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

Leaf rust (caused by Puccinia recondita f. sp. tritici) is the most widespread and regularly occurring rust on wheat. Genetic resistance is the most economical method of reducing yield losses due to leaf rust. To date, 46 leaf rust resistance genes have been designated and mapped in wheat. Resistance gene expression is dependent on the genetics of host-parasite interaction, temperature conditions, plant developmental stage, and interaction between resistance genes with suppressors or other resistance genes in the wheat genomes. Genes expressed in seedling plants have not provided long-lasting effective leaf rust resistance. Adult-plant resistance genes Lr13 and Lr34 singly and together have provided the most durable resistance to leaf rust in wheat throughout the world. Continued efforts to isolate, characterize, and map leaf rust resistance genes is essential given the ability of the leaf rust fungus to overcome deployed resistance genes.

Genetics of resistance to wheat leaf rust.

A comprehensive, critical review on the present knowledge regarding the genetics of resistance of barley to the powdery mildew fungus is presented. The review deals with six kinds of resistance: Race-specific resistance; Mlo resistance; partial resistance; induced resistance; passive resistance; and non-host resistance. Most of the sections are subdivided into: phenotype of the interaction; resistance mechanisms; and genetics. A distinction is made between three groups of genes involved in the defense of plants to diseases: those that serve exclusively to mediate resistance; those that are mobilized to strengthen the plants' defense; and those that serve exclusively functions other than disease defense, but may bring about resistance. The more than 200 gene symbols assigned to race-specific mildew resistance genes over time are summarized and revised to 85 symbols that may be considered valid.

Genetics of Powdery Mildew Resistance in Barley

Wild Aegilops species related to cultivated wheat (Triticum spp.) possess numerous genes of agronomic interest and can be valuable sources of resistance to diseases, pests and extreme environmental factors. These genes can be incorporated into the wheat genome via intergeneric crossing, following, where necessary, the development of chromosome addition and substitution lines from the resulting hybrids. The transfer of a single segment from an alien chromosome can be achieved by translocations. The Aegilops (goatgrass) species, which are the most closely related to wheat, exhibit great genetic diversity, the exploitation of which has been the subject of experimentation for more than a century. The present paper gives a survey of the results achieved to date in the field of wheat–Aegilops hybridisation and gene transfer. The Aegilops genus consists of 11 diploid, 10 tetraploid and 2 hexaploid species. Of these 23 Aegilops species, most of the diploids (Ae. umbellulata Zhuk., Ae. mutica Boiss., Ae. bicornis (Forssk.) Jaub. & Spach, Ae. searsii Feldman & Kislev ex Hammer, Ae. caudata L., Ae. sharonensis Eig, Ae. speltoides Tausch, Ae. longissima Schweinf. & Muschl.) and several polyploids (Ae. ventricosa Tausch, Ae. peregrina (Hack. In J. Fraser) Marie & Weiller, Ae. geniculata Roth, Ae. kotschyi Boiss., Ae. biuncialis L.) have been used to develop wheat–Aegilops addition lines. Wheat–Aegilops substitution lines were developed using several species, including Ae. umbellulata, Ae. caudata, Ae. tauschii, Ae. speltoides, Ae. sharonensis, Ae. longissima and Ae. geniculata. Translocations carrying genes responsible for useful agronomic traits were developed with Ae. umbellulata, Ae. comosa, Ae. ventricosa, Ae. longissima, Ae. speltoides and Ae. geniculata. A large number of genes were transferred from Aegilops species to cultivated wheat, including those for resistance to leaf rust, stem rust, yellow rust and powdery mildew, and various pests (cereal cyst nematode, root knot nematode, Hessian fly, greenbug). Many molecular markers are linked to these resistance genes. The development of new molecular markers is also underway. There are still many untapped genetic resources in Aegilops species that could be used as resistance sources for plant breeding.

Utilisation of Aegilops (goatgrass) species to widen the genetic diversity of cultivated wheat

Chromosome engineering is a useful strategy for transfer of alien genes from wild relatives into modern crops. However, this strategy has not been extensively used for alien gene introgression in most crops due to low efficiency of conventional cytogenetic techniques. Here, we report an improved scheme of chromosome engineering for efficient elimination of a large amount of goatgrass (Aegilops speltoides) chromatin surrounding Sr39, a gene that provides resistance to multiple stem rust races, including Ug99 (TTKSK) in wheat. The wheat ph1b mutation, which promotes meiotic pairing between homoeologous chromosomes, was employed to induce recombination between wheat chromosome 2B and goatgrass 2S chromatin using a backcross scheme favorable for inducing and detecting the homoeologous recombinants with small goatgrass chromosome segments. Forty recombinants with Sr39 with reduced surrounding goatgrass chromatin were quickly identified from 1048 backcross progenies through disease screening and molecular marker analysis. Four of the recombinants carrying Sr39 with a minimal amount of goatgrass chromatin (2.87–9.15% of the translocated chromosomes) were verified using genomic in situ hybridization. Approximately 97% of the goatgrass chromatin was eliminated in one of the recombinants, in which a tiny goatgrass chromosome segment containing Sr39 was retained in the wheat genome. Localization of the goatgrass chromatin in the recombinants led to rapid development of three molecular markers tightly linked to Sr39. The new wheat lines and markers provide useful resources for the ongoing global effort to combat Ug99. This study has demonstrated great potential of chromosome engineering in genome manipulation for plant improvement.

https://www.genetics.org/content/genetics/187/4/1011.full.pdf

Targeted Introgression of a Wheat Stem Rust Resistance Gene by DNA Marker-Assisted Chromosome Engineering

A partially dominant gene for adult-plant leaf rust resistance together with a linked, partially dominant gene for stem rust resistance were transferred to the hexaploid wheat cultivar 'Marquis' from an amphiploid of Aegilops speltoides × Triticum monococcum by direct crossing and backcrossing. Pathological evidence indicated that the alien resistance genes were derived from Ae. speltoides. Differential transmission of the resistance genes through the male gametes occurred in hexaploid hybrids involving the resistant 'Marquis' stock and resulted in distorted segregation ratios. In heterozygotes, pairing between the chromosome arm with the alien segment and the corresponding arm of the normal wheat chromosome was greatly reduced. The apparent close linkage between the two resistance genes, 3 ± 1.07 crossover units, was misleading because of this decrease in pairing in the presence of the 5B diploidizing mechanism. The newly identified gene for adult-plant leaf rust resistance, located on chromosome 2B, is ...

Transfer to hexaploid wheat of linked genes for adult-plant leaf rust and seedling stem rust resistance from an amphiploid of Aegilops speltoides × Triticum monococcum

P. L. Dyck

Papers

Transfer to hexaploid wheat of linked genes for adult-plant leaf rust and seedling stem rust resistance from an amphiploid of Aegilops speltoides × Triticum monococcum