The functional characterization of a key enzyme in the phosphatidylinositol (PI) signaling pathway in the model plant Arabidopsis thaliana is the focus of the research summarised in this thesis. Moreover, a particular focus is the exploration of the biological functions of Arabidopsis phophatidylinositol monophosphate 5-kinase 2 (PIP5K2) which catalyzes the synthesis of phophatidylinositol (4,5) bisphosphate, the precursor of two important second messengers (inositol 1,4,5-trisphosphate and diacylglycerol). Through molecular and genetic approaches, the author isolated and characterized the expression pattern, physiological functions and the underlying mechanism of Arabidopsis PIP5K2. It is found 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 role in salt tolerance. These findings shed new light on the crosstalk between PI signaling and auxin response, both of which have crucial regulatory roles in plant development.