Morphological and Structural Details of Tomato Seed Coat Formation: A Different Functional Role of the Inner and Outer Epidermises in Unitegmic Ovule

Abstract

In order to understand how and what structures of the tomato ovule with a single integument form the seed coat of a mature seed, a detailed study of the main development stages of the tomato ovule integument was carried out using the methods of light and electron microscopy. The integument itself it was shown to transform in the course of development into the coat (skin) of a mature seed, but the outer and inner epidermises of the integument and some layers of the integument parenchyma are mainly involved in this process. The outer epidermis cells are highly modified in later stages; their walls are thickened and lignified, creating a unique relatively hard outer coat. The fate of the inner epidermis of integument is completely different. It is separated from the other parenchyma cells of integument and is transformed into an independent new secretory tissue, an endothelium, which fences off the forming embryo and endosperm from the death zone. Due to the secretory activity of the endothelium, the dying inner parenchyma cells of the integument are lysed. Soon after the cuticle covers the endosperm, the lysis of dead integument cells stops and their flattened remnants form dense layers, which then enter the final composition of the coat of mature tomato seed. The endothelium itself returns to the location of the integument inner epidermis.

Formation of Unique Placental Seed Capsules in the Maturation Process of the Tomato Fruit

“…Tomato seeds in their structure have features in which they differ markedly from the seeds of most other Solanaceae and, first of all, in the structure of the seed coat (skin). The process of formation of the tomato seed coat at the ultrastructure level was studied in detail in our previous article [ 10 ]. In this study, we examined in more detail the outer layer of the seed coat and the features of its interaction with the shell of the seed capsule ( Figure 5 ).…”

“…These parts of the cell walls of the seed coat are markedly fragmentary thickened (cwt) compared to the outer cell wall ( Figure 6 a). At this stage of the tomato fruit maturation, the cell walls of the outer layer of the seed coat at the base and in other parts of the walls thicken by lignification [ 10 ]. After drying and release from the seed capsule, the elongated cell faces towards the outer layer of the coat crack along the ribs into separate ribbons.…”

The orchid seed coat: a developmental and functional perspective

“…Since the seed coat completely encloses the embryo, the essential functions are to supply nutrients to the developing embryo and offer physical protection during embryo development and germination. In past decades, as the published literature shows, additional information on seed coat formation and function has been elaborated, e.g., as in Brassicaceae species (Raviv et al 2017 ) and tomato (Chaban et al 2022 ), indicating the uniqueness and importance of the seed coat. The evolution and molecular control of seed coat development have also been summarized recently by Matilla ( 2019 ).…”