Email: Chunhong.chen@csiro.au

Rust disease pose a significant threat to wheat, barley, and various crops worldwide, jeopardising food security. Plant breeders employ resistance genes to prevent or reduce the damage caused by rust disease. The typical resistance genes often include receptor kinases, which recognise the ligands outside the cell and trigger defence responses, as well as nucleotide-binding site and leucine-rich repeat (NLR) proteins and tandem kinase proteins (TKP), which detect pathogen effectors within cells and initiate defence mechanism.

In this study, we report the successful cloning of an atypical leaf rust resistance gene from barley. The barley leaf rust resistance locus Rph7 has been shown to have unusually high sequence and haplotype divergence. Our investigation involved isolating the Rph7 gene using a fine mapping and RNA_seq approach, subsequently confirmed by mutational analysis and transgenic complementation. Through transient expression experiments, we observed that Rph7 induces partial cell death in Nicotiana benthamiana leaves or Golden Promise barley protoplasts.

Rph7 is a pathogen-induced, non-canonical resistance gene encoding a protein that is distinct from other known plant disease resistance proteins in the Triticeae. Structural analysis using an AlphaFold2 protein model suggests that Rph7 encodes a putative NAC transcription factor with a zinc-finger BED domain with structural similarity to the N-terminal DNA binding domain of the NAC transcription factor (ANAC019) from Arabidopsis.

Comparative analysis reveals intriguing parallels between Rph7 and the recently cloned wheat stripe rust resistance gene YrNAM aka Yr10. Despite the absence of amino acid sequence similarity at the N-terminal domains between Rph7 and Yr10, structural alignment indicates shared structural features at both the N-terminal and C-terminal domains. Consequently, Rph7 and Yr10 are classified as a novel category of rust resistance genes with a Triticeae NAC- BED (TNB) domain architecture.