The emergence of a new highly virulent race of stem rust (Puccinia graminis tritici), Ug99, rapid evolution of new Ug99 derivative races overcoming resistance of widely deployed genes, and spread towards important wheat growing areas now potentially threaten world food security. Exploiting novel genes effective against Ug99 from wild relatives of wheat is one of the most promising strategies for the protection of the wheat crop. A new source of resistance to Ug99 was identified in the short arm of the Aegilops searsii chromosome 3S(s) by screening wheat- Ae. searsii introgression libraries available as individual chromosome and chromosome arm additions to the wheat genome. For transferring this resistance gene into common wheat, we produced three double-monosomic chromosome populations (3A/3S(s), 3B/3S(s) and 3D/3S(s)) and then applied integrated stem rust screening, molecular maker analysis, and cytogenetic analysis to identify resistant wheat-Ae. searsii Robertsonian translocation. Three Robertsonian translocations (T3AL·3S(s)S, T3BL·3S(s)S and T3DL·3S(s)S) and one recombinant (T3DS-3S(s)S·3S(s)L) with stem rust resistance were identified and confirmed to be genetically compensating on the basis of genomic in situ hybridization, analysis of 3A, 3B, 3D and 3S(s)S-specific SSR/STS-PCR markers, and C-banding. In addition, nine SSR/STS-PCR markers of 3S(s)S-specific were developed for marker-assisted selection of the resistant gene. Efforts to reduce potential linkage drag associated with 3S(s)S of Ae. searsii are currently under way.

Development and characterization of wheat-Ae. searsii Robertsonian translocations and a recombinant chromosome conferring resistance to stem rust

Stem rust (caused by Puccinia graminis Pers.:Pers. f. sp. tritici Eriks. & E. Henn.), a dev- astating disease of wheat (Triticum aestivum L.), was effectively controlled worldwide for the past 50 yr by deployment of stem rust resistance (Sr) genes in wheat cultivars. However, a new stem rust race, TTKSK (known as Ug99 or TTKS) that emerged in eastern Africa, is a cause of con- cern because it has broad virulence to currently deployed Sr genes. To identify potentially new sources of effective Sr genes against Ug99, we evaluated and characterized the seedling responses to TTKSK of 62 wheat lines derived from crosses of common or durum wheat (T. turgidum L. ssp. durum) with the species Thi- nopyrum junceum, Th. intermedium, Th. bessar- abicum, Th. elongatum, Th. ponticum, Elymus rectisetus, Aegilops caudata, and Ae. speltoides. For stem rust evaluation, we fi rst tested all lines for reactions to races TTTT and TTKSK. Lines initially showing resistance to TTKSK were then retested with eight races, including three races in the TTKS lineage (TTKSK, TTKST, and TTTSK) and fi ve North American races (QFCS, QTHJ, RCRS, RKQQ, and TPMK). Thirty wheat-alien species derivatives had resistance to TTKSK, TTKST, and TTTSK. The comparisons of wheat- alien species derivatives and their parental lines for reactions to different races suggested that 12 partial amphiploids or amphiploids and four disomic addition lines may carry novel genes for stem rust resistance. These resistant lines rep- resent materials for introducing stem rust resis- tance genes into wheat breeding.

/pdf/evaluation-and-characterization-of-seedling-resistances-to-1pou9m98lo.pdf

Evaluation and Characterization of Seedling Resistances to Stem Rust Ug99 Races in Wheat-Alien Species Derivatives

Stem rust, caused by Puccinia graminis f. sp. tritici (Pgt), is one of the most serious diseases of bread (Triticum aestivum L.) and durum (Triticum durum Desf.) wheat worldwide. The discovery of new Pgt races in Africa, Ug99 and its variants, brings a new threat to global wheat production. Currently, the research of stem rust in wheat is focusing on identifying further resistance genes to control Ug99 and its derivatives. Some resistance genes, which were identified from wild relatives and chromosomal regions conferring resistance to stem rust, were also detected using QTL analysis and genome wide association studies. Additionally, development of molecular markers linked to stem rust resistance (Sr) genes is one of the focus areas of current research. These molecular markers play a key role in the genetic characterization of the new sources of resistance as well as in stacking two or more resistance genes in a single line. Pyramiding several, major and minor, stem rust resistance genes into adapted varieties as opposed to breeding varieties with a single resistance gene is considered a more effective method to combat new races. Therefore, recent progress on molecular marker development and improved donor sources are accelerating the pyramiding and deployment of cultivars with more durable resistance to stem rust.

	Key words: Molecular markers, resistance, Sr genes, stem rust, Ug99.

Status of genetic research for resistance to Ug99 race of Puccinia graminis f. sp. tritici: A review of current research and implications

The S genome of Aegilops speltoides is closely related to the B and G genomes of polyploid wheats. However, little work has been reported on the genetic relationships between the S-genome and B-genome chromosomes of polyploid wheat. Here, we report the isolation of a set of disomic substitutions (DS) of S-genome chromosomes for the B-genome chromosomes and their effects on gametophytic and sporophytic development. Ae. speltoides chromosomes were identified by their distinct C-banding and fluorescence in situ hybridization patterns with the Ae. speltoides-derived clone pGc1R-1. Although no large structural differences between S-genome and B-genome chromosomes exist, significant differences in gametophytic compensation were observed for chromosomes 1S, 3S, 5S and 6S. Similarly, chromosomes 1S, 2S, 4S, 5S and 6S affected certain aspects of sporophytic development in relation to spike morphology, fertility and meiotic pairing. The DS5S(5B) had disturbed meiosis with univalents/multivalents and suffered chromosome elimination in the germ tissues leading to haploid spikes in 50% of the plants. The effect of the Ph1 gene on meiosis is well known, and these results provide evidence for the role of Ph1 in the maintenance of polyploid genome integrity. These and other data are discussed in relation to the structural and functional differentiation of S- and B-genome chromosomes and the practical utility of the stocks in wheat improvement.

