The rapid postharvest deterioration of the roots, known as postharvest physiological deterioration (PPD), has been a major problem to the utilisation and development of cassava (Manihot esculenta Crantz) as a food and industrial crop. PPD usually occurs within two to three days after harvest and it is characterised by a blue-black discoloration of the roots, which renders the roots unpalatable and unmarketable. Scopoletin, which is synthesised de novo and released from its glucoside, scopolin, during PPD, plays a central role in this discoloration response. Interconversion of scopoletin and scopolin, which is catalysed by scopoletin-glucosyltransferase (scopoletin-GT) and scopolin-beta-glucosidase (scopolin-BG), regulates homeostasis of scopoletin in the cells. However, how this interconversion contributes to root discoloration development is poorly understood. In the present study, we identified and characterised cassava genes for the enzymes that are responsible for the interconversion of scopoletin and scopolin, subsequently manipulated their expression in transgenic cassava through scopoletin-GT RNAi gene silencing and scopolin-BG overexpression constructs. These approaches would potentially alter scopoletin and scopolin content in the root, thereby affecting PPD response. A BLAST search for homologous cassava genes revealed that scopoletin-GT and scopolin-BG are encoded by multiple genes, most of which belong to glucosyltransferase family-1 (GT1) and glycosyl hydrolase family-1 (GH1), respectively. Scopoletin-GT-down-regulated and scopolin-BG overexpressed transgenic cassava lines showed reduced not only scopolin but surprisingly also scopoletin, and delayed PPD. Additionally, other coumarins esculetin and esculin were also identified and both scopoletin-GT, MeSGT1, and scopolin-BG, BGLU23, were up-regulated during PPD development at day 4 and day 2, respectively. Our study reveals that disrupting the interconversion of scopoletin and scopolin by inhibiting scopoletin-GT and overexpressing scopolin-BG led to the decrease of both scopoletin and scopolin content and delayed PPD in cassava. These findings provide useful insights into the role of interconversion of scopoletin and scopolin in cassava PPD response and may suggest alternative ways to tackle PPD.
|Date of Award||8 Nov 2017|
|Sponsors||Ministry of Research, Technology and Higher Education|
|Supervisor||John Beeching (Supervisor) & Roderick Scott (Supervisor)|
- Cassava (Manihot esculenta Crantz)
- Postharvest Deterioration
- Gene Silencing
- Metabolic Engineering