Suppression/expression of resistance to stripe rust in synthetic hexaploid wheat (Triticum turgidum×T. tauschii)

Seventy-four hexaploid wheats, synthesized by either crossing resistantTriticum turgidum L. var.durum with susceptible/intermediateT. tauschii or susceptible/intermediateT. turgidum with resistantT. tauschii, and their parents were evaluated as seedlings in the greenhouse and as adult-plants at two field locations in Mexico for resistance to pathotype 14E14 of stripe (or yellow) rust (caused byPuccinia striiformis Westend). The seedlings of different synthetic hexaploids showed high phenotypic diversity for resistance. However, the resistance level of only 15 of the 74 synthetic hexaploid wheats were similar to the low infection types of the respective donor parents. The remaining synthetic wheats displayed either intermediate or high infection types. A similar result was also obtained in field tests, where only 18 synthetic hexaploids were resistant as adult-plants. In general, genotypes with seedling resistance were also resistant as adult-plants. A few synthetic hexaploids, which displayed intermediate or susceptible infection types as seedlings were resistant as adult-plants, indicating that additional genes for adult-plant resistance were also present. The fact that resistance of some donor parents was not expressed, or only partially expressed, in a synthetic hexaploid background suggests the presence of suppressor genes in the both the A or B, and D genomes ofT. turgidum andT. tauschii, respectively. The resistance of a donor parent was expressed in a synthetic hexaploid only if the corresponding suppressor was absent in the second parent. Moreover, the suppressors appeared to be resistance gene specific.     

Genes for resistance to stripe rust in four spring wheat varieties

Summary Four spring wheat (Triticum aestivum L.) varieties differing in origin and reaction in the seedling stage to pathotype CDL-6 (extant in California) were intercrossed and examined in greenhouse conditions in F₁, F₂, and F₃ generations. Digenic and transgressive segregation was found in all crosses. The four varieties each had infection types (1 immune, 9 susceptible) and putative resistance genes as follows: Anza, IT 7, YrA; Glennson 81, IT 2, Yr9; Yecora Rojo, IT 6, YrC; and Ollanta, IT 4–6, YrL. Anza was classified as susceptible, Yecora Rojo and Ollanta as intermediate in seedling resistance, and Glennson 81 as resistant in the seedling stage.     

Assessment of genetic diversity in synthetic hexaploid wheats and their Triticum dicoccum and Aegilops tauschii parents using AFLPs and agronomic traits

Synthetic hexaploid wheats are of interest to wheat breeding programs, especially for introducing new genes that confer resistance to biotic and abiotic stresses. A group of 54 synthetic hexaploid wheats derived from crosses between emmer wheat(Triticum dicoccum, source of the A and B genomes) and goat grass (Aegilops tauschii, D genome donor) were investigated for genetic diversity. Using the AFLP technique, dendrograms revealed clear grouping according to geographical origin for the T. dicoccum parents but no clear groups for the Ae. tauschii parents. The geographical clustering of the T. dicoccum parents was also reflected in the dendrogram of their derived synthetic hexaploids. Diversity of the T. dicoccum parents and their derived synthetic hexaploids was further evaluated by measuring 18morphological and agronomic traits on the plants. Clustering based on morphological and agronomic data also reflected geographical origin. However, comparison of genetic distances obtained from AFLP and agronomic data showed no correlation between the two diversity measurements. Nevertheless, similarities among major clusters with the two systems could be identified. Based on percentage of polymorphic markers, the synthetic hexaploids had a considerably higher level of AFLP diversity (39%) than normally observed in cultivated hexaploid wheat (12–21%). This suggests that synthetic hexaploid wheats can be used to introduce new genetic diversity into the bread wheat gene pool. This revised version was published online in July 2006 with corrections to the Cover Date.     

Resistance in spelt wheat to yellow rust

Summary Seven spelt wheat accessions of different origin were hybridized with the susceptible bread wheat cultivar Taichung 29 in order to study the genetics of their resistance to yellow rust (Puccinia striiformis Westend. f. sp. tritici). One Iranian and five European accessions were found to carry Yr5 of Triticum aestivum ssp. spelta var. album, whereas a factor for resistance in the Iranian accession 415 was confirmed to be genetically distinct from Yr5. The alleles for resistance in each of the accessions studied showed a monogenic dominant mode of inheritance. Twenty-eight spelt wheat accessions, including those studied for their resistance to yellow rust, were subjected to polyacrylamide-gel-electrophoresis to study variation for gliadin storage protein patterns. Thirteen distinct patterns were revealed, implying the presence of duplicates within the studied spelt wheat collection.     

