxic hydrocyanic, or prussic acid when root cells are ruptured. Although traditional cassava processing - boiling, pressing, sieving, toasting - helps to reduce prussic acid content and to eliminate dangers of acute poisoning, chronic cyanide toxicity has been a problem in some regions, especially in Africa, where impact has been linked to low levels of both protein and iodine in the diet.

Another disadvantage is that the freshly-harvested tubers deteriorate rapidly, thus requiring prompt processing or a very limited range of marketing for the fresh produce. Finally, although cassava has a reputation for disease and pest tolerance, it is known that yields are often significantly reduced by a variety of enemies, such as mealybugs and spidermites (Ceres No. 130), bacterial blight and mosaic viruses.

However, compensating for these handicaps, the traditional domestic cassava, Manihot esculenta, exhibits some attractive characteristics. Even with scant attention, it yields more food calories per hectare than other tropical crops such as maize, rice or sweet potato. It possesses a high level of tolerance for infertile soils and extended periods of drought. In fact, in many regions it is planted as a famine reserve crop to provide an assured food supply should other crops succumb to drought or locust plagues. In this regard, it is a particular advantage that cassava tubers can be harvested as needed at any time from nine months to two or three years after planting.

While Manihot esculenta is the only cassava widely used as a crop, nearly 100 wild species have been identified. Most appear to cross readily with the domesticated variety and are consequently considered potentially useful in breeding programs. One, M. glaziovii, has already been used as a source of resistance to African mosaic disease, which attacks M. esculenta. Another, M. tristis, when crossed with M. esculenta, has produced a hybrid with a root protein content of 12 per cent. Wild manihot can also provide genes for low cyanide content.

Given this genetic base, the challenge facing plant scientists has been to devise methods for introducing desired traits into the domesticated species or hybrids derived from it. Enter biotechnology.

"Almost every other crop has yielded to genetic engineering", says Dr Gary Toenniessen, Rockefeller's associate director of agricultural sciences. "With the particle gun, almost all cereals are transformable now". As support for cassava research has finally gained momentum, a loose international network of projects has been able to focus on specific problems and to test different genetic engineering technologies.

The International Cassava Trans Project (ICTP), a joint venture of the French Institute of Scientific Research for Cooperative Development (ORSTOM) and the Research Institute of Scripps Clinic, in the United States, is concentrating on two viruses: the cassava common mosaic virus, which is a major problem in Latin America, and the African cassava mosaic virus. Through a method known as the "coat protein mediated resistance strategy" or, more conveniently, CP, viral genes are introduced into the genetic structure of the intended host plant in such a way that they interfere with, and reduce, the spread of the viral infection. Over the past three years, the project has indicated that the chosen strategy offers good prospects for control of several strains of mosaic viruses, but further research is required to make the transformation process more efficient.

At Washington State University, another Rockefeller-supported project is using genes isolated from potatoes to provide cassava both with greater insect resistance and improved nutritional quality. Other, alternative genetic transformation techniques are being tried in separate projects in China, Denmark and the United Kingdom.

"The prime feature of genetic engineering is that you can do things that are otherwise impossible or very slow", says Guelph's Prof. Erickson. "With normal plant breeding you can't make the kinds of major changes that you can with tissue culture".

At the University of Guelph, which has a long tradition of involvement in international projects, including earlier research on cassava, Prof. Erickson sees other spinoff benefits from this kind of undertaking. The transformation system being developed is attracting the interest of postdoctoral students and scientists from other countries. His research assistant, Basdeo Bhagwat, had previously worked as a tissue culture specialist on bananas at the University of Trinidad and developed a number of techniques now being used on the cassava project. Says Prof. Erickson: "We are learning things that we hope we can apply to other crops, tropical and nontropical".

Peter Hendry