Comparing the sucrose synthase promoters in Arabidopsis thaliana and hybrid poplar

Sucrose synthase is a glycosyl transferase enzyme responsible for catalyzing the reversible cleavage of sucrose into fructose and UDP-glucose in terrestrial plants. This role in sucrose breakdown has been associated with determining sink strength, modulating phloem loading, and possibly participating in cellulose and starch biosynthesis. In Arabidopsis thaliana, all six SuSy isoforms have been observed to localize to vascular tissues other than the xylem, a major site of cellulose deposition. However, it is highly possible that SuSy plays a more significant role in cellulose biosynthesis in other plant species, such as the aspen family (Populus spp.), where several isoforms have been identified to localize preferentially to the mature xylem. Previous studies have demonstrated that promoters play essential roles in transcriptional regulation, and that the regulatory elements found in their distal and proximal regions could be fundamental to regulating spatiotemporal gene expression. As such, the primary objective of this thesis was to identify differences in the spatiotemporal localization of SuSy in A. thaliana and P. trichocarpa, by assessing the putative contribution of the promoter regions to the functional role(s) of SuSy in plant growth and development. This was investigated by first identifying essential motifs via a bioinformatic analysis of PtSuSy1, PtSuSy2, AtSuSy1, AtSuSy4 promoter sequences, and then comparing enzyme localization via histochemical staining and live-cell imaging across the vascular tissues of A. thaliana and P. trichocarpa when driven by the same promoter (PtSUS1p). Two SNBE-like motifs were identified in the distal region of the poplar SuSy promoters, and given their absence from the Arabidopsis orthologs, were selected for further analysis via promoter truncation experiments. These, along with other GUS transformants, had a sequencing error which caused a frameshift mutation preventing verified transgenic plants from producing visible GUS phenotypes. Instead, expression analysis was conducted to compare transcript abundance across vascular tissues in poplar trees. Results showed significantly higher GUS gene expression in the xylem tissues compared to the phloem, in wild-type poplar where the gene is driven by the native PtSUS1p. Although several questions remain unanswered, these findings partially support PtSUS1p as a spatiotemporal promoter capable of regulating xylem-specific gene expression.Forestry, Faculty ofGraduat