Studies Towards the Elucidation of the Function of the Cassava Brown Streak Virus HAM1h Protein

Abstract

The re-emergence of Cassava brown streak disease (CBSD) in the last twenty years is posing a huge threat to the cassava crop in sub-Saharan Africa with its rapid spread towards Western Africa, causing crop losses of up to 70% of the region’s second most important carbohydrate source. The causal agents of this disease were found to be Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), with CBSV being the more virulent. The viruses were found to be the only two of the Ipomovirus genus to contain putative HAM1 pyrophosphatases with the function of the viral HAM1h proteins currently unknown. With breeding for resistant cassava cultivars not bringing much success in the field, discovery of the function of these unique proteins may provide a specific target for the development of resistance to CBSD.
Work carried out in this thesis aimed to try to elucidate the function of the CBSV HAM1h. The viral HAM1 was first transformed into wild type and ham1 knockout yeast and used in spot assays against increasing concentrations of mutagens, to ascertain if the viral HAM1h does display the same phenotype as the overexpression of the yeast HAM1. Results from these show that the CBSV HAM1h does appear to have functionality. Secondly, the HAM1 transcript levels in Nicotiana benthamiana when infected with CBSV, were investigated to identify why the virus has its own HAM1, with results showing that the plant HAM1 is not downregulated upon CBSV infection. This suggests that the viral HAM1 acts against RNA non-canonical nucleotides and works in addition to the plant HAM1 to afford protection to high rates of mutagenesis. Further studies will need to be conducted to conclude if the CBSV HAM1h does display pyrophosphatase activity and whether this can be targeted to induce resistance to CBSD.

Date of Award	19 Mar 2019
Original language	English
Awarding Institution	The University of Bristol
Supervisor	Gary Foster (Supervisor) & Andy M Bailey (Supervisor)