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When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.
Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.
Caleosins, calcium binding proteins characterized by a single EF hand domain, are known to be involved in ABA signaling and the response to abiotic stress in plants. They are encoded by gene family with seven members in the Arabidopsis thaliana genome. Caleosins have been shown to physically interact with the alpha subunit of the heterotrimeric G proteins in vitro and in vivo. The heterotrimeric G protein complex in Arabidopsis consists of three subunits; the α subunit, GPA1, the β subunit, AGB1, and one of three γ subunits named AGG1, AGG2 and AGG3 which are known to be involved in signalling pathways associated with development and the response to abiotic stresses. Caleosins and G protein subunits are known to physically interact, however the genetic interaction of these two classes of proteins and their combined roles in the stress response of plants is poorly understood. This study confirmed in planta interaction of GPA1 and CLO4, a member of the caleosin gene family in Arabidopsis and addressed questions arising from role of the interaction in plant signaling and stress responses. The in vivo promoter activity of CLO4 showed that the gene is negatively regulated in both primary and lateral roots by abscisic acid. CLO4 was found to positively regulate stomatal development, through GPA1 related signalling and both genes were found not to be individually sufficient to control lateral root elongation in response to ABA treatment. Arabidopsis plants mutated in both clo4 and gpa1 genes showed significant reduction of total lateral root length. Gain of function of the CLO4 gene through over-expression suggests that caleosins play a role in the ABA induced stomatal closure and ABA induced inhibition of primary root elongation. In addition to Arabidopsis CLO4, this study investigated the possible GTPase activity of another member of the caleosins gene family, RD20/CLO3, toward GPA1 and showed that the GAP activity of the RD20/CLO3 protein toward GPA1 is negligible compared to the other known plant GTPase accelerating protein, RGS1, which suggests different mechanism of regulation of the heterotrimeric G protein Complex through caleosins. This study also characterized the Extra-Large G protein gene family in Triticum aestivum. Total of three Xlgs per haploid geneome, each with three homeologous copies for the total of 9 genes in the hexaploid genome were identified and characterized for expression patterns. Differential tissue specific gene expression of the members of Xlg gene family showed that the genes are expressed in seed, root, inflorescence, leaf and stem. Bioinformatics analysis on the transcript level abundance showed that, under stress conditions such as drought, heat, cold and Fusarium graminearum infection, members of the gene family altered expression which suggests transcriptional regulation of certain members of Xlg gene family in response to biotic and abiotic stress conditions.