Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2005. / Includes bibliographical references (leaves 92-94). Also available in electronic version.

Eucalyptus species form an integral part of the South African forestry industry and their uses extend from paper and pulp production to the synthesis of essential oils which are used in various cosmetic products. Throughout their lifetime these hosts are naturally challenged with various pests and pathogens, most of which cause devastating diseases. An approach to curb the spread of pathogens is to enhance the defence response of the host. Most of the information pertaining to defence against pathogens stems from studies conducted in model organisms such as Arabidopsis, however such information is scarce in woody species such as Eucalyptus. It is understood, from model systems, that once the pathogen is perceived by the host, a cascade of defences are initiated such as the activation of salicylic acid (SA), jasmonic acid (JA) and ethylene (ET) signalling pathways. These pathways in turn activate the expression of genes involved in limiting the spread of the pathogen such as pathogenesis-related (PR) proteins. Certain PR genes have also been shown to be markers of the induction of a specific pathway e.g. PR2 is a marker for the SA pathway. This study aimed to elucidate marker genes specific to the SA (PR1, PR2 and PR5) and JA (PR3, PR4 and LOX) signalling pathways in Eucalyptus grandis using the genome sequence, bioinformatics tools and sequence information from other plant species. A co-phylogenetic approach using neighbour joining analysis and maximum likelihood was used to identify and add confidence in the selection of putative orthologs. Following the selection of orthologous markers, the expression profile of these candidate genes was assessed using Reverse transcriptase quantitative PCR (RT-qPCR). Transcript profiling was conducted under mock induction of the signalling pathways as well as under pathogen stress. For the mock induction of the pathways, the expression profiles of the putative marker genes were investigated under various concentrations of the inducer and at various time points. In the interaction with Chrysoporthe austroafricana it was observed that the SA signalling pathway could have a role in facilitating resistance due to the expression profile observed for EgrPR2. In the tolerant genotype (TAG5) this gene was induced at an earlier time point as opposed to the susceptible genotype (ZG14). These putative markers could provide a diagnostic tool for the screening of pathogen challenged eucalypts to determine which signalling pathway(s) are activated against various pathogens. In addition, this research adds to our knowledge of defence responses in E. grandis by elucidating genes that can be used as targets for improving resistance. Additionally this study provides a stepping stone for understanding mechanisms to curb future tree diseases. / Dissertation (MSc)--University of Pretoria, 2014. / Genetics / Unrestricted

Mitogen-activated protein kinases (MAPKs) belong to the most conserved signaling pathways and are found in all eukaryotes, including humans where they play important roles in various diseases and cancer. Stimulation of this signal transduction pathway by microbe-associated molecular patterns (MAMP) results in a multitude of events to regulate innate immune responses in Arabidopsis thaliana stimulating large-scale changes in gene expression. Starting from a phosphoproteomic screen in Arabidopsis thaliana wild type and mpk3, mpk4 and mpk6 mutants following microbe-associated molecular pattern (MAMP) treatment, several novel chromatin-associated proteins were identified that are differentially phosphorylated by stress-induced protein kinases. Arabidopsis Ras GTPase-activating protein SH3-domain-binding protein (AtG3BP-1) is a downstream putative substrate of the MAMP-stimulated MAPK pathway that is phosphorylated by MPK3, 4 and 6 in in vitro kinase assays. AtG3BP1 belongs to a highly conserved family of RNA-binding proteins in eukaryotes that link kinase receptormediated signaling to RNA metabolism. Here, we report the characterization of the Arabidopsis homolog of human G3BP1 in plant innate immunity. AtG3BP1 negatively regulates plant immunity and defense immune responses. Atg3bp1 mutant lines show constitutive stomata closure, expression of a number of key defense marker genes, and accumulate salicylic acid but not jasmonic acid. Furthermore, Atg3bp1 plants exhibit enhanced resistance to the biotrophic pathogen Pseudomonas syringae pv. tomato. Pathogen resistance was mediated by stomatal and apoplastic immunity in Atg3bp1. More generally, our data reinforce that AtG3BP1 is a key mediator of plant defense responses and transient expression of AtG3BP1 delivered striking disease resistance in the absence of yield penalty, highlighting a potential application of this gene in crop protection.

Digital dermatitis is a top cause of lameness in dairy cattle that results in ulcerative lesions on the feet. Topical salicylic acid has been shown to provide similar efficacy to the antibiotic drugs used previously, but there is no milk withholding time established in the United States. The objective of this study was to provide data in order to establish this withholding period. A secondary objective was to evaluate outcomes among treatments. Treatment groups were topical applications of the following drugs: salicylic acid paste, salicylic acid powder, and tetracycline. The lesions were scored at day 0, day 7, and day 28 post-treatment. Milk samples were collected the day before treatment, 4 hours, 8 hours, 24 hours, 36 hours, and 48 hours post-treatment. Results indicated that most cows did not show detectable levels of salicylic acid after 24 hours.

No description available.

Salmonella enterica serovar Typhimurium is one of the most prevalent food-borne pathogens, affecting millions around the world every year, making it a threat to global health. Salmonella possesses the ability to survive the normally lethal condition of desiccation, however, discovery of the genes and mechanisms behind this phenomenon are still ongoing. Using a transposon mutagenesis approach to construct a broad transposon library, this study aimed to uncover genes that may be contributing to changes in Salmonella’s survivability under desiccation, particularly when exposed to the antimicrobial molecule salicylic acid. Building on previous findings showing salicylic acid can alter cell viability through differential gene regulation, transposon mutants were exposed to salicylic acid and subsequently desiccated to screen for mutants that displayed an alteration in survival phenotypes. This work identified a transposon mutant with an interruption of the porin-coding gene ompC that displayed an augmented survivability phenotype under these conditions, leading to further exploration into the origin of this phenomenon.

