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• Summary of On-going Research and Development Activities of CBN Members
• Activities of CBN Members by Research Area and Location

Although the current network caters for only LAC-especially the pilot sites in Brazil, Colombia, Cuba, and Ecuador-cassava farmers, end-users, and researchers all over the world are members of the worldwide CBN.

CBN members around the world are engaged in several research and development (R&D) activities aimed at making cassava more productive. By doing so, they improve the livelihoods of resource-poor farmers for whom cassava farming is their mainstay. Activities conducted by CBN members are summarized in the table and discussion below:

Summary of On-going Research and Development Activities of CBN Members

Cassava Socioeconomics

A total of 27, independent, organizations in 15 countries and a CGIAR center, spanning 5 continents-Africa, Asia, the two Americas, and Europe-are engaged in 24 on-going R&D activities in the socioeconomics of cassava. Themes address issues relating to:

• Farmer participation in technology development
• Diversification of end uses to broaden the crop's market opportunities
  
• Status of cassava production and processing
• Increasing the productivity, profitability, sustainability, and adaptability of cassava technologies in the developing world
  
• Forming policy guidelines for introducing cassava into the food security or cash crop economy
  
• Potential effects of more efficient agronomic practices and use of herbicide-tolerant, high-yielding varieties under mechanization on the competitiveness of cassava farming
• Possibilities of Substituting more Established Crops like Maize with Cassava

Postharvest Handling and Micronutrients

For cassava to become anything more than the archetypal food security crop for the peasant, the perishability of its fresh roots must be addressed, so that these can be transported over long distances and stored for long periods, like other roots and tubers such as potato, yams, and sweet potato. Also, to be accepted as a component of a safe and balanced diet, its contents of desirable micronutrients must be improved, and efficient and reliable processing and detoxifying methods must be developed and widely adapted. For these reasons, 13 organizations working on 17 different themes in 10 countries and a CGIAR center are channeling efforts toward the study of fresh-root postharvest deterioration, their processing, and "fortification" when processed to enhance nutritional value.

Starch Modification

Even though cassava starch currently represents only 2% of the 28 million tons of starch being marketed annually, probably nothing else holds as much promise for propelling cassava into the global market as does the marketing of its starch and derivatives. Cassava starch is beginning to have both regional and global significance. An outstanding example is its increased production in Southeast Asia. Once sub-Saharan Africa and South America begin to produce and export sizeable quantities of this income-generating product, its prospects for capturing a major proportion of the international market are high.

Several comparative advantages in production and use are encouraging many industries to adopt cassava starch as an alternative to traditional sources. Cassava starch is unique for its very low production costs, high purity, resistance to acid media and sheer stress, resistance to structural change under freezing, high viscosity and clear paste nature, and production of transparent gels. These qualities give cassava starch the potential to replace most modified starches used in the food industry.

Currently, 13 organizations in 9 countries and a CGIAR center are studying cassava starch under 14 themes, with progress being made in the following areas:

• Cassava starch enzymology
• Starch biosynthesis pathways
• Efficient fermentation methods for high quality in different starchy attributes
• Use of genetic engineering and induced mutagenesis to produce varieties that have enhanced starch quantities and qualities, for example, amylose-free starch
• Modification of rheological properties in cassava starch

Genomics

Cassava's shy and asynchronous flowering, coupled with the wide segregation-characteristic of out-crossing species-hinders traditional breeding, making it costly and lengthy. Typically, between 20,000 and 100,000 seedlings are screened in the first sexual generation, and it takes 8 to 10 years for an improved variety to be released. Adoption naturally takes much longer. This scenario is indeed inefficient and needs changing. Molecular genetics holds special promise for:

• Developing molecular tags that can be cheaply and rapidly deployed for identifying genotypes with sought-after traits and thereby shunting the notoriously long and costly growth cycle
• Facilitating the "pyramiding" of those genes influencing agronomic traits of importance, even when they come from different sources

As has been shown by their successful application in other more researched and better-funded crops, molecular genetics, especially marker-assisted selection (MAS), offer tools that circumvent many limitations to cassava improvement. With the discovery of the molecular basis of natural variation, molecular or DNA markers have rapidly gained importance in the study of genes, genomes, and genetic diversity. They represent a limitless source of neutral markers for the quantitative assessment of genetic diversity and signposting in gene and genome mapping. Their abundance in any organism allows them to provide resolution of genetic relationships, and have led to genome and gene mapping.

Of 10 research centers in 5 countries of Africa, Europe, LAC, and North America, the Centro Internacional de Agricultura Tropical (CIAT) and the International Institute for Tropical Agriculture (IITA) have led in the development and application of cassava molecular genetic tools. Of significance is the development of the molecular genetic framework map for cassava, its on-going saturation with markers based on the polymerase chain reaction (PCR), molecular diversity studies, and tagging of several important agronomic traits. The robust DArT, based on a DNA microarray platform, is also being developed for the rapid saturation of the map and as a fingerprinting tool.

