Molecular Studies on the Durable Wheat Resistance Gene
Lr34/Yr18/Pm38/Sr57 (subsequently referred to as Lr34) is a single wheat gene that confers durable and partial adult plant resistance against the four biotrophic diseases leaf rust, stripe rust, powdery mildew and stem rust. This phenotype is also referred to as slow-rusting or slow-mildewing and is linked to leaf tip necrosis, a morphological marker associated with senescence-like processes. Lr34 has been extensively used in wheat breeding for more than a century and no pathogen adaptation has been observed so far. Only a few genes with a similar phenotype have been identified in wheat, namely Lr46/Yr29/Pm39 and Lr67/Yr49/Pm46/Sr55. Lr34 encodes for an ATP-binding cassette (ABC) transporter protein (Krattinger et al. 2009). Members of this conserved protein family transport various substrates across biological membranes. The resistant Lr34 allele, which differs by only two amino acid polymorphisms from the susceptible Lr34 version, evolved after the domestication of hexaploid bread wheat 8,000 years ago through the acquisition of two gain-of-function mutations (Krattinger et al. 2013). An Lr34 ortholog is absent in the closely related cereal barley. We therefore stably transformed the Lr34 gene under its native promoter into barley cultivar Golden Promise (Risk et al. 2013). Interestingly, the gene conferred resistance against barley leaf rust (Puccinia hordei) and barley powdery mildew (Blumeria graminis f.sp. hordei), pathogens that are specific to barley and that do not infect wheat. In contrast to wheat, where Lr34 confers resistance only in adult plants, resistance in barley was already observed at the seedling stage. We also observed a strong leaf tip necrosis phenotype that was already visible in seedlings and that had a negative impact on plant vigor and seed setting. These results demonstrate that Lr34-resistance is transferrable to other cereals. However, tight control of Lr34 expression is necessary to avoid negative impact on yield. The use of different, tissueor age-specific promoters might allow generating barley plants with adequate levels of resistance and no impact on yield.
New Tools for Resistance Breeding Based on Pathogen Genomics
It is known that resistance which is based on R genes or quantitatively acting genes can result in a yield penalty because of the associated costs of resistance. Therefore, new approaches based on completely different resistance mechanisms should also be considered and explored actively. One new strategy might be host-induced gene silencing (HIGS) which is based on RNAi and relies on a process where the presence of an RNAi construct in wheat would result in specific gene silencing in the pathogen. The success of this strategy has already been shown in transient assays (Nowara et al. 2010; Pliego et al. 2013). This approach is not based on endogenous resistance genes in the wheat host, but on sequence information from potentially relevant genes of the pathogen. Essential genes in the pathogen might be targeted by RNAi constructs expressed in wheat, resulting in down-regulation and ideally quantitative resistance against the specific pathogen species. In our group, we have recently developed the necessary genomic tools for such approaches in the wheat powdery mildew pathogen. The complete genomic sequence of the wheat powdery mildew genome is now available and can be used for such pathogen-based resistance strategies (Wicker et al. 2013).
The approaches described in this contribution are both based on classical as well as transgenic tools to improve disease resistance breeding in wheat. The transgenic approaches will critically depend on public acceptance and a predictable, efficient regulatory framework. Given the enormous challenges for wheat production in the next decades, it is essential that the wheat scientists globally promote and explain to the public the use of classical and novel tools in breeding.
Acknowledgments This work was supported by grants from the Swiss National Science Foundation (310030B_144081/1), an Advanced Grant of the European Research Council (ERC2009-AdG 249996, Durable resistance) and a grant from the Grains Research and Development Corporation #CSP000063.