Due to the non-uniform distribution of inorganic phosphate (Pi) in the soil, plants modify their root architecture to improve acquisition of this nutrient. In this study, a split-root system was employed to assess the nature of local and systemic signals that modulate root architecture of Brassica napus grown with non-uniform Pi availability. Lateral root (LR) growth was regulated systemically by non-uniform Pi distribution, by increasing the second-order LR (2°LR) density in compartments with high Pi supply but decreasing it in compartments with low Pi availability. Transcriptomic profiling identified groups of genes regulated, both locally and systemically, by Pi starvation. The number of systemically induced genes was greater than the number of genes locally induced, and included genes related to abscisic acid (ABA) and jasmonic acid (JA) signalling pathways, reactive oxygen species (ROS) metabolism, sucrose, and starch metabolism. Physiological studies confirmed the involvement of ABA, JA, sugars, and ROS in the systemic Pi starvation response. Our results reveal the mechanistic basis of local and systemic responses of B. napus to Pi starvation and provide new insights into the molecular and physiological basis of root plasticity.