Morphological and developmental analysis of Plantago spp. seed capsules reveal key features of the dehiscence zones.

Background and aims: Shattering is a natural phenomenon displayed by dry fruits or capsules that dehisce at maturity to distribute seeds. This undesirable trait in commercially-important Plantago ovata can cause high yield losses, especially when triggered by weather events. However, the underlying internal and external triggers of capsule dehiscence are not well understood. This study aimed to characterise the morphological features of Plantago seed capsules, focusing on dehiscence zones (DZ) and structural components influencing capsule opening.

Methods: Capsule development and dehiscence in P. ovata were examined using staining, immunolabelling, and electron microscopy, with particular emphasis on the dehiscence zone between the lid and base. Polysaccharide-directed antibodies and monosaccharide profiling were used to analyse cell wall composition. Findings were compared across three Plantago relatives ranked by manually-induced dehiscence propensity.

Key results: Capsule walls are dominated by xylans and differentially-esterified pectins. The operculum (lid) shows a distinct lignification pattern absent in the capsule base. A key feature is the "operculum hook", a vertical cell layer with thickened walls enriched in xylans and lignin, connecting the lid to the base. The DZ contains two separation layers: the first formed by cuboidal cells lacking unesterified homogalacturonan with the second layer found at the junction between the operculum hook and the capsule base. Dehiscence occurs in two steps, involving abscission at these zones. Structural differences in the operculum hook, particularly cell wall thickness, vary across Plantago species and are correlated with ordinally ranked differences in manually-triggered dehiscence.

Conclusions: Capsule dehiscence in Plantago involves two sequential separation events influenced by cell wall composition and structure. Cell wall dimensions at the operculum hook base could be critical in determining dehiscence ease. These findings provide new insights into capsule development and dehiscence, which could inform future breeding strategies to reduce yield losses in P. ovata and other crops.