Browsing Doctoral Degrees (Genetics) by Author "Gouws, Liezel Michelle"
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- ItemThe molecular analysis of the effects of lumichrome as a plant growth promoting substance(Stellenbosch : University Stellenbosch, 2009-12) Gouws, Liezel Michelle; Kossmann, Jens; Kossmann, J. M.ENGLISH ABSTRACT: Through powerful signal molecules, rhizobacteria affect fundamental processes in plants. In recent years, a number of novel rhizobial molecules have been identified that positively affect plant growth and development. Previous studies have shown that Sinorhizobium meliloti, which form symbiotic relationships with leguminous plants, increases CO2 availability by enhancing root respiration in alfalfa. The active compound was identified as lumichrome, a previously unrecognized rhizosphere signal molecule that has been shown to promote plant growth in various studies. Lumichrome is a common breakdown product of riboflavin and produced by both chemical and biological factors. Various studies on lumichrome have proven its growth promoting effect in the interaction with plants. The mechanism through which lumichrome increases plant growth remains to be clarified. This study provides new insight into the molecular effects of the plant growth promoter lumichrome on the root metabolism of plants. The main aim of the work presented in this thesis was to investigate the molecular mechanism of the plant growth promoting substance lumichrome in the roots of the model plants Lotus japonicus and Solanum lycopersicon (tomato). To asses the impact of lumichrome on the root metabolism of Lotus japonicus and tomato and identify key genes involved in the growth stimulation, a comprehensive profile of differentially expressed genes, proteins and metabolites was compiled. As the effects of lumichrome as a plant growth promoter have not previously been tested on Lotus japonicus and tomato, basic growth studies were completed to determine if lumichrome indeed elicits plant growth at nanomolar concentrations, as was proven in numerous previous studies. Both Lotus japonicus and tomato showed significant increases in root biomass when treated with 5 nM of lumichrome. The treatment with lumichrome caused complex changes in gene expression. Generally, transcript profiling showed that the categories that were predominantly affected by lumichrome in both Lotus and tomato, were genes associated with RNA regulation of transcription and signaling, protein synthesis/degradation/modification and stress and defence. Proteomic studies revealed that the majority of the differentially expressed proteins were down-regulated. Lumichrome seems to largely influence proteins involved in protein folding and down-regulate proteins involved in glycolysis. Proteomics studies revealed that GS1 (Lotus) and GAPDH (Lotus and tomato) were present in lower abundance in lumichrome treated roots, therefore targeted analysis utilizing northern blots, western blots and the measurement of enzyme activities were completed to determine and verify their specific role in the lumichrome mediated growth promotion. The results indicated that GAPDH and GS1 seem to be under post-translational modification. The influence of lumichrome on the metabolome of Lotus roots was immense, however minute in tomato roots. The knowledge gained in the parallel analyses of both Lotus japonicus and tomato aided us in finding key genes involved in the growth stimulation. Overall, one of the most significant observations was that for the first time to our knowledge, six genes related to defence and pathogen responses were identified that are concurrently expressed in both Lotus and tomato. Through identifying a small number of genes involved in mediating the growth stimulation, these can be used for their functional analysis in the future, using reverse genetics to provide more insight into the molecular mechanisms that are triggered by lumichrome as a plant growth promoter.