Histogenesis and reserve dynamics during the maintenance of dormancy and germination in seeds of the basal palm Mauritiella armata.

The germination and post-seminal development of Arecaceae are notably complex due to the microscopic dimensions of the embryonic axis, the occurrence of dormancy, and the diversity of reserve compounds. In-depth information on this subject is still limited, especially in terms of the basal sub-family Calamoideae. Mauritiella armata is widely distributed in the Amazon region and is considered a key species in flooded ecosystems (veredas) in the Cerrado biome. We sought to describe histogenesis and reserve compound dynamics during the germination of M. armata, as well as the changes in incubated seeds over time. Seeds with their operculum removed (the structure that limits embryonic growth) were evaluated during germination using standard methods of histology, histochemistry, and electron microscopy. Evaluations were also performed on intact seeds incubated for 180 days. The embryos show characteristics associated with recalcitrant seeds of Arecaceae: a high water content (>80%), differentiated vessel elements, and reduced lipid reserves. Both the embryo and endosperm store abundant reserves of proteins, neutral carbohydrates, and pectins. The completion of germination involves cell divisions and expansions in specific regions of the embryo, in addition to the mobilization of embryonic and endospermic reserves through symplastic and apoplastic flows. Intact seeds show dormancy (not germinating for 180 days), but exhibit continuous development associated with cell growth, differentiation, and reserve mobilization. The anatomical and histochemical characters of M. armata seeds indicate an association between recalcitrance and dormancy related to the species' adaptation to flooded environments.

Pericarp histogenesis and histochemistry during fruit development in Butia capitata (Arecaceae).

Palm fruits show great structural complexity, and in-depth studies of their development are still scarce. This work aimed to define the developmental stages of the fruit of the neotropical palm Butia capitata and to characterize the ontogenesis of its pericarp. Biometric, anatomical, and histochemical evaluations were performed on pistillate flowers and developing fruits. The whole fruit develops in three phases: (I) histogenesis (up to 42 days after anthesis - DAA), when the topographic regions of the pericarp are defined; (II) pyrene maturation (42 to 70 DAA), when the sclerified zone of the pericarp is established; and (III) mesocarp maturation (70 to 84 DAA), when reserve deposition is completed. During pericarp ontogenesis (i) the outer epidermis and the outer mesophyll of the ovary give origin to the exocarp (secretory epidermis, collenchyma, parenchyma, sclerenchyma, and vascular bundles); (ii) the median ovarian mesophyll develops into the mesocarp, with two distinct topographical regions; (iii) the inner ovarian epidermis originates the endocarp; and in the micropylar region, it differentiates into the germination pore plate, a structure that protects the embryo and controls germination. (iv) Most of the inner region of the mesocarp fuses with the endocarp and, both lignified, give rise to the stony pyrene; (v) in the other regions of the mesocarp, carbohydrates and lipids are accumulated in a parenchyma permeated with fiber and vascular bundles. The development of the B. capitata pericarp presents high complexity and a pattern not yet reported for Arecaceae, which supports the adoption of the Butia-type pyrenarium fruit class.

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-ß-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.

Cytological and histochemical evaluations reveal roles of the cotyledonary petiole in the germination and seedling development of Mauritia flexuosa (Arecaceae).

The cotyledonary petiole (CP) completely envelops the embryo axis during embryogenesis in Arecaceae. There is little information available, however, on the roles of that structure in seed germination and initial seedling development-crucial plant life cycle phases. The study therefore sought to evaluate the roles of CP in the germination and post-seminal development of the recalcitrant seeds of Mauritia flexuosa, an ecologically and economically important neotropical palm. The CP and the embryo/vegetative axis were evaluated during germination and initial seedling development using standard morphological, anatomical, histochemical, and ultrastructural methodologies. Evaluations of dormant seeds incubated for 60 days were also performed. The CP (a) promotes seedling protrusion in the germination, extending the embryo axis outside the seed; (b) protects the vegetative axis through the development of coating rich in phenolic compounds and lignin; (c) participates in reserve translocation, with the conversion of its own proteinaceous/mucilaginous reserves into transitional starch, as well as acting in the transport of endospermic reserves; (d) favors aeration, with the formation of pathways among stomata, substomatal chambers, and intercellular spaces; (e) controls seedling morphogenesis by modulating the curvature of the vegetative axis; and (f) contributes to overcoming seed bank dormancy through cytological alterations (protein synthesis and mitochondrial proliferation). The cotyledonary petiole of palms is a unique and multifunctional structure among angiosperms, with crucial roles in germination and seedling establishment.

Roles of the haustorium and endosperm during the development of seedlings of Acrocomia aculeata (Arecaceae): dynamics of reserve mobilization and accumulation.

The mobilization of palm seed reserves is a complex process because of the abundance and diversity of stored compounds and results from the development of a highly specialized haustorium. This work focused on the important Neotropical oleaginous palm Acrocomia aculeata, with the aim of defining phases of seedling development associated with mobilization of reserves and elucidating the role of haustorium and endosperm in this process. Standard methods were performed, including biometric, anatomical, and histochemical analyses, as well as the evaluation of the activities of the enzymes endo-ß-mannanase and lipase, throughout the reserve mobilization in seeds during germination and in seedlings. Seeds of A. aculeata stored large quantities of proteins, lipids, and polysaccharides in the embryo and endosperm. The mobilization of reserves initiated in the haustorium during germination and subsequently occurred in the endosperm adjacent to the haustorium, forming a gradually increasing zone of digestion. Proteins and polysaccharides were the first to be mobilized, followed by lipids and cell wall constituents. The haustorium activates and controls the mobilization, forming transitory reserves and translocating them to the vegetative axis, while the endosperm, which also has an active role, serves as a site of intense enzymatic activity associated with protein bodies. Seedling development can be described as occurring in six phases over a long period (approximately 150 days) due to the large amount of seed reserves. This process exhibits an alternation between stages of accumulation and translocation of protein, lipid, and carbohydrate reserves in the haustorium, which favors the seedling establishment and the reproductive success of the species.