Rust Pathogen Puccinia Research Articles

Identification and inheritance of leaf rust resistance genes in the wheat cultivar ‘Marvdasht’

The objectives of this research were to examine the inheritance of leaf rust resistance genes in the Iranian wheat cultivar ‘Marvdasht’, which is highly resistant to leaf and stripe rusts, and to identify Lr genes present in this cultivar using molecular markers. The genetic basis of resistance to the leaf rust pathogen (Puccinia triticina) in ‘Marvdasht’ was studied in F2:3 populations derived from crosses of Bolani (susceptible cultivar) × Marvdasht. Isolates 84-1 and 85-28 of P. triticina, which are the predominant isolates in Iran, were used to examine the segregation of resistance originating from ‘Marvdasht’. The results indicated that resistance in ‘Marvdasht’ to Puccinia triticina isolate 84-1 was governed by two dominant seedling resistance genes Lr1 and Lr17a. Allelism studies using an F2 population derived from a cross between ‘Falat’ (Seri 82) and Marvdasht indicated that resistance in Marvdasht was not due to the resistance gene Lr26 present in ‘Falat’. With the application of a previously developed molecular marker for Lr1, the STS marker RGA-567-5, the presence of Lr1 was verified in Marvdasht. Based on bulk segregant analysis, Lr17a was mapped to the distal end of chromosome 2AS and was closely linked to microsatellite marker Xbarc212 at a distance of 3.7 cM. In conclusion, the presence of Lr1 and Lr17a was confirmed in the cultivar Marvdasht.

Suppression of Salicylic Acid-Mediated Plant Defense Responses During Initial Infection of Dyer's Woad by <i>Puccinia thlaspeos</i>

Problem statement: Puccinia thlaspeos is a microcyclic rust pathogen that is being investigated as a potential biocontrol agent of the noxious weed, dyer’s woad (Isatis tinctoria). Although, the initial events in the colonization of dyer’s woad by the rust pathogen has been elucidated using scanning electron microscopy and PCR, little is known regarding the susceptibility response of this plant to its rust pathogen. Approach: The induction kinetics and amplitude of the Salicylic Acid (SA)-responsive Pathogenesis-Related (PR) genes, PR-1, β-1, 3-glucanase and ChiA in the compatible interaction between the rust pathogen Puccinia thlaspeos and dyer’s woad were examined during the first 72 h of the infection process. Furthermore SA, an inducer of plant defense response was applied to infected plants in order to reprogram the host defense response at periods that coincided with key events of the infection process. Results: PR genes were upregulated following host penetration by the pathogen. A subsequent pathogen-mediated suppression of PR genes was seen that corresponded with haustorium formation. This was followed by a second up-regulation of these genes that was, in turn, followed by a second long-term pathogen-induced suppression of the defense response that appears to allow successful infections in dyer’s woad. Exogenous application of SA to uninoculated plants led to activation of defense responses by 8 h after treatment. In treatments where inoculated plants were treated with SA, responses differed depending on the timing of SA application. Application of SA at times corresponding to the pre-haustorial and posthaustorial phases of infection triggered an up-regulation of defense genes and increased protection against the pathogen. However, the application of SA during haustorium formation could not override the pathogen-mediated suppression of defense responses and consequently, did not offer the host increased protection. Conclusion: Although pathogen-induced PR expression was observed during rust infection of dyer’s woad, it appears to be insufficient to inhibit fungal growth during the compatible interaction between dyer’s woad and the rust pathogen. Suppression of pathogeninduced host defense responses during and after haustorium formation is postulated to be vital in the establishment of biotrophy in this system.

Artificial infection of Cirsium arvense with the rust pathogen Puccinia punctiformis by imitation of natural spore transfer by the weevil Ceratapion onopordi

Müller E, Jud P & Nentwig W (2011). Artificial infection of Cirsium arvense with the rust pathogen Puccinia punctiformis by imitation of natural spore transfer of the weevil Ceratapion onopordi. Weed Research51, 209–213.Summary Cirsium arvense is a noxious perennial weed. Because of the limited possibilities of chemical and physical control of this weed, many studies have been performed on biological methods. Successful control of C. arvense using the rust fungus Puccinia punctiformis needs the establishment of systemic infection, often connected with the weevil Ceratapion onopordi as a spore vector. To analyse whether an artificial infection of C. arvense with P. punctiformis through the plant stem is possible without the weevil, different infection tests were performed. Urediniospore solutions with different kinds of additives of weevil origin were applied. All treatments with infectious material led to an infection. Injection of pure urediniospores into the plant stem resulted in the highest infection rate (50%). Even though the infection‐via‐the‐stem‐method used in this study was successful in principle, this method is not practicable in the field. Therefore, further studies of infection techniques are needed, particularly to examine the spore stages that cause systemic infections.

