Abstract: Plasmodesmata are cytoplasmic channels crossing plant cell walls for cell-to-cell communication in plants. Despite 130 years of research on plasmodesmata since their first discovery, the molecular components of plasmodesmata structure and regulation are still not clearly understood. To isolate potential plasmodesmata mutants with altered cell-to-cell movement, a genetic screen of embryo-lethal Arabidopsis plants was performed using a tracer-loading assay. From the genetic screen a novel plasmodesmata mutant, decrease in size exclusion limit (dse), was identified by its decreased embryo specific cell-to-cell trafficking of 0.5 kDa fluorescent tracers which traffic throughout the entire embryo in wild type. The dse phenotype is due to a mutation in a single recessive gene, which maps to the lower arm of chromosome 4. The structure of cell walls and plasmodesmata in dse mutants are similar to wild type, implying dse phenotypes result from defects in plasmodesmata regulation. Decreased intercellular movement of an endogenously expressed green fluorescent protein (GFP) reporter confirms down-regulation of plasmodesmata aperture in dse embryos compared to wild type. Genetic experiments indicate that DSE is required for PTGS in Arabidopsis seedlings. A second project analyzed a GFP enhancer trap line that displays a specific pattern of GFP expression at the boundaries between the apical meristems and organs in Arabidopsis embryos, seedlings, and mature plants. The enhancer drives expression of a novel gene, ORGAN BOUNDARY (OBO), identified by TAIL-PCR in the GFP enhancer trap line. OBO is a member of a single domain gene family encoding a small protein with unknown function. Genetic ablation of the cells expressing OBO indicates that the cells are essential for maintenance of the shoot apical metistem and normal organogenesis in Arabidopsis.