The phospholipid phosphatidylinositol and its phosphorylated derivatives, collectively referred to as phosphoinositides, form the basis for a multifaceted signaling pathway regulating many different cellular processes in eukaryotic cells. Phosphatidylinositol 4,5-bisphosphate, PI(4,5)P2, assumes a central position in this complex pathway. It can serve as a precursor for the generation of second messengers but can also act as a ligand to partner proteins. In order to mediate their physiological effects properly, the location and quantity of PI(4,5)P2 and other phosphoinositides have to be tightly controlled by enzymes. In general, phospholipid kinases lead to the activation of the pathway, whereas phospholipid phosphatases attenuate or terminate the signaling cascade. The SAC domain-containing protein 9 from Arabidopsis thaliana has been identified as a putative phosphoinositide phosphatase, but very little has been published on this particular protein. In my dissertation research, I broadened our knowledge of this protein and the effects seen in Arabidopsis plants carrying the mutant allele. I used molecular, genetic, and biochemical approaches to analyze the function of the putative phosphoinositide phosphatase, SAC9. To understand its physiological role, I investigated the cellular effects of a mutation in the SAC9 gene at the light microscopy, confocal microscopy, and transmission electron microscopy levels. My studies show that AtSAC9 is a soluble protein with an apparent molecular mass of 180 kDa and that it most likely is a phosphoinositide phosphatase. Furthermore, I show that the mutation of SAC9 induced unique cell wall defects that most likely have contributed to the stuntedness of the root. However, the cortical microtubule cytoskeleton was not disturbed in elongating root cells. These data are augmented by applying a novel approach for the mathematical analysis of cortical microtubule orientation.