The roles of auxin signaling and transport in leaf initiation and vascular pattern formation in arabidopsis thaliana

The plant hormone auxin has been implicated in many diverse processes in plant development including the formation of leaves from the shoot apical meristem and subsequent leaf vascular tissue patterning. Both processes of leaf initiation and leaf vascular patterning share many commonalities including: 1. They appear to be formed by a gradual limitation process that may be synonymous with canalization of auxin transport. 2. Genetic evidence from Arabidopsis thaliana, links both processes to the MONOPTEROS (MP) gene, involved in auxin signaling and the PIN-FORMED1 (PIN1) gene, involved in auxin transport. To elucidate the potential functions of MP and PIN1 in vascular patterning, we assessed detailed MP and PIN1 expression dynamics during vascular tissue formation in A. thaliana leaf primordia. Taken together, the results presented provide novel molecular support for the canalization of auxin flow hypothesis. Since PIN1 expression appears to be regulated by MP function, it seemed possible that MP affects leaf formation exclusively as a positive regulator of auxin transport. To explore this notion further, we analyze a novel, completely leafless phenotype in mp pin1 double mutants. Subsequent analysis indicates multiple MP dependent pathways are involved in leaf initiation. Shoot meristem function, including central stem cell zone maintenance is severely disrupted in mp pin1 double mutants. Mutant analysis of characterized genes involved in central zone maintenance in mp mutant backgrounds indicate that MP functions as a negative regulator of central zone size in parallel to established mechanisms. While it is clear that MP is an important player in both processes of leaf initiation and vascular patterning, loss of function mutations in MP have a relatively limited phenotypic impact. By analyzing the temporal and spatial expression profiles of genes closely related to MP we identified the ARF3 and ARF7 genes, as candidates for putative overlapping function with MP. Subsequent mutant analysis demonstrate that ARF3 and ARF7 play novel functional roles in leaf initiation in mp loss of function mutants. The results presented in this thesis contribute to our general understanding of auxin mediated developmental processes through analysis of leaf initiation and vascular pattern formation