de la structure à la croissance cellulaire - Université Bordeaux 1

116 M. Bourdon et al.
4.2 Endoreduplication and Cell Differentiation
In the model plant Arabidopsis, the influence of endoreduplication in forming
large specialized cells was best characterized in epidermal cells of mature leaves
(Melaragno et al. 1993), during hypocotyl development in which the ploidy levels
vary according to light conditions (Gendreau et al. 1997), and in leaf single-celled
trichomes (Larkin et al. 2007). The growth of trichomes was shown to be dependent
on the succession of endocycles. The formation of a two-branched trichome cell
requires three rounds of endocycle, leading to a 16C DNA ploidy level. A supplementary
endocycle may eventually occur to give rise to the formation of a third
branch and 32C DNA content. Moreover, cell growth and differentiation of trichomes
is a genetically regulated process, since mutants affecting the nuclear
ploidy level impacts positively or negatively on trichome cell size.
As illustrated for trichomes, endoreduplication often occurs during the differentiation
of cells that are highly specialized in their morphology or metabolism. This
is the case for cells from tomato fruit pericarp and jelly-like locular tissues
(Cheniclet et al. 2005; Lemaire-Chamley et al. 2005; Chevalier 2007) as described
above, for symbiotic host cells during the formation of nitrogen-fixing root nodules
in legumes (Cebolla et al. 1999) and/or for the endosperm cells of maize kernels
(Kowles and Phillips 1985; Kowles et al. 1990).
4.3 Endoreduplication and Metabolism
There are instances where endoreduplication is linked to endogenous metabolism.
For example, nodule development on legume roots is initiated in response to interaction
with the symbiotic bacterium Sinorhizobium meliloti. During their differentiation
process, symbiotic nodule cells, programmed to fix nitrogen, develop into very large
and highly endoreduplicated cells (Cebolla et al. 1999; Vinardell et al. 2003) and
display an important transcriptional activity that is remarkably specific to the nodule
(Mergaert et al. 2003). Also, in Zea mays, endosperm cells accumulate large amounts
of starch and storage proteins, concomitantly with multiple and successive endocycles
during seed development (Lopes and Larkins 1993).
Since a correlation exists between endoreduplication and cell differentiationspecific
metabolism, it is tempting to speculate that one role of endoreduplication
would be to modulate transcriptional activity by increasing the availability of DNA
templates for gene expression as the gene copy number is obviously multiplied, and
therefore, to modulate subsequent translational and metabolic activities. However,
this hypothesis has neither been convincingly demonstrated nor negated in plant
cells. For example, Leiva-Neto et al. (2004) showed that endoreduplication levels
did not clearly impact on the expression level of some endosperm-specific genes,
which led them to propose that endoreduplication in maize endosperm functions
primarily to provide a store of nitrogen and nucleotides during embryogenesis
and/or germination.