ABSTRACTThe plant vascular system is comprised of specialized tissues known as xylem and phloem. The interconnected network of xylem vessel elements, with their elongated structures and reinforced secondary cell walls, form a hydraulic system that enables efficient and uninterrupted water transport throughout the organism, contributing to plant growth and fitness. In the first chapter, genetic and genomic methodologies are employed to illustrate the conservation and repurposing of transcriptional regulation within the xylem network in Solanum lycopersicum. Key transcriptional regulators of xylem cell differentiation are identified in the tomato root, uncovering a partial conservation of the xylem developmental master regulators (VND6 and VND7) between Arabidopsis and tomato. Furthermore, through functional validation of putative orthologs of known xylem patterning and differentiation genes, examples of conservation (HD-ZIPIII TFs) as well as a novel regulator (SlKNAT1) are revealed in the tomato xylem regulatory network. The focus of the second chapter is directed towards the function of VND6 and VND7 in the Arabidopsis inflorescence stem. An integrated approach combining glycome profiling, an in vitro immunoanalytical platform, and in situ immunolocalization was employed to identify, for the first time, the differential abundance of specific cell wall biopolymers at cellular resolution in vnd6 and vnd7 mutants. Further gene expression profiling reveals perturbed expression of multiple cell-wall associated genes in the mutant backgrounds that could partially contribute to the observed cell wall phenotypes in the mutants. The third chapter delves deeper into functional conservation within the VND transcription factor family in the tomato root. It is demonstrated that various tomato VND TFs have iii expression in the vascular tissue in the root and are sufficient to induce ectopic xylem differentiation. The results here emphasize both the evolutionary conservation and distinct variations within the VND TF family concerning xylem development between two evolutionarily distant plant species. Lastly, utilizing an inducible VND7 system, an investigation is conducted on the hysteretic or memory feature associated with a bistable switch system linked to xylem cell fate determination. The findings obtained from this study indicate the necessity of an alternative inducible system, one that possesses both the "on" and "off" features. Such a system is crucial for the effective exploration of VND7-dependent hysteresis in the xylem cell differentiation. The research presented here has broad implications for understanding the molecular mechanisms underlying the plant vascular system, which is crucial for efficient water transport and plant growth. This understanding can have applications in areas such as agriculture and forestry, where efficient water use and plant growth are essential. Additionally, the insights gained from this research can inform the development of new strategies for improving plant growth and crop yields in the face of climate change and other environmental challenges. Overall, this research has significant potential for advancing our knowledge of plant biology and contributing to the development of sustainable agriculture practices. This study expands our knowledge of the molecular mechanisms governing xylem cell differentiation. It highlights both similarities and differences in the gene regulatory network of xylem across different plant species. Additionally, this research sheds light on the

functions of VND6 and VND7 in regulating secondary cell wall composition, challengingthe earlier notions regarding functional redundancy within the VND gene family.