Genetic Resources

The available germplasm of any plant species constitutes a treasure trove for breeders, who can then exploit it to improve that plant. Harnessing the incredible genetic variation available in both cultivated and wild species, and wild relatives constitutes both a challenge and an opportunity. This is especially true for cassava, considering that both it and its relatives have, over the past 50 years, been widely dispersed from native South America to far-away Africa and Asia. With the dispersion, however, new population dynamics of pests and diseases have developed. For these to be effectively contained, the genetic base of cultivated cassava varieties must be continuously broadened.

Cassava scientists all over the world are therefore dedicating much effort to understanding, characterizing, conserving, and exploiting the crop's appreciable genetic resources. Currently, 29 different organizations in 14 countries of Africa, Asia, Europe, and LAC and two CGIAR centers are conducting 29 on-going activities involving cassava genetic resources. These activities include the genetic improvement of the crop, diversity studies of gene pools, and germplasm collection and conservation.

Tissue Culture and Transgenics

The cassava crop's future as a food staple and industrial raw material depends heavily on the production of high-yielding varieties that are resistant to biotic and abiotic stresses; on these varieties having roots that, when harvested, have a long shelf life; and on being able to satisfy the very high demand for clean planting materials.

However, cassava's heterozygous nature and long growth cycle make the development of new varieties highly inefficient. Its vegetative mode of propagation slows down adoption of improved varieties and erodes excellent varieties as the pest and disease loads accumulate. Biotechnology can help solve these constraints, particularly to the benefit of the small farmer. For example, genetic transformation can aid the transfer of single or multiple genes without dismantling the rest of the genetic structure of excellent varieties. Rapid in vitro propagation can be used to mass-produce healthy planting materials.

Cassava researchers are therefore working on the development of genetic transformation protocols and efficient regeneration systems, even for recalcitrant varieties from Africa. These activities are being complemented by research on efficient rapid propagation systems. In all, 24 organizations in 13 countries and 2 CGIAR centers are involved in 37 themes addressing these issues. Also being addressed are transformation of cassava for resistance to insect pests and viruses, reduced cyanogenic potential, improved nutritional qualities, and optimization of cryopreservation of cassava tissues and its application in germplasm conservation.

Biotic Stress

The cassava crop suffers from several yield constraints, which must be addressed before it can fulfill its potential as a future subsistence and industrial crop. Viral and bacterial diseases are major contributors to yield losses, with insect pests, nematodes, and weeds also contributing. These problems are further aggravated by the fact that cassava is vegetatively propagated, leading to an ever-increasing accumulation of pathogen loads in the propagules over successive seasons. This invariably leads to depressed yields and reduced quality of planting materials.

Of the diseases, African cassava mosaic disease (ACMD), caused by several closely related and interacting geminiviruses, is the single most important in Africa and the most important vector borne disease of any African food crop. Another major viral disease of Manihot is the cassava common mosaic virus, a potexvirus prevalent in Colombia, Brazil, and Peru, and which can account for as much as 30% of yield losses. Cassava bacterial blight, caused by Xanthomonas axonopodis pv. manihotis, is the most important worldwide disease of cassava, responsible, under certain conditions, for total crop failure in both Africa and South America. A major insect pest is the whitefly Aleurotrachelus socialis.

Biotechnology offers the potential to rapidly transfer resistance genes from resistant cultivars or wild relatives to susceptible cultivars, as in the case of CBB. The cassava research community, represented by 27 institutions in 13 countries and 2 CGIAR centers, is researching 25 themes on solving the biotic constraints to cassava production. Their multipronged approaches include developing diagnostic kits for the diseases, molecular characterization of pathotypes, several biocontrol initiatives, host-plant resistance studies, breeding, multiplication, and distribution of clean planting materials

Gene Discovery

New genomics tools promise faster discovery of genes that can be used in genetic transformation or as genetic markers to increase the efficiency and cost effectiveness of plant breeding. A crop like cassava, with all its inherent problems associated with conventional genetic improvement, needs the intervention of these novel tools to develop other efficient and reliable tools for the specific use of crop improvement. Currently, 9 institutions in 5 countries and 2 CGIAR centers in Africa, Asia, Europe, LAC, and North America are addressing 13 research themes on this subject. Some approaches include the application of proteomics to gene isolation; map-based gene cloning; development of bacterial artificial chromosome (BAC) library resources; gene expression microarray chips; isolation and characterization of tissue-specific promoters; quantitative trait loci (QTLs) studies; and the characterization of defense genes.

Activities of CBN Members by Research Area and Location