Abiotic stress poses a significant threat to crop productivity and food security. In this study, we focused on understanding the role of SIP-428, a SABP2-interacting protein, in mediating plant responses to environmental stresses. Transgenic tobacco plants overexpressing SIP-428 were subjected to salinity and drought stress. The overexpression of SIP-428 led to diminished growth under both stress conditions, indicating a negative impact on stress tolerance. Specifically, SIP-428 overexpression resulted in a reduction in catalase activity, while peroxidase activity remained unaffected. These findings suggest that SIP-428 plays a negative regulatory role on the catalase activity during abiotic stress, which may contribute to the susceptibility of plants to such stresses. Understanding the molecular mechanisms underlying SIP-428's role in stress responses could potentially lead to strategies for enhancing stress tolerance in crops, thereby promoting sustainable agricultural practices.

Plants synthesize a diverse array of phytochemicals in response to interactions with herbivores, pathogens, and commensal microbes. These phytochemicals may simultaneously enhance crop defense and quality, representing a potential pest management strategy. However, plant chemical responses to different types and levels of biotic interactions remain unclear, particularly in fruit tissues, and the feasibility of inducing these defenses through elicitor application in field environments also requires further examination. Thus, apples were used to 1) examine the impact of distinct communities of biotic interactions among plants, insects, and microbes on fruit phenolic chemistry, and 2) examine the impact of the phytohormones jasmonic acid (JA), salicylic acid (SA), and melatonin (M) on fruit phenolic chemistry and resistance against pests and pathogens. Ultimately, phenolic defenses were induced by fungal damage primarily in ripe pulp tissues, where there was also a positive relationship between fungal endophyte and phenolic diversity, supporting a broad hypothesis that chemical diversity may increase with biotic diversity. Additionally, two compounds were upregulated in response to fungal damage: chlorogenic acid and an unidentified benzoic acid. Elicitor applications did not affect phenolic chemistry, but the combined application of JA-SA analogues had some chemical or physical effect, as this treatment reduced emergence of the insect Rhagoletis pomonella. Thus, fruit induced defenses may be tissue-specific and subject to temporal, environmental, or genotypic variation. Overall, these chapters examined the relationship between biotic interactions and induced fruit chemistry, with the goal of improving understanding of plant-microbe-insect interactions and incorporating these interactions into more sustainable agricultural practices. / Master of Science / Plants may produce a diverse array of defensive phytochemical compounds in response to interactions with herbivores, pathogens, and the microorganisms that reside within plant tissues. These phytochemicals may simultaneously improve crop defenses and quality, representing a potential agricultural management strategy. However, plant chemical responses to different types and levels of biotic interactions are not well-understood, particularly in fruit tissues, and the feasibility of activating these defenses in fruits through the application of phytohormones that regulate defense pathways as a potential management strategy also requires further examination. Thus, apples were used to 1) examine the impact of distinct communities of biotic interactions among plants, insects, and microbes on fruit chemistry, focusing on phenolics, an important class of phytochemical compounds, and 2) examine the impact of the defense-activating phytohormones jasmonic acid (JA), salicylic acid (SA), and melatonin (M) on fruit phenolic chemistry and resistance against pests and pathogens. Ultimately, phenolic defenses were activated by fungal damage primarily in ripe pulp tissues, where there was also a positive relationship between fungal endophyte and phenolic diversity, supporting a broad hypothesis that chemical diversity may increase with biotic diversity. Additionally, two compounds were produced in response to fungal damage: chlorogenic acid and an unidentified benzoic acid. Phytohormone applications did not affect phenolic chemistry, but the application of the combined JA-SA analogues had some chemical or physical effect, as this treatment reduced emergence of the insect Rhagoletis pomonella. Overall, the phytochemical defenses activated by biotic interactions in fruits may occur primarily in certain tissue types, and may also vary due to environmental conditions, time of year, or plant species. These chapters examined the relationship between fruit chemistry and biotic interactions with the goal of improving understanding of plant-microbe-insect interactions and incorporating these interactions into more sustainable agricultural practices.

1. From a study of the system salicylic acid-biphenyl it was concluded that there was no compound formation in the system. The solution is very nearly ideal, having an eutectic temperature of 67.6℃ at a mole fraction of .903 for the biphenyl. 2. The system salicylic acid-diphenylamine was studied and no compound formation was found to be present. The system and no compound formation was found to be present. The system is not as ideal as the salicylic acid-biphenyl system, but follows the ideal solution curve fairly well. The system has an eutectic temperature of 48.5℃ at .926 mole fraction of diphenylamine. / M.S.

Salicylic acid binding protein 2 (SABP2) is known to play a key role in Salicylic acid mediated defense pathway. SBIP-470 is SABP2 interacting protein that might be putatively involved in transfer of lipids. SBIP-470 was cloned without the signal peptide and expressed in E. coli. In vitro lipid binding assay using recombinant SBIP-470 failed to detect lipid binding. In vitro lipid transfer assay showed recombinant SBIP-470 does not transfer phospholipid. Study has shown that SBIP-470 is highly inducible upon infection with viral as well as bacterial pathogens. Induction of SBIP-470 expression upon the TMV infection most likely depends upon the SABP2 while its expression upon non-host bacterial pathogens is most probably inhibited by the SABP2. A study of Arabidopsis knockout mutants (ltp12 mutant and ltp2 mutant) lacking the SBIP-470 homolog genes showed defects in growth phenotype, and they were found susceptible to bacterial pathogens.