TaNAC8, a novel NAC transcription factor gene in wheat, responds to stripe rust pathogen infection and abiotic stresses

The plant-specific NAC domain proteins have been shown previously to play important roles in both abiotic and biotic stresses or in diverse developmental processes. In this research, the full-length cDNA sequence of a novel NAC (for NAM, ATAF1/2, CUC2) transcription factor gene, designated as TaNAC8, was isolated from wheat ( Triticum aestivum) using in silico cloning, reverse transcription polymerase chain reaction and 3’ rapid amplification of cDNA ends PCR methods. TaNAC8 was highly homologous to the rice OsNAC8 gene and predicted to encode a protein of 481 amino acids. The encoded TaNAC8 protein contained a NAC domain in the N-terminus and a transmembrane helices motif in the C-terminus. Using the yeast one-hybrid assay, we detected that the C-terminal region of the TaNAC8 protein had transcriptional activity. TaNAC8 was expressed strongly in the developing wheat seeds, but weakly in stems and flowers. The expression of TaNAC8 in leaves was induced by the infection of the stripe rust pathogen ( Puccinia striiformis f. sp. tritici) in the incompatible interaction at 24 h post inoculation, and also by treatments with methyl jasmonate and ethylene. However, salicylic acid and abscisic acid had no any significant effect on the gene expression. Several environmental stimuli, including high salinity, PEG (polyethylene glycol) treatment, and low-temperature, also enhanced the TaNAC8 expression. These results suggest that the novel TaNAC8 protein functions as a transcriptional activator involved in wheat defense responses to abiotic and biotic stresses.

The phenotypic expression of QTLs for partial resistance to barley leaf rust during plant development

Partial resistance is generally considered to be a durable form of resistance. In barley, Rphq2, Rphq3 and Rphq4 have been identified as consistent quantitative trait loci (QTLs) for partial resistance to the barley leaf rust pathogen Puccinia hordei. These QTLs have been incorporated separately into the susceptible L94 and the partially resistant Vada barley genetic backgrounds to obtain two sets of near isogenic lines (NILs). Previous studies have shown that these QTLs are not effective at conferring disease resistance in all stages of plant development. In the present study, the two sets of QTL-NILs and the two recurrent parents, L94 and Vada, were evaluated for resistance to P. hordei isolate 1.2.1 simultaneously under greenhouse conditions from the first leaf to the flag leaf stage. Effect of the QTLs on resistance was measured by development rate of the pathogen, expressed as latency period (LP). The data show that Rphq2 prolongs LP at the seedling stage (the first and second leaf stages) but has almost no effect on disease resistance in adult plants. Rphq4 showed no effect on LP until the third leaf stage, whereas Rphq3 is consistently effective at prolonging LP from the first leaf to the flag leaf. The changes in the effectiveness of Rphq2 and Rphq4 happen at the barley tillering stage (the third to fourth leaf stages). These results indicate that multiple disease evaluations of a single plant by repeated inoculations of the fourth leaf to the flag leaf should be conducted to precisely estimate the effect of Rphq4. The present study confirms and describes in detail the plant development-dependent effectiveness of partial resistance genes and, consequently, will enable a more precise evaluation of partial resistance regulation during barley development.

The application of DNA marker and doubled-haploid technology for stacking multiple stem rust resistance genes in wheat

In wheat, the use of gene “pyramids” or “stacks” of major genes that confer resistance to all local strains of the fungal stem rust pathogen Puccinia graminis f. sp. tritici (Pgt) can increase durability of resistance where wheat cultivars with the single gene components are not widely deployed. Stacking two or more resistance genes becomes a breeding challenge, particularly when pathogen races that discriminate the genes are not available. The use of DNA markers and doubled-haploid technology provides a route for producing lines homozygous for multiple resistance genes. We have applied this approach to produce gene pyramids of two or more of the stem rust resistance genes Sr24 and new sources of SrR, Sr31 and Sr26 on reduced alien chromatin in the genetic backgrounds of Westonia and Pavon wheat. These genes, which are all derived from “alien” sources (SrR and Sr31 from rye, Sr24 and Sr26 from Agropyron elongatum) each provide resistance to all currently known pathotypes of Pgt in Australia, and SrR and Sr26 also provide resistance against all the variants of stem rust race Ug99 (TTKS group).

