Thesis (Ph. D.)--Ohio State University, 2007.

Fusarium oxysporum f. sp. vasinfectum (Fov) was first identified in Australia in 1993, and has since become one of the most significant threats to the country’s thriving cotton industry. The interaction between a unique Australian biotype of Fov and cotton hosts with varying susceptibilities to Fusarium wilt was studied. This research described the infection process and associated host defence mechanisms of two commercial cotton varieties after inoculation with Fov, and quantified their subsequent accumulation of antimicrobial terpenoids. / A rapid, reliable glasshouse bioassay that correlated with field resistance was developed for the study of Fusarium wilt of cotton. Detailed observations of the infection process obtained through light microscopy were used to formulate the disease cycle of Australian Fusarium wilt cotton. Using pathogen growth assays, varietal differences in root exudates and vascular tissues in the cotton hosts were documented. Root diffusate from the most susceptible cotton variety to Fusarium wilt, Siokra 1-4, contained a lipophilic compound that promoted the germination of Fov microconidia. On the other hand, a lipophilic compound present in diffusate from the least susceptible variety, Sicot 189, inhibited the growth of Fov germ tubes. / A bioassay using inoculated whole plants showed that Fov colonisation of the vascular tissues of Sicot 189 was restricted after 3 days. The basis for this inhibition was investigated further using light and transmission electron microscopy. Infection induced the reorganisation of contact cells in host vascular tissue, including an increase in cytoplasmic content and the partitioning of vacuoles, which was concurrent with the accumulation of materials in adjacent vessel lumens, via pits. Histochemical analysis indicated these globular materials secreted into the vessels were terpenoids. These structural and terpenoid responses in Siokra 1-4 and Sicot 189 were similar, however, they were more intense and rapid in the latter, less susceptible variety. The responses in Sicot 189 also corresponded to the time period that pathogen inhibition was observed. Thus, a correlation was demonstrated between the rapid and intense induction of both structural and biochemical responses with decreased susceptibility to Fusarium wilt. Detailed HPLC analysis of vascular tissues confirmed that terpenoids accumulated more rapidly and at higher concentrations in the less susceptible cotton variety. These findings provided strong evidence for the involvement of antimicrobial terpenoids in the determination of Fusarium wilt susceptibility of Australian cotton varieties. / This work represents the most complete survey to date of the interaction of Australian biotypes of Fov with cotton. These insights can contribute to future cotton breeding efforts and cultural management of Fusarium wilt in the field. Thus, each part of this study has advanced complementary facets of our understanding of Fov, and has provided a framework from which future studies on phytoalexins and other putative cotton defences can be studied.

M.Sc. (Biochemistry) / Plants respond to various stress stimuli by activating a broad-spectrum of defence responses that can be expressed locally at the site of pathogen infection (hypersensitive response-HR) as well as systemically in uninfected tissue (systemic acquired resistance-SAR). The ability to continuously respond to both abiotic and biotic stimuli leads to changes in the plants’ physiology, morphology and development. Therefore, there is a need to define and understand the mechanism of the plant defence system, including the mode of recognition, activation of signalling pathways and subsequent defence. In so doing, a long lasting and effective protection against various pathogens may be established. In the current study, the transcriptome status of cultured cells of Nicotiana tabacum was investigated using annealing control primer (ACP)-based differential display (DD) since it is an improved technology to compare patterns of gene expression in RNA samples, isolated from tissue / cells under different biological conditions, using a novel priming system. Here, ACP-DDRT-PCR was used in combination with a next-generation sequencing technology, namely 454 pyro-sequencing, which is the only technique that generates longer reads which are suitable for de novo assembly and annotation of non-model plants like tobacco of which the genome is not yet published in Genbank. SAR occurs following induction by biotrophic or necrotising pathogens. However, it can also be manifested artificially after chemical treatment. In this study, isonitrosoacetophenone (INAP), a novel compound that was originally isolated from extracts of citrus peel undergoing oxidative stress, was used as a chemical inducer and it was hypothesised that this compound induces defence-related responses in plants. In order to investigate this, tobacco cell suspensions were elicited with 1 mM INAP, followed by ACP-DDRT-PCR and subsequent identification of differentially expressed genes using pyro-sequencing.