To improve the sustainability of agricultural systems, an efficient use of resources such as phosphorus (P) nutrients is necessary. To reach this goal, the development of more resilient crop varieties able to cope with heterogeneous soil conditions in space and time is a promising strategy. Plants face many stresses in their natural environment and can respond to them by adjusting their phenotype (phenotypic plasticity). Integrating plastic root system traits into breeding strategies may help reach acceptable yields in low-input systems by enhancing water and nutrient uptake, thus reducing resource inputs in conventional farming systems. Bacterial bioinoculants, also considered to be a class of biostimulants, have shown great potential to increase the nutrient use efficiency of plants through diverse strategies including the modulation of root system plasticity. However, the study of plant plasticity can be challenging, particularly regarding the root system. This paper aims to encourage the integration of bioinoculants into the study of root system plasticity in response to P deficiency. We first focus on the plasticity of root architectural traits in a P-limiting context and on how bioinoculants can modulate root system plasticity and enhance P use efficiency. Then, important methodological points of attention to consider for the study of root system plasticity are highlighted.