Stem rust of wheat, caused by the fungus Puccinia graminis f. sp. tritici, is a major threat to global wheat production. The highly virulent Ug99 race group and its derivatives have spread to multiple countries. In 2024, the Ug99 variant TTKTT was detected in Nepal, endangering wheat production across South and East Asia. Cloning effective resistance genes and breeding resistant cultivars remain the most economical and sustainable strategy for controlling this disease.

 

Recently, Dr. Shisheng Chen at Peking University Institute of Advanced Agricultural Sciences, together with his collaborators, published the study “The wheat Sr8155B1 gene encodes a typical NLR protein that confers resistance to the Ug99 stem rust race group” in Plant Communications. The team successfully cloned Sr8155B1 from durum wheat (Triticum turgidum subsp. durum), a gene that confers near-immunity to several Ug99 variants.

 

Sr8155B1 provides immune or near-immune resistance to 12 of 14 tested stem rust pathotypes, including Ug99 derivatives TTKTT, TTTSK, TTKSF+, and TTKST. To isolate the gene, 4,600 F₂ individuals derived from the 8155-B1 x Rusty cross were screened for recombinants, enabling fine mapping of Sr8155B1 to a 0.03-cM interval on the short arm of chromosome 6A — corresponding to a 295-kb physical region in the Svevo reference genome containing six candidates.

 

To obtain the physical map, the resistant parent 8155-B1 was sequenced with PacBio Revio HiFi technology and subjected to genome assembly, which resulted in a contig N50 of 6.51 Mb. A single 7.69-Mb contig spanned the target interval, still containing six candidate genes. Only one of them, designated CNL1, encodes a typical CC-NBS-LRR (NLR) protein (Fig. 1B).

 

Functional validation was carried out in three complementary ways:

 

EMS mutagenesis of the resistant parent yielded a susceptible line (m27) carrying a C-to-T mutation in CNL1 that introduces a premature stop codon (Q519*) (Fig. 1C). No mutations were detected in the other five candidates.

 

CRISPR/Cas9-mediated knockout of CNL1 in the tetraploid parent 8155-B1 produced 143 independent edited events; three T₀ plants with complete knockouts and their T₁ progeny were fully susceptible (Fig. 1D).

 

A 9,968-bp genomic fragment containing CNL1 was transformed into the susceptible cultivar Fielder, converting it to resistant (Fig. 1E).

 

Collectively, these results prove that CNL1 is Sr8155B1. Expression analysis revealed that Sr8155B1 is up-regulated upon stem-rust infection (Fig. 1F). Sequence comparisons showed that Sr8155B1 shares <93 % identity with most wheat homologs. A diagnostic SNP (2096 A, encoding E699) unique to Sr8155B1 was identified; a PCR marker based on this SNP amplified only in 8155-B1 among 316 diploid, tetraploid and hexaploid wheat accessions. Wheat lines previously postulated to carry Sr8155B1 (PI 681713, Munich, Renville, Lloyd, Rugby, Belzer, Mountrail and Dilse) were confirmed positive with this marker and full-length sequencing.

 

Although Sr8155B1 maps near the known — but uncloned — genes Sr8a and Sr8b, distinct race specificities, infection types, and protein polymorphisms demonstrate that Sr8155B1 is a different locus. To facilitate deployment in bread wheat, Sr8155B1 has been successfully introgressed into hexaploid backgrounds through crossing and multi-generation backcrossing (Fig. 1G). The CC domain of Sr8155B1 triggers strong necrosis when expressed in Nicotiana benthamiana, consistent with its role as an immune receptor.

 

This work accelerates the utilization of Sr8155B1 in breeding, expanding the arsenal of genes available for durable resistance to Ug99 and related stem rust races.

Co-first authors are Dr. Tao Shen (joint PhD student, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences (CAS) & Peking University Institute of Advanced Agricultural Sciences), Dr. Guiping Wang (assistant research fellow) and Ms. Xiaohua Hao (technician). Co-corresponding authors are Dr. Shisheng Chen (Peking University Institute of Advanced Agricultural Sciences) and Dr. Zhiyong Liu (Institute of Genetics and Developmental Biology, CAS). Dr. Wenjun Zhang (University of California, Davis) participated in the project. Funding was provided by Shandong Province Key R&D Program (2024LZGC034 and 2023LZGC022) and Shandong Provincial Natural Science Foundation (SYS202206).

 

Original article: https://doi.org/10.1016/j.xplc.2025.101296