Post-harvest deterioration in cassava : from understanding towards control

The starchy storage roots of cassava (Manihot esculenta Crantz) deteriorate within 24 – 72 hours of harvest rendering them unpalatable and unmarketable. With growing urbanization and the entry of cassava into the cash economy this post-harvest physiological deterioration (PPD) has become a major constraint to the development of this important crop, affecting farmers, processors and consumers alike. The physiological changes that occur during PPD are due to the oxidation of phenolic compounds and involve the formation of reactive oxygen species (ROS), alterations in gene expression, protein synthesis and the accumulation and oxidation of a range of secondary metabolites. Biochemical and molecular data confirm that the production and reactions of ROS are central to PPD, and a model suggesting that PPD is a ROS-mediated programmed cell death (PCD) response has been proposed. This model implies that enhancing the anti-oxidant or anti-PCD status of the cassava root at or shortly after harvest may limit the reactions, damage and changes induced by ROS. We are testing this model in transgenic cassava in which we have separately introduced five anti-oxidant genes and three anti-PCD gene driven by a root-specific promoter. The resultant data will be invaluable both for confirming and refining the model, and for moving towards the identification of means by which to usefully modulate the PPD response of cassava roots, as the ultimate goal of this research is to benefit the sustainable livelihoods of resource-poor farmers.