Doubled haploids (DHs) are becoming increasingly important in crop breeding programmes but methods for producing oat DHs remain inefficient. In this study haploid and DH oat plants were produced using the oat · maize hybridization method. Factors influencing the rate of caryopsis and haploid embryo production including genotype, post-pollination plant growth regulator application and temperature were investigated. The four growth regulators tested showed significant differences in their capacity to induce caryopsis formation with dicamba producing the highest numbers of caryopses, followed by picloram, 2,4-dichlorophenoxyacetic acid (2,4-D) and gibberellic acid (GA 3 ). No significant differences were observed between these growth regulators for their effect on embryo production. The concentration of dicamba was also important and was found to influence caryopsis but not embryo production, with 50 and 100 mg/l dicamba producing significantly more caryopses than 25 or 5 mg/l. Temperature had a significant impact on both caryopsis and embryo production with the magnitude and direction of response depending on genotype. Rates of haploid embryo production observed were between 0.8% and 6.7% of the pollinated florets. The proportion of haploids, which survived and were successfully doubled with colchicine following transfer to soil was between 72% and 81%.

Inheritance of resistance to a wheat midge, Sitodiplosis mosellana (Géhin), was investigated in spring wheats derived from nine resistant winter wheat cultivars. F1 hybrids were obtained from crosses between resistant winter wheats and susceptible spring wheats, and used to generate doubled haploid populations. These populations segregated in a ratio of 1:1 resistant to susceptible, indicating that a single gene confers the resistance. The F2 progeny from an intercross among spring wheats derived from the nine resistance sources did not segregate for resistance. Therefore, the same gene confers resistance in all nine sources of resistance, although other genes probably affect expression because the level of resistance varied among lines. Heterozygous plants from five crosses between diverse susceptible and resistant spring wheat parents all showed intermediate levels of response, indicating that resistance is partly dominant. Susceptible plants were reliably discriminated from heterozygous or homozygous resistant ones in laboratory tests, based on the survival and development of wheat midge larvae on one or two spikes. This powerful resistance gene, designated Sm1, is simply inherited and can be incorporated readily into breeding programmes for spring or winter wheat. However, the use of this gene by itself may lead to the evolution of a virulent population, once a resistant cultivar is widely grown.

No abstract

The common wheat cultivar Thatcher and the backcross derivative Canthatch are moderately or fully susceptible to several races of stem rust because of a suppressor on chromosome 7DL that inhibits the expression of the relevant resistance gene(s). The incorporation of leaf rust resistance gene Lr34 into 'Thatcher' is known to enhance stem rust resistance. The effect of this gene in a 'Canthatch' background and its relationship with the 7DL suppressor were determined by replacing chromosome 7D of 'Canthatch' with 7D of 'Chinese Spring', which possesses Lr34 on the short arm. 'Canthatch' nullisomic 7D was crossed with 'Chinese Spring', followed by a succession of backcrosses to the nullisomic recurrent parent. Homozygous resistant disomic and monosomic substitution lines were recovered that exhibited the same resistant reaction as that of 'Thatcher' possessing Lr34 and as that of 'Canthatch' nullisomic 7D, in which the 7DL suppressor is absent. The results indicate that, in 'Canthatch', Lr34 permits expression of resistance genes normally inhibited by the 7DL suppressor. Furthermore, it is likely that, in 'Thatcher' and 'Thatcher' back-cross derivatives, Lr34 inactivates the 7DL suppressor.

We previously reported that treatment with aluminum (Al) leads to the accumulation of several polypeptides (12‐, 23‐, and 43.5‐kDa) in root exudates of an Al‐resistant cultivar of Triticum aestivum. In this report, we examine the segregation of the 23‐kDa, Al‐induced polypeptide and the Al‐resistant phenotype in single F2 plants arising from a cross between Al‐resistant and Al‐sensitive doubled‐haploid (DH) lines. Single plants and plant populations were screened for sensitivity/resistance to Al using synthesis of 1,3‐β‐glucans (callose) as a sensitive marker for Al injury. Callose production in the Al‐sensitive cv. Katepwa was approximately 3‐fold higher than observed in the Al‐resistant cv. Maringa, or a near‐isogenic line derived from Katepwa and Maringa (Alikat), over a broad range of Al concentrations (0–100 μM). Similar results were observed with DH lines developed from cv. Katepwa, which produced two–four times more callose than DH lines developed from cv. Alikat. When single plants from F1 and F2 populations derived from a cross between DH Katepwa and DH Alikat were scored for Al‐induced callose production after 4 days exposure to 100 μM Al, all F1 plants were Al‐resistant and F2 plants segregated approximately 3:1 for Al‐resistance/sensitivity. A backcross population derived from crossing Al‐resistant F1 with Al‐sensitive Katepwa, segregated 1:1 for Al‐resistance/sensitivity. Thus, the Al‐resistant phenotype is inherited in a monogenic, dominant fashion in our DH lines. Enhanced accumulation of the Al‐induced, 23‐kDa polypeptide in root exudates was a trait which co‐segregated with the Al‐resistant phenotype in F2 populations. This polypeptide was strongly labeled with S‐methionine after 3 days of Al exposure and 6 h labeling. When root exudate polypeptides were separated by immobilized metal ion affinity chromatography, the 23‐kDa polypeptide demonstrated significant Al‐binding capacity. This polypeptide has been purified to near‐homogeneity, providing an opportunity to isolate the gene(s) encoding this polypeptide.