The functional characterization of a key enzyme in the phosphatidylinositol (PI) signaling pathway in the model plant Arabidopsis thaliana is the focus of this thesis. Moreover, a particular focus is the exploration of the biological functions of Arabidopsis phosphatidylinositol monophosphate 5-kinase 2 (PIP5K2), which catalyzes the synthesis of phosphatidylinositol (4,5) bisphosphate, the precursor of two important second messengers (inositol 1,4,5-trisphosphate and diacylglycerol). Employing molecular and genetic approaches, the author isolates and characterizes the expression pattern, physiological functions and underlying mechanism of Arabidopsis PIP5K2. In doing so, he reveals that PIP5K2 is involved in regulating lateral root formation and root gravity response through modulating auxin accumulation and polar auxin transport, and also plays a critical part in salt tolerance. These findings shed new light on the crosstalk between PI signaling and auxin response, both of which fulfill crucial regulatory roles in plant development.