Dissecting Mla3–AVR-Rmo1 recognition and specificity

Gomez De La Cruz, Diana (2023) Dissecting Mla3–AVR-Rmo1 recognition and specificity. Doctoral thesis, University of East Anglia.

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Abstract

The plant immune system heavily relies on immune receptors known as nucleotide binding leucine-rich repeat (NLR) proteins, which recognise pathogen-secreted effectors to trigger a robust immune response. In barley, resistance to powdery mildew caused by Blumeria graminis f. sp. hordei (Bgh) is conferred by the Mildew locus a (Mla), an NLR that exists as a highly expanded allelic series. Each Mla allele governs Bgh isolate-specific resistance by recognising a corresponding AVRa effector. In addition, different alleles can confer resistance against divergent fungal pathogens. This is the case of the Mla3 allele, which not only recognises AVRa3 from Bgh, but also confers resistance to the blast fungus Magnaporthe oryzae. In this thesis, I aimed to molecularly characterise M. oryzae recognition by Mla3 and elucidate the principles governing multiple pathogen recognition by this NLR. I found that PWL2, an effector known to condition pathogenicity of M. oryzae towards weeping lovegrass, is the gene underlying AVR-Rmo1, the blast effector recognised by Mla3. Evidence indicates that barley and weeping lovegrass convergently evolved to recognise PWL2 with conserved specificity. I established that the C-terminus of Mla3 defines specificity of Pwl2 recognition and protein structure predictions suggest that this region binds to Pwl2 by mimicking the binding interface of a Pwl2 host target. By assessing copy number variation and allelic diversity, I defined that Mla3 functions in a dosage-dependent manner and postulate that polymorphisms reduce the sensitivity threshold to trigger an immune response upon effector recognition, abolishing the high dosage requirement for functional resistance. The identity of AVRa3, the Bgh effector recognised by Mla3, remains unknown. However, Pwl2 belongs to the family of MAX effectors, which is absent in Bgh, hence suggesting that Mla3 recognises structurally unrelated effectors. Altogether, these findings lay the foundation for understanding the mechanisms that shaped multiple pathogen recognition by Mla3.

Item Type:	Thesis (Doctoral)
Faculty \ School:	Faculty of Science > School of Biological Sciences
Depositing User:	Chris White
Date Deposited:	04 Jul 2023 09:19
Last Modified:	04 Jul 2023 09:19
URI:	https://ueaeprints.uea.ac.uk/id/eprint/92562
DOI:

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