A recessive resistance gene for yellow rust (Puccinia striiformis West.) in bread wheat (Triticum aestivum L.)

Summary Three lines derived from the old dirty Dutch land variety Gelderse Ris were resistant against race 66(70)EO(16) of yellow rust. It was found that this resistance was conditioned by one recessive gene provisionally coded yrGR.     

Genes for resistance to stripe rust in four spring wheat varieties

Summary The stripe rust resistant spring wheat (Triticum aestivum L.) varieties Anza, Glennson 81, Ollanta, and Yecora Rojo gave 1,2,2, and 2-gene segregations, respectively, in hybrids with susceptible Jupateco 73 when inoculated in field conditions at Davis, California USA with Puccinia striiformis West. pathotype CDL-6 and rated at post-heading stage. Intercrosses of these varieties, Anza/Yecora Rojo was not studied, permitted the following conclusions about the genes expressed in adult plants: Anza, one recessive gene; Glennson 81, two dominant genes; Ollanta, two genes, at least one is dominant; and Yecora Rojo; one dominant and one recessive gene, one of which is common with Ollanta. The resistance genes in these varieties, which expressed resistance in the seedling stage, were believed to be effective at the adult stage. Thus, seven resistance genes were identified in the four varieties. The genotypes were designated for the purposes of this study as follows: Anza, YrA YrH; Glennson 81, Yr9, YrJ, Ollanta YrL YrD; and Yecora Rojo, YrC YrD. It was recommended that these and other Yr genes be used as multiple gene complexes to increase durability of resistance to P. striiformis, an organism known to evolve virulence rapidly in field conditions. The demonstrated durability of Anza in California may be a result of its combination of resistance alleles at two loci.     

Molecular mapping and detection of the yellow rust resistance gene Yr26 in wheat transferred from Triticum turgidum L. using microsatellite markers

Yellow rust (stripe rust), caused by Puccinia striiformis Westend f. sp. tritici, is one of the most devastating diseases of wheat throughout the world. Wheat-Haynaldia villosa 6AL.6VS translocation lines R43, R55, R64 and R77, derived from the cross of three species, carry resistance to both yellow rust and powdery mildew. An F₂ population was established by crossing R55 with the susceptible cultivar Yumai 18. The yellow rust resistance in R55 was controlled by a single dominant gene, which segregated independently of the powdery mildew resistance gene Pm21 located in the chromosome 6VS segment, indicating that the yellow rust resistance gene and Pm21 are unlikely to be carried by the same alien segment. This yellow rust resistance gene was considered to beYr26, originally thought to be also located in chromosome arm 6VS. Bulked Segregation Analysis and microsatellite primer screens of the population F₂ of Yumai 18 × R55 identified three chromosome 1B microsatellite locus markers, Xgwm11, Xgwm18 and Xgwm413, closely linked to Yr26. Yr26 was placed 1.9 cM distal of Xgwm11/Xgwml8, which in turn were 3.2 cM from Xgwm413. The respective LOD values were 21 and 36.5. Therefore, Yr26 was located in the short arm of chromosome 1B. The origin and distribution of Yr26 was investigated by pedigree, inheritance of resistance and molecular marker analysis. The results indicated that Yr26 came from Triticum turgidum L. Three other 6AL.6VS translocation lines, R43, R64 and R77, also carried Yr26. These PCR-based microsatellite markers were shown to be very effective for the detection of the Yr26 gene in segregating populations and therefore can be applied in wheat breeding. This revised version was published online in July 2006 with corrections to the Cover Date.     

Transgressive segregation for resistance to yellow rust in wheat

Summary Crosses were made between wheat varieties Joss Cambier, Nord Desprez and Maris Bilbo, all classified as susceptible to yellow rust in field tests, and between Cappelle Desprez and Maris Huntsman, both classified as moderately and durably resistant. Selection for resistance to yellow rust among the progeny was carried out using races of Puccinia striiformis able to overcome all the known race-specific components of resistance in both parents of each cross. Lines with greater resistance than in both parents were obtained from each cross, those with greatest resistance being obtained from the cross between the moderately resistant parents. Three lines selected for resistance from the cross of Joss Cambier with Nord Desprez and one from the cross of Cappelle Desprez with Maris Huntsman, together with the parents, were tested in the field with 12 races of P. striiformis. Nord Desprez possessed a previously undetected race-specific component. The selected lines also displayed race-specific resistance, some of which was clearly related to race-specificity of the parents, and a component of resistance, greater than in both parents, that was effective against all 12 races. The possible origin and potential durability of this transgressive level of resistance is discussed. It is suggested that such transgressive resistance is more likely to be durable if it is derived from parents that have shown durable resistance.     

