Abstract
Red clover (Trifolium pratense L.) is a forage legume which has a high protein content and yields upwards of 10-15t DM/ha/year over three years. This high protein content can improve the performance of livestock when grown in a mixture with other forages, particularly ryegrasses in the UK. The use of red clover declined in the mid-twentieth century due to access to cheap nitrogen fertilizers and alternate sources of protein. Most varieties of red clover have moderate to low tolerance against the necrotrophic fungus crown rot (Sclerotinia trifoliorum) for which there is no economically viable chemical control. The persistency of red clover is an issue breeders experience along with delayed establishment compared to companion grasses and susceptibility to diseases and pests. These are areas which require genetic improvement. The improvement of these areas would permit red clover usage in agriculture as a shortterm cover nitrogen fixing forage crop. This thesis investigated the genetics involved in the resistance of crown rot with the ultimate aim of incorporating resistance into commercial cultivars. The first chapter of this thesis reviews the literature on red clover and its history,breeding strategies, genetics, agricultural importance, and secondary metabolites. The pathogenic fungus crown rot is also discussed along with its impact on legume crops in agriculture. The second chapter investigated the viability of an inoculation pre-screen for crown rot resistance in an ex-vitro system compared to a meristem inoculation in-vivo system. This comparison was investigated given that current methods of screening for crown rot involves destructive sampling and the high mortality rates inoculation with S. trifoliorum have on host plants. In the third chapter the potential role of polyphenol oxidase (PPO) in CR resistance was investigated. Two approaches were used. The first question was to determine if a breeding population with higher resistance to crown rot had higher PPO content. The second question used two populations with contrasting levels of PPO content to establish whether resistance to CR was correlated to PPO activity. These two questions would clarify any potential role PPO activity might have in suppressing crown rot infection in red clover. The fourth chapter used an RNA-seq approach to follow the gene expression in two red clover families with different levels of resistance to CR to investigate the genetics involved in crown rot resistance. Plants were inoculated and scored for disease, and mRNA was extracted from leaves at different timepoints for gene expression analysis.
The literature on crown rot interaction with red clover is limited. Therefore, identifying genes that influences resistance to crown rot might help to explain how crown rot suppresses plant defences during infection. In chapter five the results from all chapters were discussed and perspectives for future research given based on results generated in this thesis.
| Date of Award | 2023 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Leif Skot (Supervisor) & David Lloyd (Supervisor) |