Ultrastructure of the Epidermal Cell Wall and Cuticle of Tomato Fruit (Solanum lycopersicum L.) during Development

Abstract

The epidermis plays a pivotal role in plant development and interaction with the environment. However, it is still poorly understood, especially its outer epidermal wall: a singular wall covered by a cuticle. Changes in the cuticle and cell wall structures are important to fully understand their functions. In this work, an ultrastructure and immunocytochemical approach was taken to identify changes in the cuticle and the main components of the epidermal cell wall during tomato fruit development. A thin and uniform procuticle was already present before fruit set. During cell division, the inner side of the procuticle showed a globular structure with vesicle-like particles in the cell wall close to the cuticle. Transition between cell division and elongation was accompanied by a dramatic increase in cuticle thickness, which represented more than half of the outer epidermal wall, and the lamellate arrangement of the non-cutinized cell wall. Changes in this non-cutinized outer wall during development showed specific features not shared with other cell walls. The coordinated nature of the changes observed in the cuticle and the epidermal cell wall indicate a deep interaction between these two supramolecular structures. Hence, the cuticle should be interpreted within the context of the outer epidermal wall.

Figure 1.

Number of epidermal cells per surface unit (mm²) during Cascada fruit growth. Data are expressed as mean ± se.

Figure 2.

Electron microscopy pictures of the epidermis of Cascada fruits during the first 6 d of growth. TEM (A, B, C, D); Scanning electron microscopy (E, F). Ovary (A), bar 10 μm; 2 daa (B), bar 2 μm; 5 daa (C), bar 5 μm; 6 daa (D), bar 10 μm; ovary (E), bar 1 μm, 10 kV; 5 daa (F), bar 8 μm, 10 kV. Arrows indicate the presence of wrinkles.

Figure 3.

TEM pictures of epidermal cross sections of Cascada fruits. 8 daa (A), bar 10 μm; 9 daa (B), bar 5 μm; 10 daa (C), bar 10 μm; 15 daa (D), bar 5 μm.

Figure 4.

Evolution of cuticle and outer epidermal wall thickness during the early stages of fruit growth. Open circles, cuticle; solid circles, cell wall. Data are expressed as mean ± se.

Figure 5.

High magnification pictures of the epidermis of Cascada fruits during the cell division period. Ovary (A), bar 500 nm; 2 daa (B), bar 200 nm; 7 daa (C), bar 500 nm; 7 daa (D), bar 200 nm; 9 daa (E), bar 500 nm; 10 daa (F), bar 500 nm. Arrows indicate the location of globular structures.

Figure 6.

Changes in cuticle and outer epidermal wall thickness throughout fruit growth and ripening. Open circles, cuticle; solid circles, cell wall. Data are expressed as mean ± se.

Figure 7.

TEM pictures of epidermal cross sections of Cascada fruits during cell expansion and ripening. A, B, 20 daa, bar 10 μm (A) and 2 μm (B); C, D, 35 daa, bar 10 μm (C) and 2 μm (D); E, F, 55 daa, bar 10 μm (E) and 2 μm (F).

Figure 8.

Cell wall ultrastructure of Cascada fruit epidermis. 2 daa (A), bar 0.2 μm; 5 daa (B), bar 0.5 μm; 10 daa (C), bar 1 μm; 14 daa (D), bar 1 μm; 30 daa (E), bar 1 μm; 50 daa (F), bar 2 μm.

Figure 9.

Immunolocalization of cell wall components in epidermal cross sections of Cascada fruits. 7 daa non-esterified pectin (A); 5 daa esterified pectin (B); ovary crystalline cellulose (C); 35 daa crystalline cellulose (D). Gold particles are encircled in red. Bars = 200 nm.

Figure 10.

Photograph of Cascada fruits at the different stages of development studied in this work. Numbers indicate days after anthesis; 0 refers to anthesis. Bar = 1 cm.