A five-transgene cassette confers broad-spectrum resistance to a fungal rust pathogen in wheat

Abstract

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.

Creation and judicious application of a wheat resistance gene atlas

“…This is difficult to determine through pathogen assays due to epistasis from the recognition of multiple effectors. However, with cloned effectors, it is possible to probe the function of individual stack components using heterologous delivery systems (Liu et al, 2012) and protoplast assays (Luo et al, 2021). The improvement of transient expression systems in wheat, such as in protoplasts (Saur et al, 2019) and through virus-mediated overexpression (Bouton et al, 2018), has facilitated the rapid functional analysis of intracellular effectors.…”

A recombined Sr26 and Sr61 disease resistance gene stack in wheat encodes unrelated NLR genes

“…Due to only short 5′ and 3′ noncoding sequences being available from both Sr26 and particularly Sr61, we used heterologous regulatory sequences from Sr22 and Sr33 for transgene expression. Luo et al 30 recently showed substantial variation existed between the expression levels of a number of different NLR encoding Sr genes, with the Sr50 gene expressed at an approximately 10-fold higher level when compared with Sr22. Using Sr26 regulatory elements we saw an approximately 40% reduction in Sr61 transgene expression in resistant transgenics compared with the endogenous gene present in W3757; however, the resistance phenotype appeared unchanged.…”

“…Combining multiple R genes is a widely accepted gene stewardship strategy to enhance the durability of resistance on the basis that the simultaneous defeat of two or more effective R genes is less likely than defeat of either gene alone. One compelling reason for cloning R genes is the opportunity to combine them into transgenic cassettes to allow multiple R genes to be selected as a single "trait" in breeding 30 . The most desirable Sr genes to be combined into a transgene cassette are those with broad effectiveness against diverse Pgt races.…”

Genomic Approaches to Identify Molecular Bases of Crop Resistance to Diseases and to Develop Future Breeding Strategies

“…Good opportunities in improving pathogen resistance are offered by genetic engineering. Initially, most attempts had involved a transgenic approach in many crops [ 103 , 160 , 161 , 162 , 163 , 164 , 165 , 166 , 167 ] and some genetically engineered food crops have been approved for commercial production. This strategy followed by cross and MAS has also been applied to accumulate more genes/alleles in transgenic plants for reaching a durable resistance in crops, but it has many legislative and sustainability problems.…”