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Full nonhost resistance can be defined as immunity, displayed by an entire plant species against all genotypes of a plant pathogen. The genetic basis of (non)host-status of plants is hard to study, since identification of the responsible genes would require interspecific crosses that suffer from sterility and abnormal segregation. There are some plant/potential pathogen combinations where only 10% or less of the accessions are at most moderately susceptible. These may be regarded as marginal host or near-nonhost, and can provide insights into the genes that determine whether a plant species is a host or a nonhost to a would-be pathogen. Barley (Hordeum vulgare L.) is a near-nonhost to several rust pathogens (Puccinia) of cereals and grasses. By crossing and selection we developed an experimental line, SusPtrit, with high susceptibility to at least nine different heterologous rust taxa such as the wheat and Agropyron leaf rusts (caused by P. triticina and P. persistens, respectively). On the basis of SusPtrit and several regular, fully resistant barley accessions, we developed mapping populations. We established that the near-nonhost resistance to heterologous rusts inherits polygenically (QTLs). The QTLs have different and overlapping specificities. In addition, an occasional R-gene is involved. In each population, different sets of loci were implicated in resistance. Very few resistance genes were common between the populations, suggesting a high redundancy in barley for resistance factors. Selected QTLs have been introduced into near-isogenic lines to be fine-mapped. Our results show that the barley- Puccinia system is ideal to investigate the genetics of host-status to specialized plant pathogens.
With the TTKS family of races virulent on most genes currently providing protection against stem rust worldwide, identifying, mapping, and deploying resistance genes effective against these races has become critical. We present here a genetic map of Sr35. Both parents of our diploid mapping population (DV92/G3116, 142 SSD lines) are resistant to TTKSK, but the population segregates for resistance to TRTTF (Yemen) and RKQQC (US). Race analysis suggests that G3116 carries Sr21 and DV92 both Sr21 and Sr35. Resistance to TRTTF and RKQQC was mapped to a 6 cM interval on chromosome 3AmL between markers BF483299 and CJ656351. This interval corresponds to a 178-kb region in Brachypodium which contains only 16 annotated genes and exhibits a small inversion (including 2 genes) and a putative insertion (2 genes) relative to rice and sorghum. This map contains closely-linked markers to Sr35 and provides the initial step for this gene's positional cloning.
Association mapping detects correlations between genotypes and phenotypes in a sample of individuals based on the linkage disequilibrium and can be used to uncover new genetic variation among germplasm collections. Two hundred and five landraces collected by the English botanist A. Watkins in the 1920s were screened for rust response variation under field conditions during three crop seasons. An integrated map of 350 polymorphic DArT markers was developed. Association mapping identified the involvement of several genomic regions in controlling resistance to three rust diseases. Seven, eight and nine genomic regions, respectively, appeared to carry yet uncharacterized leaf rust, stripe rust and stem rust resistance. Three dimensional analyses indicated genetic association of leaf rust and stripe rust resistance in some accessions, whereas no such association was observed between stem rust resistance and resistance to either of the other two rust diseases. A new stripe rust resistance locus, Yr47, has been named.
Rust diseases remain a significant threat to the production of most cereals including wheat. New sources of resistance are continually sought by breeders to combat the emergence of new pathogen races. Rice is atypical in that it is an intensively grown cereal with no known rust pathogen. The resistance of rice to cereal rust diseases is referred to as nonhost resistance (NHR), a resistance mechanism that has only recently become genetically tractable. In this report, the mechanisms of rice NHR to wheat stem rust and other cereal rust diseases are explored and the potential for transferring this durable disease resistance to wheat is considered. Approaches being undertaken for the molecular-genetic dissection of rice NHR to rust are described.
Two broad categories of resistance genes in wheat have been described. One group represents the so called seedling resistance or the ‘gene for gene’ class that often provides strong resistance to some but not all strains of a rust species. The other category referred to as adult plant resistance provide partial resistance that is expressed in adult plants during the critical grain filling stage of wheat development. A few seedling rust resistance genes have been cloned in wheat and other cereals and are predominantly from the nucleotide binding site/leucine rich repeat class which is associated with localized cell death at the pathogen entry site. Until recently, the molecular basis of race non-specific, partial and slow rusting adult plant resistance genes were unknown. Gene products that differ from known plant resistance genes were revealed from the recent cloning of the Yr18, Yr36 and Lr34 adult plant genes in wheat. The available range of diverse resistance gene sequences provide entry points for developing genebased markers and will facilitate selection of germplasm containing unique resistance gene combinations.