Resistance to stripe rust in durum wheats, A-genome diploids, and their amphiploids

Stripe rust (caused by Puccinia striiformis Westend.) is a wheat disease of worldwide importance. Seedlings of 75 accessions of Triticum boeoticum, 12 of T. monococcum, 16 of T. urartu, 230 of durum wheat (T. turgidum L. var. durum), and 128 amphiploids (genome AAAABB) involving the crosses of the three diploid species (AA) with T. turgidum (AABB) were evaluated in the greenhouse for their reaction to P. striiformis race 14E14. Durum wheats and the amphiploids were also evaluated at two field locations in Mexico with the same race for their adult plant response. Resistant seedling reactions (infection types: 0-3 on a 0-9 scale) were seen for 10 (13%) accessions of T. boeticum, 19 (8%) accessions of T. turgidum and 32 (25%) amphiploids. The remaining accessions were either moderately resistant (ITs 4-6) or susceptible (ITs 7-9). The three amphiploids derived from the crosses of seedling resistant T. boeoticum and T. turgidum, were resistant as seedlings. Among the 51 amphiploids involving one resistant parent, 29 were resistant and the remaining 22 displayed intermediate to susceptible reactions. Suppressors for resistance were common in the A and AB genomes and suppression was resistance gene specific. Forty-five (20%) durums showed adequate field resistance (relative AUDPC <10% of the susceptible check ‘Morocco’). These included the 19 seedling resistant durums. Presence of genes involved in adult plant resistance was evident, because 26 of the remaining adult plant resistant durums had displayed intermediate-susceptible seedling reactions. Though the seedling reactions of the amphiploids varied from low to high, all involving the adult plant resistant durums possessed adequate field resistance. The resistant, newly produced, AAAABB amphiploids are useful genetic resources for stripe rust resistance which could be transferred to the cultivated T. turgidum. This revised version was published online in July 2006 with corrections to the Cover Date.     

Postulation of resistance genes to yellow rust in wild emmer wheat derivatives and advanced wheat lines from Nepal

Summary Seedlings of 38 wild emmer derivatives, and a total of 53 advanced wheat varieties/lines introduced from the International Maize and Wheat Improvement Centre (CIMMYT) or other sources, Nepalese breeding lines and local cultivars were inoculated with 18 different yellow rust isolates to postulate yellow rust resistance genes (Yr). Many wild emmer wheat derivatives used were resistant to all isolates indicating the presence of undescribed genes. Some derivatives carried Yr9, Yr6 and/or YrSU. Genes Yr1, Yr2, Yr6, Yr7, Yr8, Yr15, YrSU and YrA+ are no longer effective in Nepal; Yr4, Yr5, Yr9, Yr10, YrSP and YrSD are still effective; the effectiveness of Yr3 remains unclear. This study shows that stripe rust resistance in seedling stage of most Nepalese cultivars and advanced materials is based on Yr9 with combinations of Yr2, Yr6, Yr7, and YrA+, of which only Yr9 is still effective in Nepal. In many countries Yr9 has lost its effectiveness. Therefore the introduction of new Yr-genes from wild emmer wheat in Nepalese cultivars is highly important.     

Monosomic analyses of stripe rust and leaf rust resistance genes in winter wheat varieties Lüquyu and Yantar

Summary A set of 21 monosomics of Novosadska Rana-1 was used to locate the rust resistance genes of Lüqiyu, a stripe rust resistant line developed by BAU and Yantar, a leaf rust resistant wheat introduced from Bulgaria. The resistance of the former to p. striiformis race C25 was conditioned by a dominant gene located on chromosome 2B, whereas that of the latter to P. recondita race CL3 was controlled by two complementary dominant genes located on chromosomes 5A and 1D, respectively. The relationship of the stripe rust resistance gene in Lüqiyu to Yr5, Yr7 or Yr _Suwon' all located on chromosome 2B is unknown. The two complementary leaf rust resistance factors in Yantar appear to be new.     

