Most students think animals are more interesting than plants as a study topic believing that plants are inferior to animals because they are passive and unable to respond to external challenges, particularly biological invaders such as microorganisms and insect herbivores. The purpose of this study was to develop an inquiry-based learning unit, the Fighting Plant Learning Unit (FPLU), which focuses on plant defense responses to biological stimuli. The study also investigated students' perceptions of the FPLU implemented in a constructivist classroom. A total of 31 Thai science majors from grade 12 participated in this study. Multiple data-gathering techniques (quantitative and qualitative) were employed: conceptual testing, concept mapping, the Constructivist Learning Environment Survey questionnaire and semi-structured interviews. It was found that from their active participation, the students developed a better conceptual understanding of plant defense mechanisms. In addition, they appeared to have positive attitudes toward the FPLU as evidenced by their preference for inquiry-based teaching in a constructivist learning environment. Finally, their perspectives on and appreciation of plants as active organisms had been favorably changed by the experience.