The plant hormone, abscisic acid (ABA), has previously been shown to have an impact on the resistance or susceptibility of plants to pathogens. In this thesis, it was shown that ABA had a regulatory effect on an extensive array of plant defence responses in three different plant and pathogen interaction combinations as well as following the application of an abiotic elicitor. In unique studies using ABA deficient mutants of Arabidopsis, exogenous ABA addition or ABA biosynthesis inhibitor application and simulated drought stress, ABA was shown to have a profound effect on the outcome of interactions between plants and pathogens of differing lifestyles and from different kingdoms. The systems used included a model plant and an important agricultural species: Arabidopsis thaliana (Arabidopsis) and Peronospora parasitica (a biotrophic Oomycete pathogen), Arabidopsis and Pseudomonas syringae pathovar tomato (a biotrophic bacterial pathogen) and an unrelated plant species, soybean (Glycine max) and Phytophthora sojae (a hemibiotrophic Oomycete pathogen), Generally, a higher than basal endogenous ABA concentration within plant tissues at the time of avirulent pathogen inoculation, caused an interaction shift towards what phenotypically resembled susceptibility. Conversely, a lower than basal endogenous ABA concentration in plants inoculated with a virulent pathogen caused a shift towards resistance. An extensive suppressive effect of ABA on defence responses was revealed by a range of techniques that included histochemical, biochemical and molecular approaches. A universal effect of ABA on suppression or induction of the phenylpropanoid pathway via regulation of the key entry point gene, phenylalanine ammonia-lyase (PAL), when stimulated by biotic or abiotic elicitors was shown. ABA also influenced a wide variety of other defence-related components such as: the development of a hypersensitive response (HR), the accumulation of the reactive oxyden species, hydrogen peroxide and the cell wall strengthening compounds lignin and callose, accumulation of SA and the phytoalexin, glyceollin and the transcription of the SA-dependent pathogenesis- related gene (PR-1). The near genome-wide microarray gene expression analysis of an ABA induced susceptible interaction also revealed an yet unprecedented insight into the great diversity of defence responses that were influenced by ABA that included: disease resistance like proteins, antimicrobial proteins as well as phenylpropanoid and tryptophan pathway enzymes. Subtle differences were found in the number and type of defence responses that were regulated by ABA in each type of plant and pathogen interaction that was studied. This thesis has clearly identified in plant/pathogen interactions previously unknown and important roles for ABA in the regulation of many defence responses.

Stress-induced changes in the emissions of volatiles from many economically-important plants have been demonstrated in studies over the past decade. Plants such as cotton and corn change both the composition and concentration of their emissions when subjected to wounding, herbivory and pathogen attack. Terpene-accumulating plants have been overlooked as potential objects of such studies although work on conifers has shown that species rich in constitutive defences can also exhibit induced responses. The aim of this study was to investigate whether terpene-accumulating plants respond to stress by altering their foliar emissions qualitatively and/or quantitatively. Species examined included Salvia officinalis (common sage), Melaleuca alternifolia (Australian tea tree) and Ocimum minimum (Bush basil). An experimental design was developed to eliminate factors such as leaf ontogeny that can affect the obtained results and complicate their interpretation. Small-scale solvent extraction and HS-SPME-based techniques were also developed; they enabled the quantitative determination of treatment-induced changes over periods ranging from 10 minutes to 6 months. Treatment of plants included mechanical wounding, herbivory, pathogen attack and chemical elicitation. Overall, statistically significant induced-changes were observed for both leaf-oil composition and concentration. The response of the different species used varied. O. minimum exhibited the greatest compositional changes whilst M. alternifolia was the only species for which oil-concentration changes were observed. The demonstrated changes were not as great as those reported in similar studies with non-terpene producing plants. The results indicate that the high metabolic costs associated with the production and storage of constitutive defences may be responsible for the limited induction of further defensive responses / Doctor of Philosophy (PhD)