Fig. 1 Cellular aspects of endoreduplication in tomato fruit. (a) Pericarp histology in mature green fruits from two distinct tomato lines (left: Gardener's delight; right: Montfavet), showing the variability in pericarp tissue patterning resulting from differences in cell expansion. vb: vascular bundles; ie: inner epidermis. (b) Flow cytometry analysis of mature green fruit pericarp from a large-fruited line, showing the distribution of nuclei according to DNA content (C-value). (c) DAPI-stained nuclei isolated from mature green tomato pericarp (cherry line) and sorted according to their fluorescence intensity ; from left to right: 4C, 8C, 16C, 32C, 64C, 128C) ; note the increase in size and the increasing complexity of condensed chromatin distribution revealed by DAPI fluorescence. (d) FISH on two nuclei isolated from mature green pericarp tomato and flow cytometry sorted according to their ploidy class (left: 2C, right: 64C); DAPI-stained DNA appear in blue, and hybridization spots of a BAC probe specific for chromosome 7 appear in red after Texas-red revelation randomly-oriented cell divisions occur for longer periods up to 10-18 days post-anthesis (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 pre-or 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).
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