Resistance to Stem Rust Race TTKSK Maps to the rpg4/Rpg5 Complex of Chromosome 5H of Barley

Race TTKSK (Ug99) of the wheat stem rust pathogen (Puccinia graminis f. sp. tritici) is a serious threat to both wheat and barley production worldwide because of its wide virulence on many cultivars and rapid spread from eastern Africa. Line Q21861 is one of the most resistant barleys known to this race. To elucidate the genetics of resistance in this line, we evaluated the Q21861/SM89010 (Q/SM) doubled-haploid population for reaction to race TTKSK at the seedling stage. Segregation for resistance:susceptibility in Q/SM doubled-haploid lines fit a 1:1 ratio (58:71 with chi2=1.31 and P=0.25), indicating that a single gene in Q21861 confers resistance to race TTKSK. In previous studies, a recessive gene (rpg4) and a partially dominant gene (Rpg5) were reported to control resistance to P. graminis f. sp. tritici race QCCJ and P. graminis f. sp. secalis isolate 92-MN-90, respectively, in Q21861. These resistance genes co-segregate with each other in the Q/SM population and were mapped to the long arm of chromosome 5H. Resistance to race TTKSK also co-segregated with resistance to both rusts, indicating that the gene conferring resistance to race TTKSK also lies at the rpg4/Rpg5 locus. This result was confirmed through the molecular analysis of recombinants previously used to characterize loci conferring resistance to race QCCJ and isolate 92-MN-90. The 70-kb region contains Rpg5 (a nucleotide-binding site leucine-rich repeat serine/threonine-protein kinase gene), rpg4 (an actin depolymerizing factor-like gene), and two other genes of unidentified function. Research is underway to resolve which of the genes are required for conferring resistance to race TTKSK. Regardless, the simple inheritance should make Q21861 a valuable source of TTKSK resistance in barley breeding programs.

Cloning and characterization of a wheat β-1,3-glucanase gene induced by the stripe rust pathogen Puccinia striiformis f. sp. tritici

b-1,3-Glucanases are a group of pathogenesis related proteins that have been reported to be involved in plant defense against pathogens in many other plant pathogen systems. However, it was not clear if these genes play similar role in wheat (Triticum aestivum L.) against Puccinia striiformis f. sp. tritici (Pst), the stripe rust pathogen. To investigate the role of b-1,3-glucanase (EC3.2.1.39) in the resistance response of wheat (cv. Suwon11) to stripe rust, a wheat b-1,3-glucanase gene induced by Pst, designated as TaGlu, was cloned and characterized.TaGlu was predicted to encode a basic protein of 334 amino acids. Quantitative real-time PCR analyses revealed that the transcription of TaGlu was induced during both compatible and incompatible interactions with Pst, but the transcription level was much higher in the incompatible interaction than that in the compatible interaction. TaGlu also showed noticeable induction of gene expression in young green leaf tissues treated with salicylic acid, methyl jasmonate or ethylene. Immunogold labeling assays showed that the enzyme were localized mainly in the host cell wall and over the extra haustorial matrix, and the labeling densities were found significantly higher in the incompatible interaction than those in the compatible interaction.

Detached-Leaf Method for Propagating Puccinia coronata and Assessing Crown Rust Resistance in Oat.

The crown rust pathogen Puccinia coronata is an obligate biotroph with wind-disseminated propagules and numerous races. These characteristics make propagation of single-race cultures difficult. Genetic studies using single races in field and greenhouse environments are also problematic because pure cultures can easily become contaminated. In this study, we developed an isolated propagation system for P. coronata and tested its ability to assess host resistance. Oat (Avena sativa) leaf sections (10 cm each) were harvested, disinfested, and suspended in sterile plastic boxes by enclosing 3.5-cm linear sections of each leaf end between 4% agar blocks amended with various chemical constituents. The exposed sections (approximately 3 cm) were inoculated with P. coronata urediniospores suspended in water. Boxes were sealed and incubated in a lighted growth cabinet until the pathogen sporulated. Viable spores were produced on leaves in all treatments, whereas 6-benzylaminopurine (BAP) and kinetin treatments sustained the leaves longer and yielded the most viable spores. Based on these data, the BAP treatment was adopted and used for additional studies. Detached leaves of differential oat cultivars produced the same reactions as whole plants screened under standard conditions in a growth chamber. The proposed detached-leaf system should be useful for the propagation of numerous single-race cultures of P. coronata as well as evaluation of host resistance under highly controlled conditions.

