Given the potential impact of introductory science laboratories on postsecondary students' science learning, careful thinking is needed about how to support teaching assistants (TAs) to create opportunities for their students to reason about science. Research suggests that teachers are best positioned to improve their teaching practices when they recognize aspects of pedagogical discontentment and also feel they have the appropriate tools and resources to achieve their teaching goals in the future. This comparative case study examines how TAs who are pedagogically discontent or pedagogically content with their practice of supporting students to reason through talk engaged with a curriculum guide integrated with a weekly professional development (PD) program to grow their teaching practice. We found that only the pedagogically discontent TAs developed their use of ambitious talk moves to support more rigorous discussions over one semester. However, we also found that all TAs used the curriculum guide to plan opportunities for students to reason through talk. This research contributes new understandings about the role of pedagogical discontentment for TA learning of student-centered teaching practices. This research also has implications for designing TA-facing curriculum guides integrated with a weekly PD cycle to support TAs to develop teaching practices that support students to reason through talk.