Auxinic herbicide-resistant (i.e., resistant to 2,4-D and MCPA) wild radish (Raphanus raphanistrum L.) was discovered in the Western Australian wheatbelt, providing an opportunity to integrate auxinic herbicide resistance into cultivated radish (R. sativus L.) using conventional breeding methods. It was hypothesized that the inheritance of auxinic herbicide resistance in wild radish is conferred by a single, dominant nuclear gene and, therefore, will be relatively easy to introgress from wild radish to cultivated radish; and the mechanism of auxinic herbicide resistance in wild radish is through an altered target-site. Visual injury data of the F2 progeny suggested that resistance was conferred by a quantitative trait with the susceptible allele(s) exhibiting dominance with minor cytoplasmically inherited genes masking the susceptible trait. In conclusion, the resistance allele(s) were quantitative and, thus, make selection for resistance difficult. Therefore, the introgression of the resistance allele(s) was not successfully completed. To determine the mechanism of resistance, the wild radish plants resistant WARR6-26 (R) and susceptible WARR7-5 (S) were treated with radiolabeled MCPA. There was no difference in metabolism of [14C]MCPA between R and S plants. Based upon the decline in the total 14C recovered over 72 h in R and S it was clear that both were “losing” [14C]MCPA; however, R plants were losing MCPA more rapidly. It was hypothesized that because R plants exude 14C more rapidly from their roots than S plants, this accounted for the resistance of R plants.

The objective of this Ph.D. research was to identify new and novel mechanisms of wild radish (Raphanus raphanistrum L.) resistance to photosystem II (PSII) inhibitors, auxinics, and acetohydroxyacid synthase (AHAS) inhibitors. PSIIinhibitor resistance was demonstrated to be target-site based, and conferred by a Ser264 to Gly substitution of the D1 protein. Auxinic resistance was associated with reduced herbicide translocation to the meristematic regions of resistant wild radish plants. Two new resistance mutations of wild radish AHAS were discovered, including one encoding the globally rare Asp376 to Glu substitution, and another encoding an Ala122 to Tyr substitution, which has never been identified or assessed for resistance in plants previously. Characterization of the frequency and distribution of AHAS resistance mutations in wild radish from the WA wheatbelt revealed that Glu376 was widespread, and that some mutations of AHAS are more common than others. Computer simulation was used to examine the molecular basis of resistance-endowing AHAS target-site mutations. Furthermore, through the computer-aided analysis, residues were identified with the potential to confer resistance upon substitution, but which have not previously been assessed for this possibility. Results from this Ph.D. research demonstrate that diverse, unrelated mechanisms of resistance to PSII inhibitors, auxinics, and AHAS inhibitors have evolved in wild radish of the WA wheatbelt, and that these mechanisms have accumulated in some populations.