Structural and histochemical plasticity of secretory cavities reflects distinct functional strategies in Eugenia galls

Abstract Secretory cavities in Myrtaceae typically store lipophilic substances involved in plant defense. However, their structural and functional reorganization during gall development remains poorly understood, particularly regarding how gall complexity and biotic interactions influence these changes. Here, we analyzed the anatomical, histometric, and histochemical features of secretory cavities across eight gall morphotypes induced on three Eugenia species ( E. astringens , E. copacabanensis , and E. uniflora ). We hypothesized that gall induction modifies secretory cavities, which may result in functional alterations during cecidogenesis, and that the magnitude of these alterations relates to gall structural complexity. Across most morphotypes, we observed a suppression of cavities near the larval chamber and reduced secretory cavity density in gall wall, an integrated strategy that likely minimizes the inducer’s exposure to secondary metabolites. Conversely, in E. astringens fusiform galls, an associated fungal symbiont indicates that these are ambrosia galls. In these galls, we recorded higher cavity density, larger lumen area, and phenolic accumulation, suggesting a repurposing of these structures to maintain a microenvironment favorable to the fungal-insect association. The results partially supported our hypothesis: while gall complexity correlated with cavity reorganization in E. astringens , this pattern was inconsistent in the other host species, indicating that taxonomic identity, morphogenetic constraints, and biotic interactions also contribute to these traits. These findings demonstrate that secretory cavity reorganization is not a uniform process but a highly plastic response, revealing diverse functional trajectories that range from structural reduction to chemical enhancement, depending on the specific host plant-gall inducer system.