Disruption of barley immunity to powdery mildew by an in-frame Lys-Leu deletion in the essential protein SGT1

Authors: CHAPMAN, ANTONY - Iowa State University; HUNT, MATTHEW - Iowa State University; SURANA, PRIYANKA - Iowa State University; VELASQUEZ-ZAPATA, VALERIA - Iowa State University; XU, WEIHUI - Iowa State University; Fuerst, Gregory; Wise, Roger

Submitted to: Genetics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/4/2020
Publication Date: 12/22/2020
Citation: Chapman, A.V., Hunt, M., Surana, P., Velasquez-Zapata, V., Xu, W., Fuerst, G.S., Wise, R.P. 2020. Disruption of barley immunity to powdery mildew by an in-frame Lys-Leu deletion in the essential protein SGT1. Genetics. 217(2). https://doi.org/10.1093/genetics/iyaa026.
DOI: https://doi.org/10.1093/genetics/iyaa026

Interpretive Summary: Barley is an economically and culturally important crop that is vital to many industries. Like all crops, barley is under constant threat by pathogens. Barley naturally varies in its susceptibility to these pathogens, ranging from complete susceptibility to complete resistance, and this can be governed by a single gene. However, pathogens evolve rapidly and within a single growing season they can develop ways to overcome resistance with new, modified, or lost genes. Understanding how plants and pathogens interact at a genetic level gives us the possibility to pre-emptively arm against future evolutions of pathogens. The pathogenic fungus, Blumeria graminis f.sp. hordei (Bgh), causes powdery mildew disease, resulting in death of leaf tissue and a subsequent loss of photosynthetic capability. In this study, we treated a barley line resistant to powdery mildew with fast neutron mutagenesis, and screened for susceptible variants. We mapped the causal mutation in one such variant to uncover new mechanisms involved in disease resistance. We found that the mutation maps to a 6-nucleotide, in-frame deletion of Sgt1 (Suppressor of G-two allele of Skp1), a gene vital for all life. This is unexpected because mutations of Sgt1 are usually lethal, and thus, research of this genes involvement in disease resistance has been stifled. Therefore, discovery of a Sgt1 mutation that causes the host to be morphologically unaffected, while disrupting a resistance pathway, is not only a rare find, but may be invaluable in our efforts to understand this key factor in resistance signaling. Impact: The discovery of a Sgt1 mutation that only causes destruction of disease resistance, and not other basic cell processes, will allow us to determine the molecular mechanisms of how Sgt1 interacts with disease resistance proteins in barley and other cereal crops. This is critical to breeders and growers that use disease resistance to protect their crops.

Technical Abstract: 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 response to the powdery mildew fungus, Blumeria graminis f. sp. hordei. One 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. This is analogous to, but unique from, the differential requirement of Mla alleles for the co-chaperone Rar1 (required for Mla12 resistance1). 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, Sgt1'KL308–309), the structural region that interacts with MLA proteins. In nature, mutations to Sgt1 usually cause lethal phenotypes, but here we pinpoint a unique modification that delineates its requirement for some disease resistances, while unaffecting others as well as normal cell processes. Moreover, the data indicate that the requirement of SGT1 for resistance signaling by NLRs can be delimited to single sites on the protein. Further study could distinguish the regions by which pathogen effectors and host proteins interact with SGT1, facilitating precise editing of effector incompatible variants.