The wheat Lr34 gene provides resistance against multiple fungal pathogens in barley.

The Lr34 gene encodes an ABC transporter and has provided wheat with durable, broad-spectrum resistance against multiple fungal pathogens for over 100 years. Because barley does not have an Lr34 ortholog, we expressed Lr34 in barley to investigate its potential as a broad-spectrum resistance resource in another grass species. We found that introduction of the genomic Lr34 sequence confers resistance against barley leaf rust and barley powdery mildew, two pathogens specific for barley but not virulent on wheat. In addition, the barley lines showed enhanced resistance against wheat stem rust. Transformation with the Lr34 cDNA or the genomic susceptible Lr34 allele did not result in increased resistance. Unlike wheat, where Lr34-conferred resistance is associated with adult plants, the genomic Lr34 transgenic barley lines exhibited multipathogen resistance in seedlings. These transgenic barley lines also developed leaf tip necrosis (LTN) in young seedlings, which correlated with an up-regulation of senescence marker genes and several pathogenesis-related (PR) genes. In wheat, transcriptional expression of Lr34 is highest in adult plants and correlates with increased resistance and LTN affecting the last emerging leaf. The severe phenotype of transgenic Lr34 barley resulted in reduced plant growth and total grain weight. These results demonstrate that Lr34 provides enhanced multipathogen resistance early in barley plant development and implies the conservation of the substrate and mechanism of the LR34 transporter and its molecular action between wheat and barley. With controlled gene expression, the use of Lr34 may be valuable for many cereal breeding programmes, particularly given its proven durability.

Functional variability of the Lr34 durable resistance gene in transgenic wheat.

Breeding for durable disease resistance is challenging, yet essential to improve crops for sustainable agriculture. The wheat Lr34 gene is one of the few cloned, durable resistance genes in plants. It encodes an ATP binding cassette transporter and has been a source of resistance against biotrophic pathogens, such as leaf rust (Puccinina triticina), for over 100 years. As endogenous Lr34 confers quantitative resistance, we wanted to determine the effects of transgenic Lr34 with specific reference to how expression levels affect resistance. Transgenic Lr34 wheat lines were made in two different, susceptible genetic backgrounds. We found that the introduction of the Lr34 resistance allele was sufficient to provide comparable levels of leaf rust resistance as the endogenous Lr34 gene. As with the endogenous gene, we observed resistance in seedlings after cold treatment and in flag leaves of adult plants, as well as Lr34-associated leaf tip necrosis. The transgene-based Lr34 resistance did not involve a hypersensitive response, altered callose deposition or up-regulation of PR genes. Higher expression levels compared to endogenous Lr34 were observed in the transgenic lines both at seedling as well as adult stage and some improvement of resistance was seen in the flag leaf. Interestingly, in one genetic background the transgenic Lr34-based resistance resulted in improved seedling resistance without cold treatment. These data indicate that functional variability in Lr34-based resistance can be created using a transgenic approach.