Extrafloral Nectaries in Woody Plants: Toward an Integrated Framework for Ant-Mediated Plant Defense

Extrafloral nectaries (EFNs)—secretory structures that recruit predatory arthropods as indirect defenders against herbivory—have been studied almost exclusively in herbaceous systems, leaving a critical gap in our understanding of plant defense in woody plants. In trees, secondary growth, ontogenetic regulation, and seasonal phenology generate structural and temporal heterogeneity in EFN expression poorly captured by existing frameworks. We synthesize evidence across structural, chemical, and functional dimensions of EFN biology to develop an integrated framework for perennial trees. Our analysis suggests that methodological limitations in EFN detection likely bias the current chemical record of woody EFNs, while incomplete chemical characterization constrains the evaluation of their ecological functions. Existing evidence confirms that herbivory-inducible EFN secretion and ant-mediated defense operate in trees, but their generality across ontogenetic stages, seasons, and woody lineages remains unresolved. We propose Idesia polycarpa (Salicaceae) as a candidate model system for mechanistic EFN research in trees, supported by confirmed secretory activity, documented ant visitation, and a well-characterized genomic and phylogenetic context, and offer four testable predictions to guide future work on indirect defense in long-lived woody plants.