Principles of Plant Genetics and Breeding

BREEDING FOR RESISTANCE TO DISEASES AND INSECT PESTS 381
variety (or range of varieties) suitable for deployment to farmers. As described above, this is not a good option for cassava and
the other heterozygous, vegetatively propagated crops. Here the desire is to directly enhance the performance of existing farmerpreferred
varieties, landraces, and elite breeding lines, without changing their other beneficial characteristics, which may be lost
due to unpredictable segregation when sexual crossing is carried out. It is thus necessary to genetically transform each desired
cassava variety individually. This presents important challenges, as one has to correctly identify which varieties to target for such
investment and then develop the technical capacity to manipulate this germplasm within the tissue culture and genetic transformation
systems established for the crop. Greenhouse and field testing must then follow for each new transgenic variety. With support
from the United States Agency for International Development, the DDPSC is currently engaged in this process for preferred
cassava varieties from East Africa.
Future directions for transgenic cassava
Biotechnology can have a significant impact on the genetic improvement of vegetatively propagated crops, including cassava, if
sufficient resources are committed to such efforts. However, with only five research institutes capable of producing transgenic cassava
plants, it is obvious that the funding being committed to this and other crops important in developing countries such as plantains,
sorghum, millet, and sweet potato are not in proportion to their importance as sources of food security and economic well-being
for people in the tropical regions. Nevertheless, important progress has been made over the last 5–10 years in developing transgenic
programs for cassava. Genes of agronomic interest have been integrated into the crop and field trials – a critical step in the
process towards product development – that are being initiated in Africa and at CIAT, Colombia. Future programs should benefit
from mapping and other genomic-based research. BAC (bacterial artificial chromosome) libraries are being developed for the
crop and being used to identify and isolate genes responsible for resistance to CMD and bacterial blight disease. Once available
these can be integrated into farmer-preferred varieties using the transgenic technologies described above. It is also hoped to
access beneficial genes present in the wild relatives of cassava. Within this germplasm can be found traits for enhanced protein
accumulation in the roots, longer shelf-life of harvested roots, and disease and pest resistance. Cassava has an inherent capacity
for high rates of photosynthesis and the ability to accumulate large amounts of starch within its storage roots. It has already been
shown possible to shut down starch accumulation in cassava through the application of transgenic technologies. Can this unused
energy be diverted towards the accumulation of biosynthetic plastics? Could genes for apomixis be introduced to cassava to
enable true seeds to be produced, thereby revolutionizing the propagation of disease-free propagules? Reaching such goals
though traditional breeding alone is not possible. They are only feasible if robust biotechnology programs are developed for cassava.
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