Secondary-cell-wall release: a particular pattern of secretion in the mucilaginous seed coat of Magonia pubescens.

PREMISE: Plant mucilages are composed of strongly hydrophilic polysaccharides and contribute to seed germination and seedling establishment. The myxospermic seeds of Magonia pubescens, a Cerrado (neotropical savanna) tree species, produces a voluminous mucilage capsule when hydrated. The development of the mucilaginous layer, the process of mucilage secretion and its role in seed germination was not previously studied so far. METHODS: Morphological, anatomical, histochemical, and ultrastructural evaluations of the seed coat during seed development were undertaken. The formation of the mucilage capsule was followed using scanning electron microscopy, and the coat's effects on seed hydration and germination were evaluated after experimental manipulation of the seed coat. RESULTS: The mucilaginous layer is contained in the outer coat of the seed. The mucilage-secreting cells accumulate proteins in the outer periclinal region and acidic polysaccharides in the inner periclinal region of the periplasmic space. Neutral polysaccharides, which form the loosely organized secondary wall, form a folding, basket-like structure surrounding the acidic polysaccharides. The protoplast collapses at maturity. With hydration, the mucilage expands, breaks the epidermal layer, and forms a mucilage capsule around the seed. The mucilaginous seed coat does not increase the germination rate. CONCLUSIONS: Upon mucilage hydration, the secondary cell walls forming the network of neutral polysaccharides are released, along with proteins, and help retain the acidic polysaccharides-forming a peculiar architecture that imparts integrity and consistency to the mucilage capsule. As winged seeds are not usually buried, the mucilage capsule favors seedling hydration, contributing to its establishment on the soil surface.