Characterization of an early senescent mutant in Arabidopsis thaliana

grantor: University of TorontoIn an attempt to elucidate the mechanisms of CO2-mediated regulation of gene expression in plants, 10 to 14 day old T-DNA mutagenized Arabidopsis thaliana seedlings grown at ambient levels of CO2 were exposed to elevated CO2 conditions of 3000 ppm for four days. Plants exhibiting a phenotype that deviated from the wild type response in elevated CO2 conditions were placed in two broad categories: insensitive and hypersensitive plants. The hypersensitive plants were further subdivided by phenotype and one mutant exhibiting e_arly s_enescence of c_otyledons ('esc'1) was chosen for further study. The 'esc'1 mutant exhibits accelerated senescence of its cotyledons, is short in stature, and develops undulated, epinastic leaves. The mutation was not found to be closely linked with the T-DNA insertion; therefore, genetic mapping was used to identify the location of the mutation. The mutation was fine mapped to Chromosome I using molecular markers and was found to be near the G4026 marker. Structural analysis of tissue sections of the mutant with light microscopy and SEM suggested that the cells in the cotyledon appear to have collapsed prematurely. Biochemical analysis of the mutant indicated a decrease in the amount of chlorophyll and total carbohydrate levels were increased in the ' esc'1 cotyledons. Moreover, as the mutant phenotype is not rescued in ambient (360 ppm), or low (150 ppm) CO2 concentrations, it is believed that elevated levels of CO2 affect the ontogenic rate of development. Therefore, the 'esc'1 plant may not be directly involved in a CO2-mediated gene expression pathway. The mutant foliar phenotype was restored to a wild type phenotype when grown in the presence of 3% sucrose, but similar levels of glucose did not appear to rescue this mutant. In an attempt to isolate additional genes involved in this phenotype, a second mutant screen was performed. Six putative positives were identified in a tertiary screen. Of these six putative mutants, one appeared to have its phenotype rescued when grown on 3% sucrose, and thus may be allelic to the 'esc'1 locus. Taken together, these phenotypes suggest that the mutation has altered a process in carbohydrate transport or allocation, which results in accumulation of total carbohydrates in the cotyledon. This accumulation may repress photosynthetic genes, thereby prematurely initiating senescence of the cotyledons. The modified transport or allocation of carbohydrates is suggested here to cause the epinastic and deformed true leaves.Ph.D