[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] Plants being immobile have developed various adaptive responses to interpret and utilize light directionality, quantity and quality. One such adaptive response is phototropism where the plant organs bend towards a directional light source. In Arabidopsis, NPH₃ protein is absolutely required for phototropism and it interacts with the phot1 photoreceptor. Given the unique properties of this critical protein mediating phototropism, yet little is known about how phot1 signals through NPH₃. This dissertation presents results that address a better understanding on the workings of NPH₃ in mediating phototropism. In brief, it has been demonstrated that NPH₃ has reversible phosphorylation states and dephosphorylation of NPH₃ is dependent on phot1. Lastly, my dissertation work has led to the identification of three more novel components of the phototropic pathway: 1) CUL₃ (directly interacts with NPH₃); 2) a protein kinase that phosphorylates NPH₃ in dark; and 3) a protein phosphatase that dephosphorylates NPH₃ in light.