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The characterisation of resistance to Stagonospora nodorum in two Triticum tauschii accessions

Murphy, Noel Edward Anthony (1997) The characterisation of resistance to Stagonospora nodorum in two Triticum tauschii accessions. PhD thesis, Murdoch University.

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Septoria nodorum blotch is an important leaf disease of wheat that is found throughout the wheat growing regions of the world and can decrease wheat yields by up to 50%. In Western Australia it is the most important leaf disease of wheat. It is caused by the ascomycete fungus Phaeosphaeria nodorum E. Mueller (anamorph Stagonospora nodorum (Berk.) Castellani & E.G. Germano). Few spring wheat cultivars have combined good levels of resistance with other quality and agronomic traits. One of the reasons for the lack of success in developing resistant cultivars is that highly effective resistances are often polygenic and are not available in well adapted backgrounds. A potentially novel source of resistance to this fungus is the wild wheat Triticum tauschii (Goss.) Schmal. This project aimed to enhance the utilisation of resistance in Triticum tauschii by characterising the inheritance of resistance to septoria nodorum blotch in two accessions of T. tauschii, identifying a molecular marker linked to those resistances and to study a population of S. nodorum to determine the extent of the genetic diversity in the pathogen in the wheatbelt of Western Australia.

A collection of S. nodorum isolates was made by isolating ascospores from wheat straw collected from numerous locations in the wheatbelt of Western Australia. Molecular approaches were chosen to study the genetic variability of the pathogen. When compared with Random Amplified Polymorphic DNA (RAPD) and Sequence Tagged Site-Polymerase Chain Reaction (STS-PCR), Restriction Fragment Length Polymorphism (RFLP) was found to be the most appropriate and reliable method to compare genetic variability between the fungal isolates. Low copy number RFLP probes from a genomic restriction digest of a S. nodorum isolate were identified and used in the population study. A high copy number probe which produced a fingerprint was used to identify fungal clones. Considerable variation was found between the isolates in the population. Ners measure of gene diversity indicated that the level of diversity present at an individual RFLP locus was high for two of the low copy number probes. A genotypic diversity of 95% for the fingerprint haplotypes also indicated high levels of genetic variability in the population. The levels of variation of the pathogen detected in this study were similar to those reported for other populations of S. nodorum from the USA and Switzerland. Disease screening should be undertaken with a wide range of S. nodorum isolates that represent a broad range of diversity for identification of resistant germplasm.

To study the inheritance of resistance to S. nodorum in T. tauschii, two accessions (AUS21712, RL5271) that had been identified previously as resistant to septoria nodorum blotch were used in two crosses with a common susceptible T. tauschii accession (CPU110889). The progeny of these crosses were screened for resistance against a single isolate of S. nodorum in the F1, F2 and F3 generations. Two components of resistance, the incubation period and the lesion number per square centimetre, were also scored in the F1 and F2 generations. The resistant parents were found to differ from the susceptible parent in both of the components. Incubation period was found to be unreliable in the F1 and F2 disease screening as parental variation was not always observed. The lesion number per square centimetre was more reliable but had a lower broad sense heritability than disease severity and was more heavily influenced by the environment. The average response of the FI for Disease Score was intermediate between the two parents indicating incomplete dominance for both crosses. In the F2 up to 100% of all the plants could be classified as a parental type for the traits measured, a result consistent with a simply inherited resistance. In the progeny screening of the F3 generation up to 20% of the families with high levels of resistance were recovered, and this was an indication that resistance to S. nodorum was simply inherited. In Cross B the genotypic ratio of a sub-population of F3 families was not significantly different from the ratio expected for a single gene. The results were consistent with a single gene controlling resistance in Cross B and in Cross A the resistance appears to be simply inherited.

Progeny of the two crosses also provided the genetic information and plant material required to search for a molecular marker linked to resistance using an F2 Bulked Segregant Analysis. The DNA extracted from 8 to 12 F2 individuals were bulked together to form bulks of homozygous resistant and homozygous susceptible F2 plants. These bulks were analysed for polymorphisms with RAPD primers. Three marker bands potentially linked to resistance were identified in both crosses. The three RAPD bands were cloned, labelled and used as RFLP probes against parental DNA that had been digested with nine different restriction enzymes. In Cross A the RAPD probe produced a polymorphism between the parents when the parental DNA was digested with HindIII. In Cross B one of the RAPD probes did not produce a polymorphism between the parents with any of the restriction enzymes used but a second RAPD marker produced a polymorphism when the parental DNA was double digested with HindIII and MseI. A specific band produced by the HinIII/MseI double restriction digest was found to segregate with resistance when tested against DNA from F2 individuals. The band’s presence or absence was correlated with resistance or susceptibility for fourteen of the seventeen (82.4%) F2 plants tested. The remaining RAPD markers require further development before their linkage to resistance can be tested.

The possibility of introgressing the resistance genes studied in this work into the commercial bread wheat cultivars is discussed. Tightly linked molecular markers could be useful in the introgression of these resistances into the commercial bread wheats.

Item Type: Thesis (PhD)
Murdoch Affiliation: Division of Science
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Loughman, Robert
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