Floral nectary and pericarp galls of Sapium glandulosum: Structural convergence and cell wall remodeling

Galls are novel plant organs induced by specialized organisms that redirect host plant development. Although the host organ may impose structural constraints on gall formation, some gall-inducing insects can produce similar gall phenotypes in distinct plant tissues. Herein, we investigated the morphology, anatomy, and cell wall immunocytochemistry of galls induced by Neolithus fasciatus (Hemiptera: Triozidae) in the floral nectaries and fruit pericarps of Sapium glandulosum (Euphorbiaceae). Morphological and anatomical analyses were performed using historesin-embedded sections, while immunocytochemical analyses were conducted with monoclonal antibodies targeting homogalacturonans (HGs) and rhamnogalacturonan-I (RG-I) epitopes. Gall induction in floral nectaries caused extensive tissue reorganization, including the loss of secretory epidermal cells and nectar production, whereas fruit galls developed from the exocarp and parenchymatous mesocarp. Despite originating from anatomically and functionally distinct organs, both gall types exhibited a remarkably similar structural organization, consisting of a uniseriate epidermis, a compartmentalized cortex with outer chlorophyllous and inner metabolically active parenchyma, collateral vascular bundles, laticifers, and a single nymphal chamber. Immunocytochemical analyses revealed a predominance of highly methyl-esterified HGs in both gall systems, indicating the maintenance of flexible and metabolically active cell walls. In addition, RG-I containing (1→4)-β-D-galactan side chains were particularly abundant in the inner cortex of the galls. These findings demonstrate that N. fasciatus exerts strong control over gall morphogenesis, promoting anatomical convergence in distinct reproductive structures while preserving organ-specific signatures in the pectic composition of cell walls.