Genetic compensation abilities of Aegilops speltoides chromosomes for homoeologous B-genome chromosomes of polyploid wheat in disomic S(B) chromosome substitution lines.

Durum wheat (Triticum turgidum L., 2n = 4x = 28; AABB genomes) is the preferred wheat for preparing pasta products. Current durum cultivars have little resistance to Fusarium head blight (FHB), or scab, a ravaging fungal disease. In our Durum Germplasm Enhancement (DGE) project, we previously showed that chromosome 1E of diploid wheatgrass Lophopyrum elongatum (Host) A. Love (2n = 2x = 14; EE genome) is an excellent source of FHB resistance. Through hybridization of this grass with the durum cultivar Langdon, coupled with manipulation of chromosome pairing and selection, we added the chromosome 1E pair into the durum genome and produced a fertile durum 1E addition line, called DGE-1. Also by substituting 1E for the durum homoeologues 1A or 1B, we produced, two fertile disomic substitution lines, 1E(1A) and 1E(1B), which were designated as DGE-2 and DGE-3, respectively. The level of FHB resistance of the genetic stocks was studied in the greenhouse and in the field at Prosper, North Dakota. In the greenhouse, the 1E addition showed 32.15 % infection and was better than Langdon at 58.73 %, but not significantly different from 1E(1A) disomic substitution at 45.60 % infection. The 1E(1B) disomic substitution (88.29 % infection) had the highest FHB infection. Under natural conditions in the field, the observed infection rates were essentially similar. Among the three genetic stocks, the 1E addition DGE-1 had the lowest level of infection (at 10 %) followed by the substitution 1E(1A) at 20 %, and the substitution E(1B) at 70 %. The parental cultivar Langdon had 15 % FHB infection. The genetic stocks of durum may help produce several more alien addition and substitution lines involving different chromosomes and may help locate sources of FHB resistance.

Durum wheat genetic stocks involving chromosome 1E of diploid wheatgrass: resistance to Fusarium head blight

In the past 50 years, a number of stem rust (Sr) resistance genes identified from wild relatives of wheat have been incorporated into wheat genomes through chromosome engineering. Some of these genes, including Sr25, Sr26, Sr32, Sr37, Sr39, Sr40, Sr43, Sr44, SrR, and three unnamed novel Sr genes from Aegilops speltoides and Haynaldia villosa, are effective against Ug99. However, the alien chromosomal segments possess deleterious genes in addition to the Sr genes. To enhance the utility of these Sr genes in wheat breeding, we have been eliminating deleterious linkage drag associated with these Sr genes through homoeologous recombination. The ph1b mutant was used to induce recombination between the alien chromosomal segments and their homoeologs in wheat. Recombinants with reduced alien chromatin were identified and characterized through stem rust testing, molecular marker analysis, and fluorescent genomic in situ hybridization (FGISH). To date, several resistant lines with modified or shortened alien chromosomal segments have been developed. Lines containing modified alien chromosome segments with genes SrR, Sr26, Sr32, and Sr39 are undergoing backcrossing and field evaluation for yield and quality characteristics in wheat breeding programs.

/pdf/chromosome-engineering-to-enhance-utility-of-alien-derived-xi886x2w5l.pdf

Chromosome engineering to enhance utility of alien-derived stem rust resistance

Although many wild relatives in the Triticeae tribe have been exploited to transfer stem rust resistance genes to wheat, the derived germplasms have often not been immediately useful in wheat breeding programs. Too frequently, large chromosome segments surrounding desirable genes also harbor deleterious genes that result in unacceptable yield or quality. Recombination between chromosomes of wheat and chromosomes of distant relatives is very rare due to genetic restrictions on chromosome pairing in polyploid wheat. However, chromosome pairing can be manipulated by utilizing mutant stocks that relax this tight genetic control. The ph1b mutant produced by E.R. Sears over 30 years ago is an invaluable chromosome engineering tool, readily employed in the age of high-throughput molecular genetics. Shortened translocations have already been produced for stem rust resistance genes Sr26 and SrR using ph1b-induced homoeologous recombination. We are currently using induced-homoeologous recombination to reduce the sizes of alien chromosome segments surrounding TTKSK-effective genes Sr32, Sr37, Sr39, Sr40, Sr43, Sr47, SrTt3, Sr2S#1 and SrAeg5 to eliminate linkage drag putatively associated with these genes. Additional TTKSK-effective genes Sr44, SrHv6, SrAsp5, and SrAse3 were first targeted for development of compensating translocation stocks and then for shortening the size of each alien segment. Population development is also underway to characterize several potentially new sources of resistance.

Cytogenetic manipulation to enhance the utility of alien resistance genes.

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Chromosome engineering to enhance utility of alien-derived stem rust resistance

Cytogenetic manipulation to enhance the utility of alien resistance genes.