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Antony Chapman - Final Oral Exam

Dec 15, 2020 - 2:00 PM
to Dec 15, 2020 - 5:00 PM

Genetics and Genomics Final Oral Exam

Antony Chapman

“You Should Be Dead! How a non-lethal mutation may unravel the extent of SGT1 requirement in disease resistance”

December 15, 2020

2:00 PM

Major Professor:  Roger Wise

Co-Major Professor:  Steve Whitham

Home Department: Plant Pathology and Microbiology


Barley (Hordeum vulgare L.) Mla (Mildew resistance locus a) and its nucleotide-binding leucine-rich-repeat receptor (NLR) orthologs protect many cereal crops from diseases caused by fungal pathogens. However, large segments of the Mla pathway and its mechanisms remain unknown. To further characterize the molecular interactions required for NLR-based immunity, we used fast-neutron mutagenesis to screen for plants compromised in MLA-mediated immunity to the powdery mildew fungus, Blumeria graminis f. sp. hordei.

One unique variant, m11526, contained a novel mutation, designated rar3 (required for Mla6 resistance3), that abolishes race-specific resistance conditioned by the Mla6, Mla7 and Mla12 alleles, but does not compromise immunity mediated by Mla1, Mla9, Mla10 and Mla13. We used bulked-segregant-exome capture and fine mapping to delineate the causal mutation to an in-frame Lys-Leu deletion within the SGS domain of SGT1 (Suppressor of G-two allele of Skp1, Sgt1ΔKL308-309), the structural region that interacts with MLA proteins. In nature, mutations to Sgt1 usually have lethal phenotypes, but this mutant displays no physiological abnormalities.

We then used Yeast-2-Hybrid, site-directed mutagenesis, and parallel reaction monitoring mass spectrometry to uncover how and why the Sgt1ΔKL308-309 mutation selectively disrupts MLA-based disease resistance. The mutation appears to weaken the intermolecular interactions between MLA and SGT1, causing a reduction of MLA protein to below a threshold necessary for effective resistance. Our work, combined with recent advances with other NLRs, suggests a new cyclical model for the interaction between MLA, SGT1, and their co-chaperones, which may apply to all dimeric CC-NLRs.

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