de la structure à la croissance cellulaire - Université Bordeaux 1

124 M. Bourdon et al.
CaMV 35S promoter as a primary approach were generated. While the Pro35S:
SlCCS52B AS plants showed no evident phenotype at all, the Pro35S:SlCCS52A AS
plants displayed smaller fruits than wild-type plants (Fig. 2b). The ploidy level of
the Pro35S:SlCCS52A AS fruits was reduced and correlated with a decrease in mean
cell size and an increased cell number.
Gain-of-function transgenic tomato plants overexpressing SlCCS52A were also
generated. In the most extreme case, the phenotype of the Pro35S:SlCCS52A OE line
OE3 exhibited gross changes in leaf development and morphology (Fig. 2c). The
plant phenotype was characterized by the appearance of under-developed (small and
curly) leaves, resembling those of Arabidopsis plants overexpressing CDK inhibitors
(Wang et al. 2000; De Veylder et al. 2001; Schnittger et al. 2003), thus suggesting
that cell division was deeply impaired in these plants. Interestingly, the ploidy level in
these plants was increased towards high DNA levels. Five more SlCCS52A overexpressing
plants were analyzed for which the overall phenotypes were less affected
allowing subsequent analyses, especially fruit growth characteristics (Fig. 2c).
In accordance with the level of SlCCS52A expression, which was not significantly
different from that of WT, fruits from line OE25 did behave like WT. On the
contrary, the growth of fruits from the four other lines expressing greater levels of
SlCCS52A transcripts displayed a slower kinetics. However, fruits from these transgenic
lines at 35 DPA tended to reach almost the same size as WT fruit.
To explain these data, the effects of a SlCSS52A overexpression during very
early fruit development are essentially, likely to affect the cell division process thus
leading to very small fruits. Thereafter, and in accordance with the functional
specificity of SlCSS52A in the control of endoreduplication, fruit growth is then
accelerated during the endoreduplication-driven cell expansion phase to recover an
optimum final fruit size close to that of WT. A kinetic study of the appearance of
highly polyploid nuclei during fruit growth in these Pro35S:SlCCS52A OE lines does
support strongly this hypothesis (Mathieu-Rivet et al. submitted).
The functional analysis of endoreduplication-promoting genes, such as WEE1
and CCS52A thus demonstrates the physiological role of endoreduplication in
fleshy fruit growth, since a reduction in cell size originating from a decrease in
DNA ploidy levels impacts the whole fruit development and final fruit size.
We cannot make short work of the above stated causality dilemma, and whether
endoreduplication is as a driving regulator for cell expansion. Nonetheless WEE1
is thought to control the endocycle G-phase length as to allow the sufficient cell
growth prior to commitment to the next nuclear DNA amplification, and accordingly
cell enlargement becomes determining for subsequent DNA reduplication.
At the organ level, manipulating endoreduplication through CCS52A overexpression
highlights the function of endoreduplication as an ultimate driving force for
fruit growth, and even an enhancer of cell growth rate facilitating and/or accelerating
fruit growth or bigger fruit size. As shown in our cytological analysis in
Table 1, endoreduplication is strong when fruit development is short and, therefore,
endoreduplication is a way to get big fruits rapidly as supported by our
molecular data.