Background/Question/Methods Root phenotypic plasticity has been proposed as a target for the development of more productive crops in variable environments. Root traits were phenotyped on a large maize association panel in the field with and without water deficit. Phenotypic plasticity of these traits was quantified as the relative change in trait value under water-deficit compared to control conditions. Genome-wide association mapping was performed on traits and their plasticity. Results/Conclusions A number of root traits expressed phenotypic plasticity. For example, metaxylem number, root cortical aerenchyma, stele diameter, multiseriate cortical sclerenchyma, root angle, and lateral branching density and length were plastic in response to drought. The genetic control of phenotypic plasticity was highly quantitative, and loci associated with plastic responses were distinct from loci that control trait expression in stress and non-stress. However, plasticity was not necessarily a characteristic of an organism as a whole, but rather a characteristic of a specific trait. In drought, many of these plastic responses were adaptive and were associated with deeper rooting, yield stability, and greater plant performance. Phenotypic plasticity of root traits is heritable and genetically controlled and is a widespread and important phenomenon for the strategic capture of nutrients and water. The fitness landscape of root phenotypic plasticity is complex and dependent on many variables including the environment and management practices. Understanding the adaptive value and genetic control of root plasticity will provide an array of tools to better understand adaptive strategies for enhanced plant productivity.