Cassava (Manihot esculenta) and cassava brown streak disease

: an investigation into interaction and infection

Abstract

Cassava is an emerging, future-proofed, and globally-important crop which already acts as a critical source of calories for 800 million people and is seeing increased commercial interest for industrial production of bioethanol and high-quality starch. Cassava production is threatened by cassava brown streak disease, caused by Cassava brown streak virus and Ugandan cassava brown streak virus (U/CBSVs). U/CBSVs cause yield losses in cassava of up to 70%, with incidences as high as 80-100% within its endemic range. CBSD is, therefore, considered to be one of the top seven most devastating contemporary plant diseases.

Research into U/CBSVs, therefore, is pertinent. This thesis reports attempts to explore CBSD in a three-pronged approach:

Firstly, efforts to construct a UCBSV infectious clone that can be reconstituted in planta are described, using a yeast recombination-based approach. Although these were ultimately unsuccessful, obtained data highlighted instability issues within UCBSV infectious clones and strategies are proposed for resolving these issues in future construction attempts, hopefully, enabling future reverse genetics and resistance research into this pathogen.

The U/CBSV genome encodes a novel gene encoding for an ITPase (HAM1). A similar ITPase is encoded by cassava, alongside other characterised members of the Euphorbiaceae family. These plant HAM1s include a C-terminal nuclear localization signal hypothesized to act as an anti-viral strategy. This thesis describes several attempts to isolate HAM1 genes present in other euphorbiaceous plants to study the nuclear localization signal. These findings highlight the challenges of working with both U/CBSVs and the Euphorbiaceae.

Finally, the host range of CBSV was explored, examining susceptibility of diverse candidate species through a large-scale mechanical inoculation experiment. Whilst results were not definitive, they highlighted the potential of several previously unexplored plant species to act as reservoirs for this virus. This may be valuable in informing future phytosanitary measures and CBSD management strategies.

Date of Award	24 Jan 2023
Original language	English
Awarding Institution	University of Bristol
Sponsors	Bristol Centre for Agricultural Inno
Supervisor	Gary Foster (Supervisor) & Andy M Bailey (Supervisor)