A review of the preprint “The wheat powdery mildew resistance gene Pm4 also confers resistance to wheat blast” by O’Hara et al. 2023
Wheat blast, caused by the hemibiotrophic fungus Magnaporthe oryzae pathotype triticum (MoT), is a devastating and emerging disease that is spreading globally and threatening major wheat-producing regions. Only a handful of genes that provide resistance against MoT have been identified, which prompted O’Hara and colleagues to search for new sources of resistance in wheat germplasm. The authors conducted pathogen infection assays using a MoT isolate encoding the broadly recognised AVR-Rmg8 effector to screen for resistance specificities in a large panel of wheat accessions. Unexpectedly, genomic analysis narrowed down the source of resistance to a gene called Pm4, previously shown to confer isolate-specific disease resistance to wheat powdery mildew. The authors find that although allelic variation in AVR-Rmg8 results in specific recognition by wheat Rmg7 or Rmg8, both of the AVR-Rmg8 effector alleles are detected by the majority of the Pm4 alleles tested.
The identification of single NLRs capable of recognising multiple pathogens has prompted interest in how convergent evolution has shaped resistance specificities in crop species (Lozano-Torres et al. 2012; Vos et al. 1998). For example, the barley CNL MLA3 is capable of recognising the Magnaporthe oryzae effector PWL2 and confers resistance to rice blast and strains of barley powdery mildew containing the AVRA3 effector. Not only does the current study present a similar example of host specificity barrier conferred by a disease resistance gene to the blast fungus, it is also the second example of resistance to blast and mildew conferred by the same gene (Brabham et al. 2023). It also emphasizes the importance of searching for resistance specificity in regions where the target pathogen was not expected to have evolved. This is a well-written study that contributes to a growing recognition that crop breeding efforts need to focus on multiple pathogens in order to prevent selection pressure for resistance breakdown to other diseases.
We suggest that considering the following aspects could improve the manuscript:
1. Line 226: Differences in genetic background between the wheat accessions may affect MoT infection phenotypes, especially under differing temperatures. It is advisable to generate transgenic lines expressing Pm4 alleles in an isogenic susceptible background to validate the results.
2. Line 249: Similar to point #1, genome-wide genetic differences between the tested wheat accessions might confound the interpretation of the observed pathotypes. Considering that both V1 and V2 alleles of Pm4 are necessary for resistance, the reported alternate splicing of transcripts may vary among these wheat accessions. Have the transcript levels for both transcripts been compared between all accessions carrying Pm4 resistance?
3. Please consider using a heterologous plant system for validating the resistance mediated by Pm4 alleles to test whether the recognition mechanism is direct or indirect.
4. Simplifying gene nomenclature will make it much easier for the reader to further investigate these sequences. In addition, access to the corresponding nucleotide sequences could be made easier.
The Plant Microbe Interactions Journal Club at MPIPZ
The author declares that they have no competing interests.