Proportional reasoning is the ability to use multiplicative thinking and make multiple comparisons. It is known to be challenging for many students and at the same time, many teachers require support to develop sufficient subject matter knowledge and pedagogical content knowledge to teach the diverse concepts that underpin proportional reasoning. The data reported in this paper are drawn from the first year of a broader study aiming to promote the teaching and learning of elements of proportional reasoning across the curriculum by engaging primary school teachers in ongoing professional development that includes the implementation of a series of mathematics interventions, each of which included a research component. This paper focuses on the impact of implementing the interventions on the teachers' mathematical knowledge for teaching. Three structured interventions were implemented by eight teachers (Years 3, 4, 5) during each of three school terms. Data collected showed that engaging in this scaffolded type of practitioner research, the structured nature of the interventions, and reflection on the outcomes of each intervention promoted teachers' subject matter knowledge and pedagogical content knowledge.