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Title: Multiple pathogen recognition at the Mla locus in barley
Author: Brabham, Helen
ISNI:       0000 0004 8509 6442
Awarding Body: University of East Anglia
Current Institution: University of East Anglia
Date of Award: 2019
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The majority of NB-LRR-encoding resistance genes recognise single pathogen species; few NB-LRRs have the capacity to recognise multiple pathogens. The Mla locus has over 30 described alleles conferring isolate-specific resistance to Blumeria graminis f. sp. hordei (Bgh), contains three NB-LRR encoding gene families (RGH1, RGH2, and RGH3), and is associated with resistance to multiple pathogens including Puccinia striiformis f. sp. hordei (QRps1H) and Pyricularia oryzae (Rmo1). In addition, sensitivity to the Bipolaris victoriae toxin victorin (Lov1) is in coupling with Mla3. In Arabidopsis thaliana, sensitivity to victorin is mediated by an NB-LRRdependent plant immune response. I performed two high-resolution recombination screens and confirmed the genetic coupling of Mla3, Rmo1, and Lov1 in the Mla3 haplotype, and narrowed the interval for QRps1H in the Mla12 haplotype. Using sequence capture and RNAseq, copy number variation and high expression levels for Mla3 (RGH1; three copies), with one expressed copy (Mla3 D 6) containing a 6 base pair deletion in the LRR region; and presence of a gene fusion between RGH2 and Exo70F1 in the Mla3 haplotype were discovered. Characterisation of diverse barley accessions found substantial allelic variation in RGH2 including presence/absence of the integrated Exo70F1. Across Poaceae species, shared interspecific conservation in the RGH2-Exo70F1 integration was found. I hypothesise that balancing selection has maintained allelic variation at Mla as a trans-species polymorphism due to the role of multiple pathogen recognition, preserving interspecific diversity during speciation over 24 My. Evaluation of stable transgenic barley found that Mla3 conditions Bgh and P. oryzae resistance, whereas Mla3 D 6, RGH2, and RGH3 do not confer resistance to either pathogen. Resistance to P. oryzae is dependent on the copy number of the Mla3 transgene present. This work suggests that MLA has the potential to recognise multiple plant pathogen effectors—hypothesised due to conserved molecular structures or shared host targets.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available