Additive gene action for resistance to Puccinia striiformis F.SP. Tritici in Triticum dicoccoides

Summary Seven single-plant selections of wild emmer, with temperature-sensitive minor-effect genes for stripe rust resistance, were intercrossed in eight combinations. The resulting progenies were studied for a possible additive gene action.The transgressive segregation towards resistance in F₂ observed in all the combinations indicates that additive gene action for resistance indeed occurs in wild emmer. The common occurrence of this phenomenon in random combinations suggests further that several minor-effect genes are involved.Following selection of the most resistant plants in F₂, a marked shift towards resistance was noted in F₃, which demonstrates a positive response to selection. In some instances, additive resistance selected for (in F₂) at the high temperature-profile was expressed (in F₃) also at the low temperature-profile. This kind of resistance, when utilized in breeding programmes, promises therefore to be effective over a range of temperatures.     

Use of the gene pools of Triticum turgidum ssp. dicoccoides and Aegilops squarrosa for the breeding of common wheat (T. aestivum), through chromosome-doubled hybrids

Summary Triticum turgidum ssp. dicoccoides (wild emmer wheat, AABB, 2n=28) and Aegilops squarrosa (goat grass, DD, 2n=14) comprise a rich reservoir of valuable genetic material, which could be useful for the breeding of common wheat (T. aestivum, AABBDD, 2n=42). Many accessions of both wild species, most of them selected for resistance to stripe rust, were used to make amphiploids. Two strategies were applied: (1) the production of autopolyploid cytotypes of the wild species, followed by hybridisation, and (2) the production of allotriploid interspecific hybrids, followed by doubling of the number of chromosomes. The first route was unsuccessful because of failure of the crosses between the autopolyploid cytotypes, possibly due to incongruity between the two species and to reduced fertility in the autopolyploid cytotypes. The second route yielded the desired synthetic hexaploids. However, the rate of success of the crosses was low and there were great differences between years, and within years between crosses. Embryo rescue was applied to obtain the primary hybrids (2n=21), which were highly sterile and had on average 0.3 bivalents and 20.4 univalents per pollen mother cell. Various abnormalities were recorded. Doubling of the number of chromosomes sometimes occurred spontaneously or was brought about by colchicine treatment. The large scale of the interspecific hybridisation programme ensured that one-third of the female and one-sixth of the male accessions were represented in the synthetic hexaploids.     

Yr15 — a new gene for resistance to Puccinia striiformis in Triticum dicoccoides sel. G-25

Summary In a comparative study of reaction patterns and by analysis of segregation ratios in cross progenies, Triticum dicoccoides Koern. sel. G-25 was shown to possess a yet unknown gene for resistance to yellow rust. It is suggested to assign provisionally the symbol Yr15 to this gene.     

Effects of moisture stress on leaf appearance,tillering and other aspects of development in Triticum tauschii

Summary A glasshouse study was conducted to describe the dynamics of leaf and tiller appearance of four accessions of T. tauschii (Tt 04, Tt 17, Tt 65 and Tt 74) and to determine the influence of moisture stress (treatments were high and low moisture, imposed seven days after transplanting) on these and other aspects of development in this wild wheat.Under high moisture conditions, accessions differed greatly in flag leaf dimensions, culm length and seed number per spike, the values being lower in Tt 04 than in the other accessions. Low moisture strongly reduced values for these traits, with Tt 04 being least affected, but overall, there was no apparent association between the values obtained for these variables in the high moisture conditions and the effects of moisture stress. For three of the four accessions, final leaf number on the main culm was significantly lower in the low moisture treatment than in the respective control (P<0.05), but the differences between treatments (ca. 0.5 leaves or less) were very small. Maximum tiller number, on the other hand, was strongly reduced by low moisture, and initiation of tillering was inhibited until water was reapplied. There were no apparent after-effects of the moisture regime on the rate of subsequent tiller appearance.The four accessions differed in their leaf appearance rates, giving phyllochron values (117–142° Cd leaf^-1) within the range reported for hexaploid wheat. Low moisture tended to increase phyllochron, but in only one accession was this effect significant. Thus, depending on the accession, low moisture did not affect, or slightly decreased (by ca. 15–20%) the rate of leaf appearance. These effects were similar to those reported for cultivated wheat suggesting that there would be little scope for using these accessions of T. tauschii in breeding for stress tolerance.     

