Investigating The Functions of Two Transcription Factors in Vascular Development and Wall Biosynthesis in Arabidopsis thaliana

Plant vascular system is important for plant growth and development. There are three different cell types in vascular tissues, xylems, phloems and procambial or cambial. In the presence of cambium, plants continuously generate new vascular tissues. During the differentiation process, xylem fibers develop lignified secondary cell walls which provide abundant resources for biofuel industry. However, the underlying mechanisms of vascular development and secondary cell wall formation are still elusive. Our objectives are to identify new transcription factors function in vascular initiation and biomass deposition in Arabidopsis. In this thesis, we identified two activation-tagging mutants, hva-d with high vascular activity and stp-2d showing secondary wall thickening in pith cells. A novel mutant, hva-d, showed more developed vascular bundles. Histochemical analysis indicated cambium and phloem activity in hva-d were also elevated. The phenotype results from elevated expression of HVA. HVA functions as a transcriptional repressor and interacts with TPL. The results demonstrated HVA was an important regulator involved in vascular development. Another dominant mutant, stp–2d showed secondary wall thickening in pith cells (STP). Activation of microRNA 165b (miR165b) is responsible for the STP phenotype. The expression of three class III HD–ZIP transcription factor genes, including AtHB15, was repressed in the stp–2d mutant. MicroRNA-resistant mtAtHB15 complemented stp-2d defects and it indicated that repression of AtHB15 accounted for the abnormal lignification in stp-2d mutant. These results illustrated a microRNA165b-AtHB15 mediated regulatory pathway functions upstream of the secondary cell wall master regulators