Gene expression analysis of the wheat response to infection by Fusarium pseudograminearum

Crown rot (CR) of wheat, caused by Fusarium pseudograminearum ( Fp) and other Fusarium species, is an important disease globally. To understand the host response to challenge by Fp, we examined gene expression changes in the wheat stem base following inoculation with macroconidia using the Affymetrix GeneChip Wheat Genome Array. Induced genes included mainly those with defensive functions such as genes encoding anti-microbial proteins as well as oxidative stress-related proteins, signalling molecules, and proteins involved in both primary and secondary metabolism. Comparison of genes induced by Fp and the biotrophic rust pathogen Puccinia triticina revealed substantial overlap in most functional classes of induced genes, except for oxidative stress-related genes which were specifically induced by the necrotroph, Fp. Differential expression of selected Fp-induced genes was confirmed and further analysed using real-time quantitative RT-PCR on an inoculation time-course of wheat cultivars Kennedy and Sunco. Interestingly, several genes were induced earlier, and to higher levels, in the partially CR-resistant cultivar Sunco than in susceptible Kennedy. Many Fp-induced genes were also activated by methyl jasmonate and benzothiadiazole, an analogue of salicylic acid, suggesting that these signalling molecules may be involved in activating defences during crown rot. Most of the genes identified here that were induced by Fp were also induced by deoxynivalenol (DON), a toxin produced by Fp during CR. In particular, DON induced several genes encoding glucosyltransferases that may be involved in DON detoxification. To initiate functional characterisation, one of these wheat glucosyltransferase genes was over-expressed in Arabidopsis thaliana, however this did not result in improved tolerance to DON. This study is the first comprehensive analysis of the wheat transcriptome during CR and provides new insights into the host processes potentially involved in plant defence against this pathogen.

Exploiting regulatory variation to identify genes underlying quantitative resistance to the wheat stem rust pathogen Puccinia graminis f. sp. tritici in barley

We previously mapped mRNA transcript abundance traits (expression-QTL or eQTL) using the Barley1 Affymetrix array and 'whole plant' tissue from 139 progeny of the Steptoe x Morex (St/Mx) reference barley mapping population. Of the 22,840 probesets (genes) on the array, 15,987 reported transcript abundance signals that were suitable for eQTL analysis, and this revealed a genome-wide distribution of 23,738 significant eQTLs. Here we have explored the potential of using these mRNA abundance eQTL traits as surrogates for the identification of candidate genes underlying the interaction between barley and the wheat stem rust fungus Puccinia graminis f. sp. tritici. We re-analysed quantitative 'resistance phenotype' data collected on this population in 1990/1991 and identified six loci associated with barley's reaction to stem rust. One of these coincided with the major stem rust resistance locus Rpg1, that we had previously positionally cloned using this population. Correlation analysis between phenotype values for rust infection and mRNA abundance values reported by the 22,840 GeneChip probe sets placed Rpg1, which is on the Barley1 GeneChip, in the top five candidate genes for the major QTL on chromosome 7H corresponding to the location of Rpg1. A second co-located with the rpg4/Rpg5 stem rust resistance locus that has been mapped in a different population and the remaining four were novel. Correlation analyses identified candidate genes for the rpg4/Rpg5 locus on chromosome 5H. By combining our data with additional published mRNA profiling data sets, we identify a putative sensory transduction histidine kinase as a strong candidate for a novel resistance locus on chromosome 2H and compile candidate gene lists for the other three loci.

Pathotypic evolution in Puccinia striiformis in India during 1995 - 2004

Yellow rust pathogen (Puccinia striiformis) distribution was monitored in different wheat-production zones (north-western plains zone, north hills zone, and south hills zone) of India during the last decade (1994–2004). Studies indicated the predominance of 3 pathotypes: 46S102 (46E149), 47S103 (47E148), and 46S119 (46E149+Yr9) in the north-western plains zone. These pathotypes were detected for the first time in 1995–96 and are evolving frequently. Another pathotype (78S84), although present in low frequency, was first detected from northern India and is virulent on Yr9 and Yr27, posing a major threat to cv. PBW343, which carries these genes. The high altitudes of the northern hills where very primitive wheat cultivars are grown showed less variability for the yellow rust pathogen. Continued breeding efforts and replacement of cultivars have resulted in low specialisation in the yellow rust pathogen in the south hills zone and pathotype 38S102 predominated in this area during the last decade.

