Ultrastructural aspects of metabolic pathways and translocation routes during mobilization of seed reserves in Acrocomia aculeata (Arecaceae)

Seeds of several palms are reserve-rich, and have a complex and poorly understood mobilization. The massive endosperm and embryo of Acrocomia aculeata Lodd. ex Mart. possess large amounts of proteins, polysaccharides, and lipids. We evaluated cell features related to reserve mobilization during germination and initial development of seedlings of A. aculeata. Samples of the haustorium (the cotyledonary blade) at different developmental stages and of the entire digestion zone of the endosperm were processed by standard methods for ultrastructural evaluation. The haustorium reserve mobilization begins when germination starts, and proteins are the first to be metabolized, followed by polysaccharides and lipids. Haustorial cells present lipid bodies associated with glyoxysomes and protein vacuoles, which are involved in lipid mobilization. The digestion zone of the endosperm comprises cell layers adjacent to the haustorium, in which reserve mobilization begins after the protrusion of the cotyledonary petiole. The endosperm reserve mobilization is similar to observed in the haustorium, firstly proteins, polysaccharides, and lipids in the end. The abundant carbohydrates in the cell walls of endosperm are hydrolyzed, and the cells lose integrity, while digestion products accumulate around the haustorium. The sinuosities observed in the plasma membrane and the organelles predominating in the epidermal cells of the haustorium are consistent with absorption and transitory storage of reserves, and there is no evidence of the secretion of enzymes that can act in the mobilization of endosperm reserves. Therefore, the endosperm has a storage and self-degradation function, and the products of hydrolysis are transported towards the haustorium via the apoplastic route.