Anatomy, histochemistry and ultrastructure of seed and somatic embryo of Acrocomia aculeata (Arecaceae)

Abstract

Macaw palm (Acrocomia aculeata (Jacq.) Lodd. ex Mart.) is a tropical species with multiple uses, including oil supply for biodiesel production. However, structural and physiological studies of the seed are still scarce, as well as in vitro propagation techniques. The aim of this study was to characterize the anatomy, histochemistry and ultrastructure of the seed and the in vitro somatic embryo of A. aculeata, gaining insight into relationships between the post-harvest seed behavior and the somatic embryo conversion to plant. The zygotic embryo and the endosperm show high quantities of protein and lipids, stored in protein and lipid bodies, respectively. The cell wall storage polysaccharides occur in the thickened cell wall of the endosperm. The absence of vacuoles and the few organelles, except for the storage ones, indicate low metabolic state of the zygotic embryo, which suggests a orthodox behavior of the seed at maturity. The somatic embryo shows a shoot meristem with few leaf primordia, vacuolated cells and, occasionally, amyloplasts , but not lipid or protein reserves. These characteristics, common in somatic embryogenesis, could be associated with the maturation phase, and, consequently, with the low conversion of these embryos into plants. Key words: macaw palm, zygotic embryo, endosperm, storage reserves Anatomia, histoquímica e ultra-estrutura da semente e do embrião somático de Acrocomia aculeata (Arecaceae)RESUMO: A macaúba (Acrocomia aculeata (Jacq.) Lodd. ex Mart.) é uma espécie tropical com múltiplos usos, incluindo a geração de óleo para a produção de biodiesel. Entretanto, estudos estruturais e fisiológicos da semente ainda são escassos, assim como as técnicas de propagação in vitro. Caracterizaram-se a anatomia, a histoquímica e ultraestrutura da semente e do embrião somático de A. aculeata obtido in vitro, visando a obter relações entre o comportamento pós-colheita da semente e a conversão em planta a partir do embrião somático. O embrião zigótico e o endosperma apresentam alta quantidade de proteínas e lipídios, estocados em corpos protéicos e lipídicos, respectivamente. Os polissacarídeos de reserva da parede celular estão presentes nas paredes celulares espessadas do endosperma. A ausência de vacúolos e as poucas organelas visualizadas, exceto as organelas de reserva, indicam baixo estado metabólico do embrião zigótico, o que sugere comportamento ortodoxo da semente madura. O embrião somático apresenta meristema apical com poucos primórdios foliares, células com vacúolos conspícuos e, ocasionalmente, amiloplastos, mas não há reservas protéicas ou lipídicas. Essas características, comuns na embriogênese somática, podem estar associadas à fase de maturação dos embriões e, consequentemente, com a baixa conversão desses embriões em plantas. Palavras-chave: macaúba, embrião zigótico, endosperma, substâncias de reserva

Enzyme activity and reserve mobilization during Macaw palm ( Acrocomia aculeata ) seed germination

“…Starch likewise was not detected by histochemical tests in the embryos, nor in the in endosperm tissues of macaw palm seeds (Moura et al 2010). During early seedling development events (day 22 to 29) there was a considerable increase in starch content as opposed to lipid content, indicating the gluconeogenic origin of the polysaccharides.…”

“…Gluconeogenesis is common in oilseeds, and is induced after visible germination (Graham 2008 (Oo & Stumpf 1983), Cocos nucifera (Sugimuma & Murakami 1990), and B. capitata (Oliveira et al 2013a). Because of the abundant mannose and galactose contents in the endosperm tissue, we assume that galactomannan is a storage compound for macaw palm seeds, and Moura et al (2010) suggested that the cell walls of macaw palm endosperm were composed of a type of mannan. Seed mannans and galactomannans are, in fact, important reserve compounds in the endosperm cell walls of many Arecaceae species because of this conversion into living sugars and possibly involvement in gluconeogenesis (Alang et al 1988;Buckeridge et al 2000;Buckeridge 2010).…”

“…In addition to storage lipids, macaw palm seeds (endosperm and embryo) contain other storage compounds such as protein (Moura et al 2010); the endosperm has cells with thickened noncellulosic and non-lignified walls (Moura et al 2010) composed of non-soluble carbohydrates characteristic of mannan -although how macaw palm seeds use these embryo and endosperm storage compounds and how they are mobilized during germination is still unclear. Macaw palms only multiply through seeds (Ellis et al 1985), but their dormancy (Ribeiro et al 2011;Oliveira et al 2013b) will hinder the establishment of future commercial plantations.…”

Somatic embryogenesis in Acrocomia aculeata Jacq. (Lodd.) ex Mart using the thin cell layer technique

“…Additionally, Moura et al (2010) found that somatic embryos had no proteins or lipids as storage compounds, which may be essential for conversion. Few somatic embryos progressed and failures occurred during their conversion into a plant, such as the failure of leaf primordium, or roots or both to develop or differentiate.…”

Do recalcitrant seeds really exist?

“…Among them are anatomical and biochemical modifications, which allow seed tissues to survive through a period of anabiosis. For example, some recent studies have characterized the cell wall folding and ultrastructure of the cell containing fewer organelles as well as low vacuolated cells in vegetative tissues, such as the so-called orthodox seeds (Moura et al 2010) and also the resurrection plants (Moore et al 2008(Moore et al , 2009). However, considering desiccation sensitive seeds, adaptations on the cell wall or subcellular structure are impaired.…”