Abstract
Olive (Olea europaea L.) tree is one of the most extensive and important agricultural crop in Mediterranean countries due to its beneficial health and nutritional properties and its high economic value. Currently, olive tree constitutes the sixth most important cultivated plant in the world, spreading from the Mediterranean region of origin to new production areas such as Australia, South and North America and South Africa. However, the mobilization processes of storage materials i.e. reserve proteins during seed germination, which are largely involved in essential physiological process including plant growth and development, remain poorly understood. Morphometric and immunohistochemistry analyses of protein bodies contained in olive seed storage tissues, cotyledon and endosperm, were performed by using different microscopy techniques, including light (bright-field and fluorescence) microscopy and transmission electron microscopy. Furthermore, we used legumin-like proteins (11S-type globulins) as a molecular marker to study the mobilization of reserve proteins from PBs of cotyledons at germinating seedling stages by using immunofluorescence assays. Results demonstrated that cotyledon and endosperm are characterized by distinct PBs populations containing legumin-like proteins, distinctly discriminated by the number of PBs per cell and tissue, size, immunofluorescence and histochemical staining. These features reflect differential PBs biogenesis during development and maturation processes in olive seed tissues endosperm and cotyledon, in relation to proteins (polypeptides) final composition, SSPs processing and/or packaging during seed maturation. Three different mobilization patterns of legumin-like proteins were identified for the first time in cotyledon PBs during seedling germinating process. Mature proteins composition and/or processing, cell types and enzyme composition and/or differential activation have been discussed as key features determining how proteins mobilize from PBs for further degradation in the cotyledon.
Highlights
The effective mobilization of seed storage reserves and their degradation during seed germination is a crucial physiological process in the plants life cycle [1].Seed Storage proteins (SSPs) in dicotyledonous plants, mainly legumin-like (11S globulins) and vicilin-like (7S globulins) proteins, accumulate in specialized storage cells of the embryonary axis[2], and are protected against uncontrolled premature degradation through two major mechanisms, 1) structural features that prevent cleavage by proteases simultaneously present in the same compartment [3,4] and 2) protein mature forms being transported and deposited into large membrane-bounded specialized compartments like PBs and protein storage vacuoles (PSVs) protecting them from cytoplasmic proteases [2]
We have studied the morphometric and immunohistochemical characteristics of the PBs populations in mature seed reserve tissues, endosperm and cotyledon, and the differential cytological changes that occur in olive cotyledon during seed germination, with a focus in PBs proteins mobilization toward a better understanding of the physiological process accompanying stored proteins degradation along seed in vitro germination stages
Mature olive seeds consist of a brown seed coat (Figure 1(A)) and a relatively thick layer of white endosperm, which surrounds a white embryo (Figures 1(B) and (C))
Summary
The effective mobilization of seed storage reserves and their degradation during seed germination is a crucial physiological process in the plants life cycle [1].Seed Storage proteins (SSPs) in dicotyledonous plants, mainly legumin-like (11S globulins) and vicilin-like (7S globulins) proteins, accumulate in specialized storage cells of the embryonary axis (cotyledon and endosperm)[2], and are protected against uncontrolled premature degradation through two major mechanisms, 1) structural features that prevent cleavage by proteases simultaneously present in the same compartment [3,4] and 2) protein mature forms being transported and deposited into large membrane-bounded specialized compartments like PBs and protein storage vacuoles (PSVs) protecting them from cytoplasmic proteases [2]. The effective mobilization of seed storage reserves and their degradation during seed germination is a crucial physiological process in the plants life cycle [1]. SSPs mobilization and degradation are key events during seed germination [7]. These are complex and multi-stages processes of developing plants, for which efficiency of reserve mobilization, and of seedling establishment itself, depends on the extent of reserve accumulation during seed development and maturation [8]. The major metabolic events occurring during seed germination are different depending of the specie, as well as their genetically determined diversity seed morphology, physiological maturity, developmental pattern and chemical reserves [2,10,11]
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