Unravelling the mechanism and signals originating from roots and able to contribute to stomatal control has been the subject of research for decades. In this commentary, Tardieu summarizes and discusses many studies and models on root-shoot signals, including work in the same issue by Visentin et al. which shows that strigolactones in roots are a component of the systemic signal in tomato. Tardieu points out that the nature of the signals reported in the literature is different in different species and experimental systems, and notes, “In my own analysis, none of the control systems considered here can be ruled out based on obvious artefacts or errors of protocols. One must, therefore, live with a diversity of hypotheses.” He concudes with a call for more multi-species meta-analyses and dynamic models. (Summary by Mather Khan) New Phytol.