A number of stem rust resistance genes derived from wild relatives of wheat appeared to be more effective against race TTKSK (Ug99) of Puccinia graminis f. sp. tritici than Sr genes of wheat origin. In an attempt to identify sources of stem rust resistance genes effective against TTKSK, we evaluated several cultivated and wild relatives of wheat for resistance to TTKSK and other stem rust races with broad virulence in seedling tests. Preliminary results indicated that TTKSK resistance could readily be found, but frequencies of resistance varied among the species. Aegilops speltoides had the highest frequency of resistance (nearly 100%). Other species having high frequencies of TTKSK resistance included triticale (77.7% of 567 accessions), Triticum urartu (96.8% of 205 accessions), and T. monococcum (61% of 1020 accessions). Frequencies of TTKSK resistance in other species were: 14.7% in Ae. tauschii (456 accessions), 15% in T. timopheevii (298 accessions), and 17% in T. turgidum ssp. dicoccoides (157 accessions). Based on specific infection types to several races, we postulated that novel genes for resistance to TTKSK are present in some of these species. Accessions with putatively new resistance genes were selected to develop crosses for introgressing resistance into wheat and for developing mapping populations.
Rust fungi can cause devastating diseases in agriculture and are particularly important pathogens of wheat. We have been using the flax (Linum usitatissimum) and flax rust (Melampsora lini) model system to study disease resistance mechanisms to this important class of pathogens. Rust resistance in flax and other plants is mediated by the plant innate immunity system in which highly polymorphic resistance (R) proteins act as receptors that recognize specific avirulence (Avr) proteins produced by the pathogen. This race-specific resistance is characterised by Flor’s “gene-for-gene” model, first proposed based on the flax rust system. In gene-for-gene resistance, recognition between the R and Avr proteins initiates defense responses leading to host resistance to infection, including a localised necrosis or hypersensitive response. Nineteen different rust resistance genes have been cloned from flax, including 11 allelic variants of the L locus, which all encode cytosolic proteins with conserved nucleotide-binding (NB) and Leucine-rich repeat (LRR) domains. Four families of Avr genes, AvrL567, AvrM, AvrP123 and AvrP4, have been identified in the flax rust pathogen and all encode small secreted proteins. Rust Avr proteins are secreted from haustoria, specialized infection structures that penetrate the host cell wall, and are translocated across the host plasma membrane and into the host cytoplasm. These proteins are probably members of a suite of disease ‘effectors’ involved in manipulating host cell biology to facilitate infection, but have become targeted for recognition by the host immune system. As yet the mechanism of Avr protein transport is unknown, but could prove to be a useful target for novel disease control strategies. Recognition of at least two of these Avr proteins is based on direct interaction with the cytoplasmic NB-LRR R proteins. One interesting observation from the flax rust system is that all of the virulent rust strains retain intact copies of the Avr genes, but have altered their sequences sufficiently to escape recognition. Thus it may be possible to re-engineer R genes to recognise new Avr gene variants. We are currently identifying haustorially expressed secreted proteins from wheat stem rust as candidate Avr/effector proteins.
Variants of Puccinia triticina race BBG/BN, separately overcoming three resistance genes, were identified from durum wheat (Triticum turgidum ssp. durum) fields in northwestern Mexico since its introduction in 2001. Major genes available for use in breeding programs are limited and an alternative strategy is required. Previous studies indicated that slow rusting resistance in eight CIMMYT durums was determined by 2 to 3 minor genes with additive effects. Twenty-eight 4-way crosses were made between these lines with the aim of developing new germplasm with enhanced levels of resistance through pyramiding diverse minor genes. Plants in F1 (4-way) through F3 generations were selected for slow rusting under high leaf rust pressure at the Cd. Obregon and El Batan field sites in Mexico and spikes from selected plants were harvested as bulks. Plants in the F4 generation were individually harvested and1,843 advanced lines obtained, among which 106 lines with enhanced resistance, and desirable agronomic and grain characteristics were selected for non-replicated yield and leaf rust evaluation trials at Obregon during the 2007-2008 season. The best 19 lines, exhibiting near-immunity but with the presence of a few susceptible type pustules, parents and susceptible checks were evaluated for leaf rust resistance under very high disease pressure in replicated trials sown on two dates (16 May and 6 June) at El Batan during 2008. Spreader rows of susceptible cultivar ‘Banamichi C2004’, sown as border and as hills on one side of each plot, were inoculated with P. triticina race BBG/BP. Leaf rust severities, and host responses to infection were determined from weekly readings, and area under the disease progress curves (AUDPC) were calculated. Several lines were identified with significantly lower final leaf rust severity responses and AUDPC values than the most resistant parent in each cross. Our results show that enhanced levels of slow rusting can be generated by pyramiding diverse genes present in different parents. The trial is being repeated during the 2008-2009 season at Obregon to validate the results. In addition these lines are being used for transferring slow rusting resistance into high yielding, superior quality adapted backgrounds using the single-backcross approach.