Use of Triticum tauschii to improve yield of wheat in low-yielding environments

Triticum tauschii (Coss.) Schmal. is an ancestor of bread wheat (T. aestivum). This species has been widely used as a source ofsimply-inherited traits, but there are few reports of yield increases due tointrogression of genes from this species. Selections from F₂-derivedlines of backcross derivatives of synthetic hexaploid wheats (T.turgidum / T. tauschii) were evaluated for grain yield in diverseenvironments in southern Australia. Re-selections were made in theF₆ generation and evaluated for grain yield, yield componentsincluding grain weight, and grain growth characters in diverse environmentsin southern Australia and north-western Mexico. Re-selection was effectivein identifying lines which were higher yielding than the recurrent parent,except in full-irrigation environments. Grain yields of the selectedderivatives were highest relative to the recurrent parent in thelowest-yielding environments, which experienced terminal moisture deficitand heat stress during grain filling. The yield advantage of the derivativesin these environments was not due to a change in anthesis date orgrain-filling duration, but was manifest as increased rates of grain-filling andlarger grains, indicating that T. tauschii has outstanding potential forimproving wheat for low-yielding, drought-stressed environments.     

The reaction of x Tritordeum and its Triticum spp. and Hordeum chilense parents to rust diseases

Summary Hexaploid and octoploid tritordeums and their parents Hordeum chilense and Triticum spp. were screened for resistance to isolates of wheat and barley yellow and brown rusts. All H. chilense lines were highly resistant to both wheat and barley brown rust, few lines were susceptible to wheat yellow rust while susceptibility to barley yellow rust was common. In general the resistance of tritordeum is predominantly contributed by the wheat parent and apparently the genes for resistance in H. chilense are inhibited in their expression by the presence of the wheat genome.Abbreviations WYR wheat yellow rust - WBR wheat brown rust - BYR barley yellow rust - BBR barley brown rust     

Temperature-sensitive genes for resistance to Puccinia striiformis in Triticum dicoccoides

Summary Twenty-six selections of wild emmer collected at different sites in Israel and three entries obtained from Turkey were studied for the possible presence of temperature-sensitive genes controlling reaction to Puccinia striiformis. In tests carried out in Israel with a local P. striiformis isolate at two temperature regimes, 16 selections showed a change in infection rating toward resistance at the higher temperature regime. In comparable tests performed in the United States with a Montana P. striiformis isolate of different virulence pattern-including 14 of the same selection—ten entries displayed a temperature-sensitive reaction. Although slightly different temperature regimes were used in Israel and the United States, the results obtained were in general agreement. The shift toward resistance observed in the seedling tests at the higher temperature-profile was also evident in the field in the mature plant stage with increasing spring temperatures.     

Control and inheritance of resistance to yellow rust in Triticum aestivum-Lophopyrum elongatum chromosome substitution lines

A set of T. aestivum-L. elongatum chromosome substitution lines was tested for yellow rust resistance at the seedling stage. Inheritance of the resistance and esterase-5 (Est-5) variation were studied. The results demonstrated that L.elongatum carried a new gene(s) conferring yellow rust resistance. This gene was dominant and located on chromosome 3E of L. elongatum. The biochemical locus encoding Est-5was also located on chromosome 3E, and co-segregated with theYr gene(s) in the wheat background. The transmission frequencies of chromosome 3E in 3E(3A) × CS, 3E(3B) × CS and 3E(3D) × CS hybrids were scored.None of the hybrids transmitted the alien chromosome at thetheoretical maximum rate, but the transmission frequencies ofchromosome 3E in F₂ populations of 3E(3A) × CS and 3E(3D) × CS were significantly higher than in thatof 3E(3B) × CS.     

Gametocidal effects and resistance to wheat leaf rust and stem rust in derivatives of a Triticum turgidum ssp. durum/Aegilops speltoides hybrid

Summary Genes for leaf rust and stem rust resistance and segregation distortion (Gc), that seemed to derive from an Aegilops spetroides ssp. ligustica accession, were transferred to common wheat. While the advanced backcrosses had normal meioses and 42 chromosomes, high levels of male and female sterility, abnormal endosperm development and chromosome aberrations were evident. These effects were more pronounced in Gc-heterozygotes than in homozygotes. Gametes without Gc genes did not survive, and the Gc-associated defects were always inherited with the resistance. Since the resistance genes were effective against local pathotypes of the leaf rust and stem rust pathogens, an attempt was made to disrupt the Gc-system through irradiation, treatment with the mutagen N-nitroso-N-methyl-urea or growing the material at elevated temperatures. A very low frequency of the treated material showed slightly better fertility and seed development. However, these effects did not persist in subsequent generations and were apparently not strong enough to allow the recovery of segregates which had lost the Gc gene(s).