Inside a Dual Secretory Cell: Ultrastructural Insights into Mucilage and Phenolic Secretion in Mimosa Species (Leguminosae)

The co-occurrence of mucilage and phenolic compounds within the same secretory cell is rarely documented in plants. Recently, such cells were reported in vegetative and floral organs of sensitive legumes (Mimosa), but without detailed subcellular analysis. To address this gap, we used transmission electron microscopy to examine the organelles involved in biosynthesis, the intracellular sites of metabolite storage, and the secretion processes across floral and foliar organs in five Mimosa species. Secretory epidermal cells of sepals, petals, and leaf blades produce both mucilage and phenolics, with no significant differences between organ types. Dictyosomes, rough endoplasmic reticulum, and plastids predominated in the cytoplasm of the secretory cell during biosynthesis. Dictyosomes may mediate mucilage production, the rough endoplasmic reticulum may be involved in phenolic synthesis, and plastids may contribute to the biosynthesis of both compounds. These metabolites are stored in distinct cellular domains: phenolics accumulate in a large vacuole near the outer periclinal wall, while mucilage is deposited between the microfibrils of the inner periclinal wall. This spatial separation is evident by the distention of the inner periclinal wall due to mucilage accumulation. The absence of karyokinesis and phragmoplast formation during metabolite segregation confirms that these secretory cells have two different functional domains, forming a uniseriate rather than biseriate epidermis. Notably, the inclusion of several species in the ultrastructural analyses enhances the significance of these findings.