Wild relatives of common wheat, Triticum aestivum, and related species are an important source of disease and pest resistance and several useful traits have been transferred from these species to wheat. C-banding and in situ hybridization analyses are powerful cytological techniques allowing the detection of alien chromatin in wheat. C-banding permits identification of the wheat and alien chromosomes involved in wheat-alien translocations, whereas genomic in situ hybridization analysis allows determination of their size and breakpoint positions. The present review summarizes the available data on wheat-alien transfers conferring resistance to diseases and pests. Ten of the 57 spontaneous and induced wheat-alien translocations were identified as whole arm translocations with the breakpoints within the centromeric regions. The majority of transfers (45) were identified as terminal translocations with distal alien segments translocated to wheat chromosome arms. Only two intercalary wheat-alien transloctions were identified, one induced by radiation treatment with a small segment of rye chromosome 6RL (H25) inserted into the long arm of wheat chromosome 4A, and the other probably induced by homoeologous recombination with a segment derived from the long arm of a group 7 Agropyron elongatum chromosome with Lr19 inserted into the long arm of 7D. The presented information should be useful for further directed chromosome engineering aimed at producing superior germplasm.

Characterization of wheat-alien translocations conferring resistance to diseases and pests: current status

The recent advances in alien gene transfer from distantly-related species into wheat are reviewed in the present paper The main achievements during the last ten years include the great expansion of the range of wide hybridization and development of new techniques for production and characterization of wheat-alien chromosome translocations Updated results of wide hybridization since 1983 and comprehensive characterization of wheat-alien translocation lines in our laboratory are compiled The future outlook for alien gene transfer in wheat is also discussed

Recent advances in alien gene transfer in wheat

Genomic in situ hybridization was used to identify alien chromatin in chromosome spreads of wheat, Triticum aestivum L., lines incorporating chromosomes from Leymus multicaulis (Kar. and Kir.) Tzvelev and Thinopyrum bessarabicum (Savul. and Rayss) Love, and chromosome arms from Hordeum chilense Roem. and Schult, H. vulgare L. and Secale cereale L. Total genomic DNA from the introgressed alien species was used as a probe, together with excess amounts of unlabelled blocking DNA from wheat, for DNA:DNA in-situ hybridization. The method labelled the alien chromatin yellow-green, while the wheat chromosomes showed only the orange-red fluorescence of the DNA counterstain. Nuclei were screened from seedling root-tips (including those from half-grains) and anther wall tissue. The genomic probing method identified alien chromosomes and chromosome arms and allowed counting in nuclei at all stages of the cell cycle, so complete metaphases were not needed. At prophase or interphase, two labelled domains were visible in most nuclei from disomic lines, while only one labelled domain was visible in monosomic lines. At metaphase, direct visualization of the morphology of the alien chromosome or chromosome segment was possible and allowed identification of the relationship of the alien chromatin to the wheat chromosomes. The genomic in-situ hybridization method is fast, sensitive, accurate and informative. Hence it is likely to be of great value for both cytogenetic analysis and in plant breeding programmes.

Genomic in situ hybridization to identify alien chromosomes and chromosome segments in wheat.

Investigations of the world wheat genepool conducted by Ukraine's National Centre for Plant Genetic Resources produced information on the geographic origins and genetic background of numerous wheat lines possessing the 1BL/1RS wheat-rye translocation, 1B(R) substitution and 1AL/1RS translocation. Wheat-rye translocations can determine high productivity, adaptive possibilities, and disease and insect resistance in wheat. Data on 330 cultivars and lines carrying wheat-rye translocations and/or substitutions were analysed. Based on this information, wheats were classified according to continent and country of origin and year of release. This clarified how translocation sources of different genetic and geographic origins have been used in breeding programmes all over the world.

Importance of wheat-rye translocations for breeding modern cultivars of Triticum aestivum L.

The barley Mla locus confers multiple resistance specificities to the obligate fungal biotroph, Blumeria (= Erysiphe) graminis f. sp. hordei. Interspersed within the 240 kb Mla complex are three families of resistance gene homologs (RGHs). Probes from the Mla-RGH1 family were used to identify three classes of cDNAs. The first class is predicted to encode a full-length CC-NBS-LRR protein and the other two classes contain alternatively spliced, truncated variants. Utilizing a cosmid that contains a gene corresponding to the full-length candidate cDNA, two single-cell expression assays were used to demonstrate complementation of AvrMla6-dependent, resistance specificity to B. graminis in barley and wheat. The first of these assays was also used to substantiate previous genetic data that the Mla6 allele requires the signaling pathway component, Rar1, for function. Computational analysis of MLA6 and the Rar1-independent, MLA1 protein reveals 91.2% identity and shows that the LRR domain is subject to diversifying selection. Our findings demonstrate that highly related CC-NBS-LRR proteins encoded by alleles of the Mla locus can dictate similar powdery mildew resistance phenotypes yet still require distinct downstream signaling components.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-313x.2001.00982.x

The MLA6 coiled‐coil, NBS‐LRR protein confers AvrMla6‐dependent resistance specificity to Blumeria graminis f. sp. hordei in barley and wheat

Powdery mildew resistance of Amigo wheat has been analyzed and mapped by combining F 2 segregation data with cytogenetic and storage protein analyses. It is concluded that the powdery mildew resistance of Amigo is regulated by one gene with conditioned dominance. Cytogenetic and electrophoretic analyses revealed that the complete wheat chromosome arm 1AS is missing in Amigo and has been replaced by the rye chromosome arm 1RS. No susceptible recombinant was found among 1,034 F 2 plants of crosses of Amigo with Pm3 resistant lines; Pm3 is known to be located on 1AS (...)

Chromosomal location of the powdery mildew resistance gene of Amigo wheat.

Progenies of a tetraploid 1BL/1RS wheat-rye translocation line, CV 256, selected from the cross ‘Cando’ x ‘Veery’, were analyzed by means of Giemsa C-banding. CV 256 is cytologically stable for the presence of the 1BL/1RS translocation but still segregating for A- and B-genome chromosomes of ‘Cando’ and ‘Veery’. In CV 256, nucleolar activity of the 1RS NOR locus is suppressed, as judged by the absence of a secondary constriction in that rye segment and the capability of organizing nucleoli. PAGE analysis of prolamins confirmed the presence of two 1RS secalins in all single seeds analyzed. SDS-PAGE analysis of reduced glutenins of single seeds indicated that some seeds contained the ‘Cando’ Glu-B1 locus (subunits 6+8), some contained the ‘Veery’ Glu-B1 locus (subunits 7+9) while others contained all four subunits, indicating that the material was heterozygous. Pm8 resistance is expressed in the tetraploid 1BL/1RS translocation line based on the reactions of six well-defined powdery mildew isolates. However, Pm8 resistance is not expressed in the hexaploid wheat cultivars ‘Olymp’, ‘Heinrich’ and ‘Florida’, which also contain the 1BL/1RS translocation. Obviously, the existence of the 1BL/1RS translocation is not a proof for the expression of the associated genes. PAGE results did not show a clear linkage between powdery mildew resistance and the presence of 1RS secalins.

Cytological characterization, powdery mildew resistance and storage protein composition of tetraploid and hexaploid 1BL/1RS wheat-rye translocation lines.

B. Friebe

Papers

Chromosomal location of the powdery mildew resistance gene of Amigo wheat.

Cytological characterization, powdery mildew resistance and storage protein composition of tetraploid and hexaploid 1BL/1RS wheat-rye translocation lines.