de la structure à la croissance cellulaire - Université Bordeaux 1

118 M. Bourdon et al.
The rate of organ growth could be also under the influence of genome size. From
the data presented in Table 1, no obvious relationship between genome size and
length of fruit growth could be observed. For instance, pear and apple (nonendoreduplicating
species) have respectively a small (C-value ¼ 0.55 pg) and large (Cvalue
¼ 4.50 pg) genome size and display similar fruit growth lengths of around 21
weeks. The same observation is also true for endoreduplicating species, such as
tomato (C-value ¼ 1 pg) and pepper (C-value ¼ 4 pg), whose fruit both develops in
6–8 weeks. Thus, the positive correlation between genome size and life cycle
shown by Bennett (1972) may not be applicable if confined to fruit development,
at least in this range of genome size.
We also examined if the ability to exhibit endoreduplication was favored by a
small genome size, owing to the fact that a higher nuclear DNA content as a result
of autopolyploidy requires more time to replicate (Bennett 1972). From the 22
different fruit species analyzed in Table 1, it seems that the occurrence and extent of
endoreduplication is not related to genome size, since endopolyploidizing fruits
display genome sizes ranging from 0.16 to 4 pg and nonendopolyploidizing ones
from 0.43 to 4.50 pg.This result is in accordance with the findings of Barow and
Meister (2003) who observed only a low negative correlation between genome size
and endopolyploidization, which was contrary to a previous claim based on fewer
species (Nagl 1976).
5 Molecular Control of Endoreduplication
5.1 The Canonical Cell Cycle
The canonical eukaryotic cell cycle is composed of four distinct phases: an undifferentiated
DNA presynthetic phase with a 2C nuclear DNA content, termed the
G1 phase; the S-phase, during which DNA is synthesized, with a nuclear DNA
content intermediate between 2C and 4C; a second undifferentiated phase (DNA
postsynthetic phase) with a 4C nuclear DNA content, termed the G2 phase; and the
ultimate M-phase or mitosis. Mitotic cell division is the ultimate step in the cell
cycle, and leads to the transmission of the genetic information from one mother
cell to two daughter cells. The classical cell cycle thus involves the accurate
duplication of the chromosomal DNA stock in S-phase and its subsequent equal
segregation in each nascent cell as a result of mitosis.
The progression within the four phases of the plant cell cycle is regulated by a
class of conserved heterodimeric protein complexes consisting of a catalytic subunit
referred to as cyclin-dependent kinase (CDK) and a regulatory cyclin (CYC)
subunit whose association determines the activity of the complex, its stability, its
localization and substrate specificity (Inzé and De Veylder 2006). The canonical
A-type CDK (CDKA; Joubès et al. 2000) harboring the PSTAIRE hallmark in the
cyclin-binding domain displays a pivotal role during the cell cycle as it participates