Next Generation Science Standards (NGSS) call for integrating knowledge and practice in learning experiences in K-12 science education. "PlantingScience" (PS), an ideal curriculum for use as an NGSS model, is a computer-mediated collaborative learning environment intertwining scientific inquiry, classroom instruction, and online mentoring from scientists. With implementation in hundreds of classrooms across the United States, science teachers have noted PS as successful in motivating their students to engage in classroom inquiry investigations. To investigate the role of the online scientist-mentors in motivating students in the PS learning environment, we used self-determination theory (SDT) to guide a multiple case study of 10 student inquiry teams engaged in PS in a rural public school in a large Southwestern U.S. state. We used online dialogues between scientist-mentors and their assigned student inquiry teams to answer research questions about the motivational support provided by scientist-mentors and the association between this support and students' engagement in scientific inquiry. Analyses revealed a general relationship between scientist-mentor motivational support and student inquiry engagement. Differences were observed in the specific ways in which scientists supported autonomy, competence, and relatedness. Student inquiry engagement corresponded to high support in relatedness, whereas correspondence with autonomy support was low.