This grounded theory study explored the pedagogical content knowledge (PCK) of experienced agriculture teachers in the plant sciences. The central phenomenon that emerged during data collection and analysis was the influence of beliefs on shaping participants' PCK. This finding guided subsequent collection and analysis that resulted in the following central research question: what shapes experienced agriculture teachers' PCK in plant sciences? The data presented here focused on the most emergent category shaping PCK, integrated belief systems, which included participants' beliefs about the purpose of agricultural education, beliefs about plant science education, and beliefs about teaching and learning in agricultural education. A substantive level theory was developed that illustrated the relationships between the three belief components on participants' PCK. These findings support further investigation into how beliefs are shaping agriculture teachers' PCK in plant sciences and other agriculture content areas.