de la structure à la croissance cellulaire - Université Bordeaux 1

108 M. Bourdon et al.
randomly-oriented cell divisions occur for longer periods up to 10–18 days postanthesis
(Gillaspy et al. 1993; Tanksley 2004). Moreover, two different modified
genetic backgrounds affect cell divisions in tomato pericarp, excluding cell-layer
forming divisions (Cong et al. 2002; Jones et al. 2002). Commonly, tissues closest
to the ovules (e.g., placenta in tomato or fruit surface in strawberry) cease division
earlier than other tissues (Coombe 1976). All these data indicate spatially and
temporally complex regulation of cell divisions in growing fruit.
After anthesis, the locular cavities in fruit are usually filled as a result of intense
cell division activity from one or more of the locule surface areas (e.g., placenta in
tomato, septum in banana and grape, endodermis in banana and orange, and seed
aril in lychee). Cell expansion then contributes to the filling of the locule, which
behaves in concert with the neighboring flesh to form fruit pulp or, as in tomato, to
form a jelly-like tissue with distinct properties from the pericarp (Coombe 1976).
In conclusion, the fruit as a whole is composed of cells, which were present at
anthesis and of newly formed cells during fruit growth. The ratio between both kinds
of cells is a function of the number of doublings at these two phases. After anthesis,
80–97% of fruit cells in apple, strawberry, peach, apricot and tomato are produced,
whereas ca. 70% of fruit cells are formed before anthesis in cucumber and blueberry
(Cano-Medrano and Darnell 1997). The modulation of cell division in fruit either preor
post-anthesis has repeatedly been associated with strong variations in fruit size.
As an illustration, strong differences in overall anticlinal, but not periclinal, cell
division in the pericarp are associated with varying levels of fw2.2 transcripts
corresponding to the major quantitative trait locus (QTL) for tomato fruit size,
which accounts for as much as a 30% difference in fruit fresh weight between
small-fruited and large-fruited tomatoes (Cong et al. 2002; Liu et al. 2003).
2.4 Cell Expansion During Fruit Growth
In eukaryotic cells, cell enlargement results from two processes: cell growth by
increase in cytoplasmic volume and cell expansion through vacuolation. Cell
growth by cytoplasmic volume increase may occur in all types of cells and
is responsible for moderate increases in cell volume, by less than tenfold
(Sugimoto-Shirasu and Roberts 2003). Cell expansion through vacuolation is a
specific property of plant cells because of their large vacuolar compartment, and
it leads to an increase in cell volume by more than one hundredfold (Sugimoto-
Shirasu and Roberts 2003). Cell expansion through vacuolation typically starts in
young organs, once cells have stop dividing, exit the cell cycle, and differentiate.
Some of the largest cells found in plants occur in the flesh of ripe fruit, with cell
length between 150 and 700 mm, and in some cases more than 1 mm. The volume of
cells in ovary wall is only ca. 10 3 –10 2 nL at anthesis whereas it is in the range of
1–10 nL, up to 100 nL in mature fruit (Coombe 1976). Because cell volume may be
increased by 10 2 –10 4 times during the growth of fleshy fruit, this phenomenon