Abstract
The endothelium is an additional cell layer, differentiating from the inner epidermis of the ovule integument. In tomato (Solanum lycopersicum L.), after fertilization, the endothelium separates from integument and becomes an independent tissue developing next to the growing embryo sac. In the absence of fertilization, the endothelium may proliferate and form pseudo-embryo. However, the course of the reorganization of endothelium into pseudo-embryo in tomato ovules is poorly understood. We aimed to investigate specific features of endothelium differentiation and the role of the endothelium in the development of fertilized and unfertilized tomato ovules. The ovules of tomato plants (“YaLF” line), produced by vegetative growth plants of transgenic tomato line expressing the ac gene, encoding chitin-binding protein from Amaranthus caudatus L., were investigated using light and transmission electron microscopy. We showed that in the fertilized ovule of normally developing fruit and in the unfertilized ovule of parthenocarpic fruit, separation of the endothelium from integument occurs via programmed death of cells of the integumental parenchyma, adjacent to the endothelium. Endothelial cells in normally developing ovules change their structural and functional specialization from meristematic to secretory and back to meristematic, and proliferate until seeds fully mature. The secretory activity of the endothelium is necessary for the lysis of dying cells of the integument and provides the space for the growth of the new sporophyte. However, in ovules of parthenocarpic fruits, pseudo-embryo cells do not change their structural and functional organization and remain meristematic, no zone of lysis is formed, and pseudo-embryo cells undergo programmed cell death. Our data shows the key role of the endothelium as a protective and secretory tissue, needed for the normal development of ovules.
Highlights
One of the basic features of the ovule development in tomato is the formation of new tissue—the endothelium—differentiating from the inner layer of singular integument, and assembling an extra boundary tissue around the embryo sac
The cross-section of the ovule at the same stage of development demonstrates that the endothelium cells expand perpendicular to the axis of the embryo sac (Figure 1b)
Given the great attention to various aspects of the development of parthenocarpy and unequivocal interest to tomatoes as a model object for various fields of research [19,27], we aimed to investigate the consecutive stages of the formation and growth of endothelium and surrounding tissues during the development of tomato ovules and to analyze the structural changes of the endothelium during formation of pseudo-embryo in the ovules of parthenocarpic fruit of transgenic tomatoes, which we used as model plants in our previous study [8]
Summary
One of the basic features of the ovule development in tomato is the formation of new tissue—the endothelium—differentiating from the inner layer of singular integument, and assembling an extra boundary tissue around the embryo sac. If fertilization does not occur, the endothelium may separate, grow, and proliferate independently of other tissues This eventually leads to the disruption of the embryo sac and development of a pseudo-embryo—the additional tissue—which substitutes the embryo. This unique feature of the endothelium in plants from the Solanaceae family was observed during the development of ovules in hybrid plants [1,2,3]. According to these studies, rapidly proliferating endothelium may induce death of embryo sacs and developing embryos. Pseudo-embryos were often detected in the ovaries of parthenocarpic tomato fruits [4,5,6,7], but it is still unclear how the proliferating endothelium contributes to pseudo-embryo formation
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