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
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
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
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".