Determination of the key resistance gene analogues involved in Ascochyta rabiei recognition in Chickpea

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Ford, Rebecca

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Love, Christopher

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2018-06
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Abstract

Chickpea (Cicer arietinum L.) is an important cool season food legume, playing a significant role in global food security. However, the production of chickpea is severely constrained by foliar Ascochyta blight disease caused by the fungus Ascochyta rabiei (syn. Phoma rabiei). The disease results in substantial yield losses annually and has become a primary biotic constraint to production in Australia. Several disease management options have been developed to reduce or control the pathogen, including host plant resistance. However, for host resistance to be effective, the plant must quickly recognise the pathogen and instigate initial defence mechanisms at the point of contact. Previous research has shown that the most resistant host genotypes are able to recognise the pathogen the fastest. Resistance Gene Analogues (RGAs) are key factors in the recognition of plant pathogens and the signaling of inducible defences. They comprise a large gene family with conserved domains and structural features; classified as either nucleotide binding site leucine rich repeat (NBS-LRR) or transmembrane leucine rich repeat (TM-LRR) groups. In chickpea, they are reported to recognise A. rabiei with varying knowledge of their identities and putative functions. In this study, a suit of RGA loci were chosen from both published literature and from homologous sequences within the NCBI database for further investigation. All RGA candidates were members of the NBS-LRR family group. Following their validation in the chickpea genome through traditional PCR, and qPCR primer optimization, 10 of the target RGA were selected for differential expression analysis in response to A. rabiei. This was performed in a set of four chickpea genotypes including two resistant cultivars (ICC 3996 and PBA Seamer), one moderately resistant cultivar (PBA HatTrick) and one susceptible cultivar (Kyabra). Expression of each locus was assessed via qPCR at 2, 6, and 24 hours after A. rabiei infection with a previously characterised highly aggressive isolate. As a result, all loci were differentially transcribed in response to pathogen infection in at least one genotype and at least one time point after inoculation. Among these, transcription of RGA 8, RGA 10, RGA 21 and RGA 23 was significantly and consistently increased in the resistant genotype ICC 3996 immediately following inoculation. Further bioinformatics in-silico analyses of these four RGA indicated they all function through ADP binding, in different parts of pathogen recognition pathway. These represent clear targets for future functional validation and potential for selective resistance breeding and/or for introgression into elite cultivars that are quickly able to recognise and respond to A. rabiei.

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Thesis (Masters)

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Master of Science (MSc)

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School of Environment and Sc

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The author owns the copyright in this thesis, unless stated otherwise.

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Ascochyta rabiei

Chickpea

Disease management options

Resistance gene analogues

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