Abstract
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.
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Acknowledgements
The authors thank the Centro de Microscopia da Universidade Federal de Minas Gerais—CM/UFMG for the electron microscopy analyses.
Funding
The authors received from the Fundação de Amparo à Pesquisa do Estado de Minas Gerais—Fapemig financial support for the research project (Process: APQ-00468-1) and from Conselho Nacional de Desenvolvimento Científico e Tecnológico—CNPq the Master’s degree scholarship awarded to ACFM, under the Programa de Pesquisa Ecológica de Longa Duração—PELD-VERE (Process: 441440/2016-9), and research productivity grants awarded to LMR (Process: 304627/2015-1), MOMS (Process: 304801/2016-0), and YRFN (Process: 306375/2016-8).
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ESM 1
Longitudinal sections of the medullary region of the radicles and roots of Mauritia flexuosa submitted to histochemical tests. The drawings on the left represent stages of embryonic development after the cultivation of a seed without an operculum; red indicates the cotyledonary petiole and the germinative button. (a, e, i, m, q) Proteins, revealed by red staining with Xylidine-Ponceau (XP). (b, f, j, n, r) Mucilage, revealed by black staining with tannic acid. (c, g, k, o, s) Starch, revealed by black staining with Lugol’s solution. (d, h, l, p, t) Neutral polysaccharides, revealed by magenta staining with PAS. (a, b) Cells with large protein and mucilaginous reserves, respectively. (c) Absence of starch. (d) Neutral polysaccharide reserves in the cell walls and vacuoles (black arrows). (e, f) Vacuolation (white arrows), indicating early mobilization of proteins and mucilage. (g, h) Accumulations of starch grains. (i–l) Protein residues, indicating the intensification of mobilization associated with starch accumulation and vacuolation (white arrow). (m) Protein residues and vacuolation (white arrow). (n–p) Accumulations of mucilage associated with partial starch mobilization. (q–t) Total protein mobilization associated with vacuolation (white arrow) and starch accumulation. mu, mucilage; nu, nuclei; pi, phenolic idioblast; pt, protein; st, starch; ve, vessel element. (PNG 7036 kb)
ESM 2
Longitudinal sections of the cotyledonary petioles of embryos and seedlings of Mauritia flexuosa submitted to histochemical tests. The drawings on the left represent the stages of embryonic development after cultivation of a seed without an operculum; red indicates the cotyledonary petiole and the germinative button. (a, e, i, m, q) Proteins, stained red with Xylidine-Ponceau (XP). (b, f, j, n, r) Mucilage, stained black by tannic acid. (c, g, k, o, s) Starch, stained black with Lugol’s solution. (d, h, l, p, t) Neutral polysaccharides, stained magenta with PAS. (a, b) Cells with large protein and mucilaginous reserves, respectively. (c) Absence of starch. (d) Reserves of neutral polysaccharides in the cell walls and vacuoles (black arrows). (e, f) Vacuolation (white arrows), indicating early mobilization of proteins and mucilage. (g, h) Accumulations of small starch grains. (i–l) Voluminous vacuoles (white arrows) and protein residues, indicating the intensification of mobilization, associated with starch accumulations. (m) Protein residues and vacuolation (white arrows). (n–p) Accumulations of mucilage, associated with the beginning of starch mobilization. (q–t) Total protein mobilization, vacuolation (white arrows), and partial mobilization of starch; residues of polysaccharides can be observed in the vacuoles (black arrows). mu, mucilage; nu, nuclei; pi, phenolic idioblast; pt, protein; rp, raphide; st, starch. (PNG 6908 kb)
ESM 3
Longitudinal sections of the medullary regions of Mauritia flexuosa radicles obtained from dormant seeds 60 days after sowing and submitted to histochemical tests. The drawing to the left represents the morphology of the embryo; the opercular integument is indicated in brown, and the cotyledonary petiole in red. (a) Proteins, stained red with Xylidine-Ponceau (XP). (b) Mucilage, stained black with tannic acid. (c) Starch, stained black with Lugol’s solution. (d) Neutral polysaccharides, stained magenta with PAS. (a, b) Cells containing abundant protein and mucilage reserves, respectively. (c) Absence of starch. (d) Neutral polysaccharides stored in the cell wall. mu, mucilage; nu, nuclei; pt, protein. (PNG 1393 kb)
ESM 4
Longitudinal sections of the cotyledonary petiole of Mauritia flexuosa obtained from dormant seeds 60 days after sowing and submitted to histochemical tests. The drawing to the left represents the morphology of the embryo; the opercular seed coat is highlighted in brown, and the cotyledonary petiole in red. (a) Proteins, stained red with Xylidine-Ponceau (XP). (b) Mucilage, stained black with tannic acid. (c) Starch, stained black with Lugol’s solution. (d) Neutral polysaccharides, stained magenta with PAS. Cells containing abundant protein and mucilage reserves respectively. (c) Absence of starch. (d) Neutral polysaccharides stored in cell walls and vacuoles (black arrows). mu, mucilage; pt, protein. (PNG 1390 kb)
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Ferreira Moura, A.C., Ribeiro, L.M., Mazzottini-dos-Santos, H.C. et al. Cytological and histochemical evaluations reveal roles of the cotyledonary petiole in the germination and seedling development of Mauritia flexuosa (Arecaceae). Protoplasma 256, 1299–1316 (2019). https://doi.org/10.1007/s00709-019-01375-1
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DOI: https://doi.org/10.1007/s00709-019-01375-1