Host resistance

The identification of R-genes using traditional map-based approaches is a long, laborious process, not to mention the time required for subsequent development of cultivars incorporating the new resistances. Breeders seek to reduce the length of breeding cycles, and researchers require new tools to accelerate discovery and understanding of mechanisms associated with durable resistance, especially adult plant resistance (APR). A new method for rapid generation advancement, known as ‘speed breeding’, significantly reduces the length of breeding cycles, provide increased recombination during line development and enable selection in early generations. The speed breeding protocol uses controlled temperature regimes and 24h light to accelerate plant growth and development. Phenotyping methods adapted for use in the speed breeding system permit year-round evaluation of APR to rust pathogens within 5 weeks from time of sowing. RNA sequencing (RNA-Seq) technology has revolutionized gene expression profiling in plants. We previously used RNAseq to identify novel transcripts and miRNAs associated with seedling resistance (Lr28) leading to identification of transcription factors and miRNA families (e.g. miR36, miR37 and miR39) involved in signalling and defense response (Kumar et al. J. Nuc. Acids 2014:570176). In this study we report the application of speed breeding and RNAseq technologies for the purpose of rapidly identifying transcripts and miRNA associated with APR. Wheat landraces harbouring novel sources of resistance were grown under speed breeding conditions and sampled for RNA at key growth stages, before and after inoculation, which enabled discovery of differentially expressed miRNAs. Our next steps are aimed at validating these genetic factors associated with APR in order to better understand the signalling pathways and deliver tools to assist the assembly of robust wheat cultivars for the future.

The shortage of stem rust resistance genes effective against the Ug99 group prompted recent efforts to increase the number of resistance genes available to breeders. We are fortunate that many new and/or cytogenetically improved rust resistance genes are now being shared with the global wheat breeding community by their developers. If we are poor stewards of these resources, the new resistance genes will eventually be defeated, and we will waste the efforts and investments that have been made. However, if we are good stewards, we should have enough resistance to achieve sustainable, durable resistance. Stewardship can be defined as the careful and responsible management of something entrusted to one’s care. What should we do to safeguard the new resistance genes? Diversification of resistance is often suggested as a way to reduce the risk of large scale epidemics. Although diversification is generally a good idea, it cannot be at the expense of leaving new genes exposed and vulnerable. A durable combination (pyramid) must be designed so that the component genes protect each other. They should reduce the probability of simultaneous pathogen mutations to virulence and they should avoid stepwise erosion of the pyramid by preventing significant reproduction of any new race that is virulent on component genes. We need pyramids to be immune or nearly immune not only to current races, but to anticipated mutants. This objective should be achievable with three or more major genes or a combination of major and minor genes. Successful gene stewardship will depend on several things. On the technical side, we will need very good markers for each gene. Each breeding program will require strong genotyping support to assemble and then validate pyramids. Most importantly, successful stewardship will require that we organize our user community to cooperate more closely. We will need to decide which genes require special stewardship and which do not. Every user of the stewardship pool resource will need to participate in earnest. It only takes one cultivar with an unprotected gene to give the pathogen a stepping stone to greater virulence. As they say, a chain is only as strong as the weakest link

The number of designated stem rust resistance genes has increased by ~10 over the past four years. Translocations involving several broadly-effective alien resistance genes with limited or no previous agricultural deployment were enginneered to reduce the likelihood of linkage drag, and the foundations of adult plant resistance were established. This progress resulted from international collaboration, increased global coordination, and critical financial support. By buidling on these initial accomplishments and improving genetic and genomic resources over the next four years we expect to achieve: 1. more than 10 additional formally designated stem rust resistance genes conferring resistance to Ug99-complex races, 2. robust/diagnostic DNA marker haplotypes identified for most sources of resistance, 3. multiple linkage blocks of two or more resistance genes to enhance gene pyramiding efforts, and 4. knowledge of numerous additional sources of resistance complelely or partially identified. Never before have so many resources and supporting tools been available to combat the wheat rusts. It is an opportune time for the international community to strategically deploy and responsibly steward our genetic resources for durable control of wheat stem rust.