Reserve mobilization dynamics and degradation pattern of mannan-rich cell walls in the recalcitrant seed of Mauritia flexuosa (Arecaceae)

Most seeds store reserves, which mobilization after germination is complex and diversified among plant species. Information on the reserve mobilization in recalcitrant seeds (i.e., intolerant to desiccation) is scarce. The aim of this work was to characterize the dynamics of reserve mobilization and the degradation pattern of the endospermic cell walls in the recalcitrant seeds of the neotropical palm Mauritia flexuosa. Biometric, anatomical, histochemical, ultrastructural and immunocytochemistry assessments were performed in the endosperm and haustorium (structure of the seedling involved in reserve mobilization), during germination and throughout seedling development. Endo-beta-mannanase activity was assessed. The main reserves stored in the seeds are mucilage in the living protoplast and, mainly, heteromannans in the thick cell walls of the endosperm cells. The reserve mobilization extends for about 180 days, in four phases. During germination, the embryonic reserves are catabolized, which induces the mobilization of the endosperm by establishing the flow of water and carrying substances to the haustorium. After germination, the cells of the endosperm actively control the process of their degradation, which results in the formation of the digestion zone. The growth of the haustorium promotes the crushing of endospermic cells and facilitates the entry of substances via the apoplastic route. The pattern of degradation of endospermic cells involves three phases: 1) mobilization of the vacuolar content by symplastic route; 2) increased vacuole turgor pressure, directing the content of the cytoplasm to the cell walls; 3) degradation of cell walls.