Two modes of pathogen recognition by plants

More than 50 years ago, J. B. S. Haldane (1) presciently predicted that polymorphism in proteins governing host–pathogen interactions was driven by the rapid rates of microbial evolution. He suggested that slowly evolving host species could only manage to dodge microbial infection by generating high levels of polymorphism in the relevant immune system genes. These ideas were in part inspired by observations of variability in resistance of wheat to the wheat rust pathogen Puccinia graminis . Soon thereafter, H. H. Flor (2) published seminal studies delineating the genetic basis of resistance in flax to the rust fungus Melampsora lini . Flor documented genetic polymorphism and recognition specificity. Flor (3) also proposed the widely accepted “gene-for-gene” theory of disease resistance, which predicted that successful disease resistance is triggered only if a resistance ( R ) gene product in the plant recognizes a specific “avirulence” ( Avr ) gene product from the pathogen. An evolutionary corollary of this model is that pathogens are under evolutionary pressure to evolve diverse Avr proteins that no longer interact genetically with the cognate R proteins but presumably maintain their core function in the furtherance of a successful parasite life cycle. Conversely, natural selection favors evolution and maintenance of allelic diversity at plant R loci to expand recognition specificity. In this issue of PNAS, Dodds et al . (4) provide evidence for direct, allele-specific interaction between alleles of a particular R protein and the corresponding pathogen-encoded Avr proteins. In a remarkable arc of history, they do so by using cloned alleles from flax and flax rust loci that were defined genetically five decades ago by Flor (2, 3, 5, 6). Combined with other recent breakthroughs, this work suggests that plant R proteins can function either by directly detecting … ‡To whom correspondence should be addressed at: Department of Biology, University of North Carolina, CB# 3280, 108 Coker Hall, Chapel Hill, NC 27599. E-mail: dangl{at}email.unc.edu

Neighbourhood of host plants influences oviposition decisions of a stem-boring weevil

Summary The spatial arrangement of suitable host plants in the field may significantly constrain insects to find optimal hosts. Plant neighbours around a focal host plant can either lead to lower (associational resistance) or higher (associational susceptibility) herbivore loads. We tested whether the spatial arrangement of hosts of different suitability for the larval development of the shoot-base boring weevil Apion onopordi affects oviposition decisions in the field. Host plants in our study were healthy creeping thistles (Cirsium arvense; suboptimal hosts) and thistles infected by a rust pathogen (Puccinia punctiformis; optimal hosts). For analysis, we used nearest neighbour methods that disentangle the spatial distribution of organisms that are dependent on the position of other species (e.g. phytophagous insects and their host plants). Although theory predicts that the small-scale spatial infestation pattern can have major consequences for the population dynamics in insect–plant systems, field studies quantifying spatial pattern of phytophagous insects are rare. The spatial arrangement of host plants clearly influenced oviposition pattern in A. onopordi. In contrast to previous studies, we demonstrated that not the rust infection itself determined if a plant was infested by weevils, but rather the density of rusted shoots within a certain neighbourhood. We found strong indications for associational susceptibility of healthy thistle shoots to weevil oviposition when growing in the neighbourhood of rusted thistles. Weevil-infested plants were spatially aggregated, indicating that A. onopordi is limited in its dispersal ability within patches. Other stem-boring insects on creeping thistle were affected in their oviposition decisions by other factors than A. onopordi. Thus, it may be difficult to find general rules for oviposition choice in phytophagous insects. Our study showed that the spatial arrangement of host plants in the field critically determines oviposition choice and should thus be included as constraint in theories of optimal host selection.

