Guillaume did his PhD in Xavier Draye's lab, on water relations in the soil-plant-atmosphere continuum. Already at that time his research combined plant physiology, image analysis, phenotyping and modelling. After the completion of his doctorate, he moved to Liege, to work with Claire Périlleux on carbon relations in the root system at the floral transition. Again, during that project, he combined experimental and modelling work. Guillaume was appointed as Assistant Professor in a joint position between the Forschungszentrum Jülich and the Université catholique de Louvain in 2016 to decrypt complex processes using whole plant models.