Abstract
Cassava (Manihot esculenta Crantz) is a major world crop, whose storage roots provide food for over 800 million throughout the humid tropics. Despite many advantages as a crop, the development of cassava is seriously constrained by the rapid post-harvest physiological deterioration (PPD) of its roots that occurs within 24-72 h of harvest, rendering the roots unpalatable and unmarketable. PPD limits cassava's marketing possibilities in countries that are undergoing increased development and urbanisation due to growing distances between farms and consumers. The inevitable wounding of the roots caused by harvesting triggers an oxidative burst that spreads throughout the cassava root, together with the accumulation of secondary metabolites including phenolic compounds, of which the coumarin scopoletin (7-hydroxy-6-methoxy-2H-1-benzopyran-2-one) is the most abundant. Scopoletin oxidation yields a blue-black colour, which suggests its involvement in the discoloration observed during PPD. Feruloyl CoA 6'-hydroxylase is a controlling enzyme in the biosynthesis of scopoletin. The cassava genome contains a seven membered family of feruloyl CoA 6'-hydroxylase genes, four of which are expressed in the storage root and, of these, three were capable of functionally complementing Arabidopsis T-DNA insertion mutants in this gene. A RNA interference construct, designed to a highly conserved region of these genes, was used to transform cassava, where it significantly reduced feruloyl CoA 6'-hydroxylase gene expression, scopoletin accumulation and PPD symptom development. Collectively, our results provide evidence that scopoletin plays a major functional role in the development of PPD symptoms, rather than merely paralleling symptom development in the cassava storage root.