High-resolution mapping of the barley leaf rust resistance gene Rph5 using barley expressed sequence tags (ESTs) and synteny with rice

The rapidly growing expressed sequence tag (EST) resources of species representing the Poacea family and availability of comprehensive sequence information for the rice (Oryza sativa) genome create an excellent opportunity for comparative genome analysis. Extensive synteny between rice chromosome 1 and barley (Hordeum vulgare L.) chromosome 3 has proven extremely useful for saturation mapping of chromosomal regions containing target genes of large-genome barley with conserved orthologous genes from the syntenic regions of the rice genome. Rph5 is a gene conferring resistance to the barley leaf rust pathogen Puccinia hordei. It was mapped to chromosome 3HS, which is syntenic with rice chromosome 1S. The objective of this study was to increase marker density within the sub-centimorgan region around Rph5, using sequence-tagged site (STS) markers that were developed based on barley ESTs syntenic to the phage (P1)-derived artificial chromosome (PAC) clones comprising the distal region of rice chromosome 1S. Five rice PAC clones were used as queries in a blastn search to screen 375,187 barley ESTs. Ninety-four non-redundant EST sequences were identified from the EST database and used as templates to design 174 pairs of primer combinations. As a result, 9 barley EST-based STS markers were incorporated into the 'Bowman' x 'Magnif 102' high-resolution map of the Rph5 region. More importantly, six markers, including five EST-derived STS sequences, were found to co-segregate with Rph5. The results of this study demonstrate the usefulness of rice genomic resources for efficient deployment of barley ESTs for marker saturation of targeted barley genomic regions.

Reaction of barley seedlings with different stem rust resistance genes to Puccinia graminis f. sp. tritici and Puccinia graminis f. sp. secalis

Seedling tests are desirable for disease resistance screening in breeding programs because a large number of lines can be evaluated in a short period of time and with modest space requirements. This study was undertaken to identify pathotypes of the wheat stem rust pathogen (Puccinia graminis f. sp. tritici) and cultures of the rye stem rust pathogen (Puccinia graminis f. sp. secalis) that might be useful for detecting resistance genes in barley (Hordeum vulgare) at the seedling stage. Barley accessions with different resistance genes (Rpg1, Rpg2, Rpg3, rpg4, and rpgBH) were assessed for their infection types (ITs) to 13 pathotypes of P. graminis f. sp. tritici and two cultures of P. graminis f. sp. secalis at two temperature regimes (18–21 °C and 24–25 °C). The P. graminis f. sp. tritici pathotypes HKHJ and QCCJ were effective for specifically detecting Rpg1 and rpg4, respectively, and will facilitate resistance breeding efforts. No cultures were found to be effective for specifically detecting the resistance of Hietpas-5, PI 382313, or Black Hulless at the seedling stage. Some pathotypes or cultures did, however, produce low ITs on these sources and will therefore be useful for genetic studies involving crosses with susceptible barleys

PCR Markers for Triticum speltoides Leaf Rust Resistance Gene Lr51 and Their Use to Develop Isogenic Hard Red Spring Wheat Lines

New leaf rust resistance genes are needed in wheat (Triticum aestivum L.) to provide additional sources of resistance to the highly variable and dynamic leaf rust pathogen Puccinia triticina Eriks. Leaf rust resistance gene Lr51, located within a segment of Triticum speltoides Taush chromosome 1S translocated to the long arm of chromosome 1B of bread wheat, is resistant to the current predominant races of leaf rust in the USA. The objectives of this study were to determine the genetic length of the translocated 1S segment, develop a PCR marker for Lr51, and use this marker to generate isogenic lines for this gene. Characterization of two translocation lines (F‐7‐3 and F‐7‐12) with 10 molecular markers indicated that F‐7‐3 has an interstitial T. speltoides chromosome segment of 14 to 32 cM long including loci XAga7 and Xmwg710, whereas line F‐7‐12 has a complex series of translocations among chromosomes of homeologous group 1. On the basis of the DNA sequence of the A, B, D, and S alleles of the XAga7 locus, we designed a cleavage amplified polymorphic sequence (CAPS) marker for the S genome allele. Primers S30‐13L and AGA7‐759R preferentially amplified the XAga7 1S (819 bp) and 1B alleles (783 bp). These amplification products can be separated in agarose gels after digestion with PstI or BamHI restriction enzymes. This CAPS marker was validated in a collection of 32 common wheat cultivars and was used to develop three pairs of hard red spring isogenic lines from the donor parent F‐7‐3. These isogenic lines will be valuable for future assessment of the effect of this chromosome introgression on agronomic performance and end‐use quality.

