Principles of Plant Genetics and Breeding

378 CHAPTER 20
Introduction
Nigel J. Taylor
Industry highlights
Genetic improvement of cassava through biotechnology
International Laboratory for Tropical Agricultural Biotechnology (ILTAB), Donald Danforth Plant Science Center,
St Louis, MO 63132, USA
The vast majority of the world’s farmers reside in developing countries where they cultivate crops on a hectare or less of land for
on-farm consumption and for sale in the local marketplace. Delivering enhanced plant varieties to these farmers is central to
improving their food security, health, and economic well-being. The starchy root crop cassava (Manihot esculenta), otherwise
known as tapioca, manioc, yucca, and mandioca, is an important component of agricultural systems throughout much of the
world’s tropical regions. After rice and maize, cassava is the most important source of dietary calories in the tropics, and worldwide
is cultivated over an area only 7% less than that of the Solanum potato (FAO 2004). Cassava is grown for its large swollen
storage roots, which develop to store large amounts of starch within specially developed xylem parenchyma. Resistance to
drought, tolerance to poor soils, and a flexible harvest window – the roots can be dug up when needed any time between 9 and 24
months after propagation from woody stem cuttings – makes the crop attractive to small-scale and subsistence farmers. As a result
cassava is widely cultivated in Africa, where it is the most important staple crop after maize, and by resource-poor farmers in Latin
America and tropical Asia. In addition, cassava is grown on a commercial scale for animal feed and as a source of starch for industrial
applications and the food processing industries in Asia, southern India, and increasingly in South America.
Why use biotechnology for the genetic improvement of cassava?
Conventional breeding in cassava is complicated by strong heterozygosity and inbreeding depression. Breeding programs consist of
bringing together preferred parents, crossing these and screening the first generation of resulting offspring for desired traits. Preferred
individuals are then propagated vegetatively to generate sufficient clones to allow for further screening and multilocation testing
(Ceballos et al. 2004). Large numbers of crosses must be carried out and thousands of offspring examined in order to identify candidate
lines. Most importantly, the inability to perform backcrossing greatly hinders the capacity to introgress multiple desired traits into a
single genetic background. Breeding programs in Africa and South America have been successful in developing and releasing
cassava varieties with an enhanced harvest index and resistance to disease. However, there is general acceptance that transgenic
technologies could hold the key to realizing the full genetic potential of cassava. Modern biotechnology allows the direct integration
of beneficial traits into a plant genome. In the case of cassava, this could be utilized to work hand in hand with breeding programs to
integrate traits missing from otherwise high performing germplasm, or employed to directly improve existing varieties and landraces
already favored by farmers. An example of the latter approach is provided by ongoing programs utilizing transgenic technologies
to generate resistance to cassava mosaic disease in East African cassava; the aim being to increase yields in highly susceptible landraces
and in some cases prevent such germplasm from being abandoned by farmers. Transgenic technologies also offer the possibility
to integrate beneficial traits that do not exist within the species or its sexually compatible relatives, into cassava. Examples
of such genes include those imparting resistance to herbicides and the Bt family of genes that provide resistance to insect pests.
Development of a transgenic capacity in cassava
The use of genetic transformation to improve a crop plant requires two technologies. Firstly, molecular biologists identify genes
with the potential to impart desired traits and then generate “gene constructs” that will allow effective expression of this genetic
material in the target species. Most often this involves fusing the coding sequence to a suitable promoter sequence that will direct
expression of the gene(s) in the appropriate tissues. For example, because cassava is a root crop, significant resources have been
directed at identifying promoters that drive gene expression in this organ and to the discovery of new promoter sequences, free
from existing intellectual property rights, which are capable of carrying out this function (Verdageur et al. 1998; Zhang et al.
2003). Secondly, technical capacity must be developed to deliver the new genetic material through the cell wall and facilitate its
integration into the plant’s genome. The latter requires that tissues be manipulated in culture to generate totipotent cell lines. It is
these totipotent cells that act as the target for transgene insertion, and which after selection for the successful integration events,
can be regenerated to produce whole genetically transformed plants.
Efforts to develop transgenic technologies in cassava were initiated in the late 1980s. However, it was not until 1996 that the
first transgenic cassava plants were reported (Li et al. 1996; Schöpke et al. 1996). This delay resulted from an inability to produce
suitable totipotent target tissues for transgene insertion. Although techniques for producing somatic embryos existed before 1996,
these tissues proved to be ineffective for the recovery of transgenic cassava plants via microparticle bombardment or
Agrobacterium tumificiens. The multicellular origin and highly organized nature of these embryogenic structures meant that only
chimeric embryos and plants could be recovered when this target tissue was used for genetic transformation. Two breakthroughs
concerning the in vitro manipulation of cassava somatic embryos were instrumental in allowing the first transgenic cassava plants