The Arabidopsis fruit develops from a gynoecium composed of two fused carpels, which, upon fertilization, grows to become a typical silique that contains the developing seeds. After seed maturation, dehiscence takes place, and valves detach from the central septum freeing the seeds (Bowman et al., 1999; Ferrándiz et al., 1999; Fig. 1)
The dehiscence zone (DZ) is a region that is no more than a few cells wide extending the entire length of the fruit at the boundaries between the valves and the replum (i.e. external septum). At maturity, the DZ can be considered to consist of a non-lignified separation layer (SL), placed between a region of lignified cells in the valve and the lignified vasculature of the replum (Fig. 1).
The differentiation of specialized tissues in the fruit is apparent from floral developmental stages 11/12 in Arabidopsis (according to Smyth et al., 1990), which is shortly before fertilization of the ovules. After fertilization (stage 13), longitudinal creases, corresponding to SL differentiation, clearly delimit the valves from the replum. Cells at the SL are morphologically different from adjacent non-separating cells. The SL cells differentiate early during silique development as small, isodiametric cells with dense cytoplasm and many plasmodesmatal connections, and appear to remain in a state of arrested development while the adjacent cells continue to differentiate, expand, and vacuolate. Cells of the inner epidermis of the valves (ena, according to Spence et al., 1996) expand and remain thin-walled, whereas cells of the inner subepidermal layer (enb) develop thickened walls (stage 17A, Ferrándiz et al., 1999), which subsequently lignify (stage 17B). Patches of a few mesocarp cells adjacent to the SL also become lignified (stage 17B). Simultaneously, the main vascular strands that run along the replum enlarge and show much additional lignification. Yellowing and desiccation of all tissues occurs, and cells of ena finally disintegrate (stage 18). During the final stages of fruit development, detachment of the valves from the replum proceeds (stage 19). Cell separation takes place by degradation of the middle lamella at the fracture surface of the SL. Spence et al. (1996) describe the cells at the SL as remaining intact. However, studies in Brassica napus by Meakin and Roberts (1990b) and the more recent work from Rajani and Sundaresan (2001) in Arabidopsis, support the idea that autolysis of SL cell protoplasts also takes place and could be needed for the release of cell-wall-degrading enzymes. Pod shatter appears to occur by a combination of cell wall loosening at the SL and the tensions established by the differential mechanical properties of the lignified tissues at the enb layer and the valve margin patches, and of the drying exocarp and mesocarp cells (Meakin and Roberts, 1990a; Spence et al., 1996).
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