Inheritance of seedling resistance to stripe rust (Puccinia striiformis f. sp. tritici) in ‘Otane’ and ‘Tiritea’ wheat ( Triticum aestivum)

Two spring wheat (Triticum aestivum) cultivars, ‘Tiritea’ and ‘Otane’, were crossed and the reactions of the F1‐derived double haploid (DH) population to the stripe rust pathogen (Puccinia striiformis f. sp. tritici) at the seedling stage were examined in glasshouse and field experiments. Both parental cultivars were susceptible to stripe rust pathotype 106E139A+, buttransgressive segregation in the DH progenies indicated that both parents possess resistance genes. The distribution of DH lines fitted a trigenic ratio with epistatic gene action. In a proposed model, resistant DH lines were produced when all three loci involved were homozygous recessive, moderately resistant DH lines were produced when the t gene was homozygous recessive with at least one recessive allele at either the B or the C locus, and susceptible DH lines were produced when either t, b, or c were present alone or had a dominant gene at the B and the C loci. The susceptible phenotype of all F1 progenies indicated that the resistance factors segregated were all recessive. The susceptible reaction of both cultivars could be attributed to the presence of genes that may suppress the expression of resistance genes when they coexist in the parents. The use of these cultivars as parents in different wheat breeding strategies is briefly discussed.

Resistance gene-analog polymorphism markers co-segregating with the YR5 gene for resistance to wheat stripe rust.

The Yr5 gene confers resistance to all races of the stripe rust pathogen ( Puccinia striiformis f. sp. tritici) of wheat in the United States. To develop molecular markers for Yr5, a BC(7):F(3) population was developed by backcrossing the Yr5 donor ' Triticum spelta album' (TSA) with the recurrent parent 'Avocet Susceptible' (AVS). Seedlings of the Yr5 near-isogenic lines (AVS/6* Yr5), AVS, TSA, and the BC(7):F(3) lines were tested with North American races of P. striiformis f. sp. tritici under controlled greenhouse conditions. The single gene was confirmed by a 1:2:1 segregation ratio for homozygous-resistant, heterozygous and homozygous-susceptible BC(7):F(3) lines. Genomic DNA was extracted from the parents (the Yr5 near-isogenic line and AVS) and 202 BC(7):F(3) lines. The resistance gene-analog polymorphism (RGAP) technique was used to identify molecular markers. The parents and the homozygous-resistant and homozygous-susceptible BC(7):F(3) bulks were used to identify putative RGAP markers for Yr5. Association of the markers with Yr5 was determined using segregation analysis with DNA from the individual BC(7):F(3) lines. Of 16 RGAP markers confirmed by segregation analysis with 109 BC(7):F(3) lines, and nine of the markers confirmed with an additional 93 BC(7):F(3) lines, three markers co-segregated with the resistance allele and three markers co-segregated with the susceptibility allele at the Yr5 locus. The other four markers were tightly linked to the locus. Analysis of a set of Chinese Spring nulli-tetrasomic lines with three markers that co-segregated with, or were linked to, the susceptibility allele confirmed that the Yr5 locus is on chromosome 2B. Of five RGAP markers that were cloned and sequenced, markers Xwgp-17 and Xwgp-18 that co-segregated with the Yr5 locus were co-dominant and had 98% homology with each other in both DNA and translated amino-acid sequences. The two markers had 97% homology with a resistance gene-like sequence from Aegilops ventricosa and had significant homology with many known plant resistance genes, resistance gene analogs and expressed sequence tags (ESTs) from wheat and other plant species. The markers Xwgp-17 and Xwgp-18 also had significant homology with the NB-ARC domain that is in several genes for plant resistance to diseases, nematode cell death and human apoptotic signaling. These markers should be useful to clone Yr5 and combine Yr5 with other genes for durable and superior resistance for the control of stripe rust.

Variability in isolates of Puccinia polysora in Brazil

The main objective of this work was to evaluate the variability of the southern rust pathogen Puccinia polysora in Brazil, based on its virulence on a set of maize (Zea mays) cultivars. Sixty single pustule isolates, from different areas of occurrence of southern rust, were evaluated for their virulence to 50 maize experimental hybrids. Six cultivars showed a clear distinction between susceptible and resistant reaction, and were used to characterize the variability of the pathogen. Seventeen virulence patterns were identified among the 60 isolates tested. The most frequent virulence patterns identified, were observed in all locations of sampling, which suggests the absence of geographical differentiation among prevalent populations of P. polysora in Brazil.