de la structure à la croissance cellulaire - Université Bordeaux 1

106 M. Bourdon et al.
Fruit growth relies on a spatially and temporally organized pattern of cell
division and of cell expansion. Remarkably, fleshy fruit tissues may have very
various ontogenic origins (Coombe 1976), although they all share similar characteristics,
with large, highly vacuolated cells with thin walls. In tomato, the ovary
wall has seemingly a simple organization at anthesis, with ca. 11 layers of small
isodiametric cells including two epidermal cell layers, and vascular bundles in the
central pericarp (Cheniclet et al. 2005). During fruit organogenesis and growth, the
fertilized ovary acquires a complex pattern of cells with various sizes and shapes,
and various metabolic differentiations (Cheniclet et al. 2005; Gillaspy et al. 1993;
Mohr and Stein 1969; Smith 1935) (Fig. 1a). How this spatio-temporal pattern of
development is related to gene expression, metabolic profiles and cellular characteristics,
such as endopolyploidy has only just started to be described (Lemaire-
Chamley et al. 2005). This apparent slow progress has been due to the difficulty in
correlating various cellular and molecular data at the level of given cell types in
three-dimensionally complex organs, such as fruits. How this complexity relates to
hormonal and nutritional regulation during fruit growth also remains largely ignored.
Many data have been reported on the kinetics of hormone content in various
fruit materials (reviewed in Gillaspy et al. 1993; Srivastava and Handa 2005), but
their use for a proper understanding of their action in relation to growth mechanisms
remains rather elusive. Fruit growth requires the combined presence of several
growth-promoting plant hormones, such as auxin, gibberellins, cytokinins and
brassinosteroids (Cowan et al. 2005; Ozga et al. 2002; Srivastava and Handa
2005). Many of these hormones appear to originate from the developing seeds,
with a particular role for the endosperm in the secretion of these compounds.
2.3 Cell Division During Fruit Growth
Active cell division within the flesh is usually restricted to an initial period of
1–2 weeks after pollination and fertilization (e.g., cucurbits, tomato), 3 weeks in
apple, 4–7 weeks in Japanese pear, peach, and plum. Cell divisions do not occur in
the pericarp of Corynth grape, Rubus, or some Ribes species. However, divisions
continue in avocado and in strawberry throughout the life of the fruit (Coombe
1976; Crane 1964; Nitsch 1965). In tomato and as in many fleshy fruits, cell
divisions occur in various cells and with various division planes to allow fruit
growth, but they also occur in discrete cell layers with definite division planes for
specific purposes. This is the case for tomato, where the two epidermal cell layers of
the pericarp undergo anticlinal divisions, whereas the two subepidermal layers
undergo several rounds of periclinal divisions leading to an increase in the number
of pericarp cell layers to a varying extent according to the tomato line. These
various types of cell divisions are differently regulated because cell-layer
forming cell divisions occur only within 5–8 days post-anthesis in various tomato
lines (Cheniclet et al. 2005; Cong et al. 2002; Mazzucato et al. 1998), whereas