The Arabidopsis disease resistance gene RPSP is involved in recognition of bacterial pathogens carrying the avirulence gene avrRpt2, and the RPMl resistance gene is involved in recognition of pathogens carrying avrRpml or avrB. We identified and cloned two Arabidopsis genes, AIGl and AIG2 (for a_vrRpt2-~nduced gene), that exhibit RPSP-and avrFfptbdependent induction early after infection with Pseudomonas syringae pv maculicola strain ES4326 carrying avrRpt2. However, ES4326 carrying avrRpml or avrB did not induce early expression of AIGl and AIG2. Conversely, ES4326 carrying avrRpml or avrB induced early expression of the previously isolated defense-related gene ELI3, whereas ES4326 carrying avrRpt2 did not. The induction patterns of the AIG genes and EL13 demonstrate that different resistance gene-avr gene combinations can elicit distinct defense responses. Furthermore, by examining the expression of AlGí and €LI3 in plants infiltrated with a mixed inoculum of ES4326 carrying avrRpt2 and ES4326 carrying avrRpmí, we found that there is interference between the RPSP-and RPM1-mediated resistance responses.
Breeding wheat with durable resistance to the fungal pathogen Puccinia graminis f. sp. tritici (Pgt), a major threat to cereal production, is challenging due to the rapid evolution of pathogen virulence. Increased durability and broad-spectrum resistance can be achieved by introducing more than one resistance gene, but combining numerous unlinked genes by breeding is laborious. Here we generate polygenic Pgt resistance by introducing a transgene cassette of five resistance genes into bread wheat as a single locus and show that at least four of the five genes are functional. These wheat lines are resistant to aggressive and highly virulent Pgt isolates from around the world and show very high levels of resistance in the field. The simple monogenic inheritance of this multigene locus greatly simplifies its use in breeding. However, a new Pgt isolate with virulence to several genes at this locus suggests gene stacks will need strategic deployment to maintain their effectiveness.Pgt continues to overcome resistant wheat cultivars, with three new, highly virulent isolates emerging in the last 20 years, and the disease reappearing in Europe and the UK 1 . Two classes of Pgt resistance genes have been cloned from wheat: all-stage resistance (ASR) genes and adult plant resistance (APR) genes 2 . ASR genes (for example, Sr22 (ref. 3 ), Sr35 (ref. 4 ), Sr45 (ref. 3 ) and Sr50 (ref. 5 )) generally encode nucleotide-binding, leucine-rich repeat (NLR) proteins that recognize a specific Pgt molecule (an effector) introduced into host plant cells by the fungus to promote parasitism, whereupon a plant defense response is activated 2 . The presence, absence or allelic variation of the fungal effector determines which Pgt isolates an ASR gene is effective against. ASR genes are extremely valuable for crop protection but, when deployed singly, often show transient resistance, as pathogen effectors rapidly evolve to avoid recognition. Combining ASR genes increases their durability, and theoretical estimates suggest that the chance of a single Pgt isolate gaining virulence for five or more ASR genes in wheat is infinitesimally small 6 .The second gene class, APR genes, can be remarkably durable and, in some cases, effective against multiple pathogen species. However, these genes generally provide partial resistance that is often insufficient for crop protection during severe pathogen epidemics.
The wild relatives and progenitors of wheat have been widely used as sources of disease resistance (R) genes. Molecular identification and characterization of these R genes facilitates their manipulation and tracking in breeding programmes. Here, we develop a reference-quality genome assembly of the wild diploid wheat relative Aegilops sharonensis and use positional mapping, mutagenesis, RNA-Seq and transgenesis to identify the stem rust resistance gene Sr62, which has also been transferred to common wheat. This gene encodes a tandem kinase, homologues of which exist across multiple taxa in the plant kingdom. Stable Sr62 transgenic wheat lines show high levels of resistance against diverse isolates of the stem rust pathogen, highlighting the utility of Sr62 for deployment as part of a polygenic stack to maximize the durability of stem rust resistance.
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