Cassava (Manihot esculenta Crantz) is one of the most important staple crops worldwide, and is the most important in many arid regions, such as sub-Saharan Africa.
One of the most valued characteristics of this crop is its capacity to produce reasonable root yields under adverse climatic and soil conditions.
Compared with other staple foods, cassava also offers the advantage of a flexible harvesting date, allowing farmers to keep the roots in the ground until needed. In addition to the important role cassava plays in food security, there is a growing demand for cassava roots by the starch, food, animal feed and ethanol processing industries. However, several factors affect the relative efficiency of cassava to satisfy these needs. Cassava is generally grown in marginal environments, some distance away from the processing centers, and transport infrastructure is poor. Cassava roots are also bulky, containing approximately 65% water, and have a very short shelf life due to a process known as post-harvest physiological deterioration (PPD).
PPD renders the roots unpalatable and unmarketable within 2-4 days of harvest. This severely limits the marketing options by increasing the likelihood of root losses, raising the overall marketing costs, and restricting access to more distant urban markets or processing centers. The processes involved in PPD resemble typical changes associated with the plant’s response to wounding, and triggers a cascade of biochemical reactions, in which reactive oxygen species are central. Specific genes involved in PPD have been identified and characterized, and their expressions evaluated. Although this knowledge means that there could be a potential genetic solution to this problem, very little progress has been made to overcome it.
This study started with an accident. A few roots from a cassava clone (belonging to a new generation of high-carotene germplasm) were left on a shelf for more than two months between the end of 2008 and beginning of 2009. When inspected these roots did not show any symptoms of PPD. This result prompted the planning and execution of a study whose results are presented in this article.
Four different sources of tolerance to PDD have been identified. One comes from the only Manihot species native of the United States (M. walkerae). A second source was induced by mutagenic levels of gamma rays which putatively silenced one of the genes involved in PPD genesis. A third source was a group of high-carotene clones. It is postulated that the antioxidant properties of carotenoids protects the roots from PPD (basically an oxidative process). Finally tolerance was also observed in a waxy-starch (amylose-free) mutant. It is expected that tolerance to PPD co-segregated with the starch mutation and is not a pleiotropic effect of the latter.
The economic relevance of these discoveries is expected to be huge. It will benefit the poorest of the poor, widening and strengthening the markets for cassava, reducing marketing costs and losses along the marketing or value addition process. These are the bottlenecks that prevent cassava having a larger impact on the livelihoods of the communities that depend on it, as identified by FAO’s Global Cassava Initiative which culminated in 2000.
This study highlights the importance of germplasm collections and their screening, the usefulness of inbreeding cassava (in search of recessive traits) and the potential of induced mutations particularly with the advent of molecular tools such as TILLING (Targeting Induced Local Lesions in Genomes).
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