de la structure à la croissance cellulaire - Université Bordeaux 1

114 M. Bourdon et al.
Even if polyteny seems to be widespread in various plant organs, differences in
chromatin organization can be observed between cell types during endoreduplication.
These differences lie mainly in a more or less complete pairing of sister
chromatids and in the state of chromatin condensation. Sister chromatid pairing
can be observed along the entire length of chromosomes, such as for giant hair cells
in Bryona anthers (Barlow 1975) or just in part of them, such as the embryo
suspensor cells in Phaseolus coccineus (Nenno et al. 1994). Recently, Schubert
et al. (2006) provided evidence for a differential alignment of sister chromatids
pairing along Arabidopsis chromosome 1, and thus, for polyteny in Arabidopsis
vegetative tissues. In addition, the condensation state of centromeres in Arabidopsis
endoreduplicated cells was shown to be variable and cell type-dependent (Fang and
Spector 2005). Decondensation could also account for a greater range of chromatin
movement in endoreduplicated cells (Kato and Lam 2003). Hence, further investigations
are needed to determine the biological significance of chromosome and
chromatin organization in endoreduplicated cells.
Since endoreduplication appears to be the preferred mode of endopolyploidization
in most plant organs and notably in tomato fruit, we shall use this term from
here onwards.
4 Proposed Physiological Roles for Endoreduplication
4.1 Endoreduplication and the Determination
of Cell- and Organ Size
The frequent positive correlation between endoreduplication and cell size in
many different plant species, organs, and cell types (Joubès and Chevalier 2000;
Sugimoto-Shirasu and Roberts 2003; Kondorosi and Kondorosi 2004) is commonly
interpreted as endoreduplication as the driver for cell expansion. The successive
rounds of DNA synthesis during endoreduplication induce a consequent hypertrophy
of the nucleus. This can influence the final size of the cell, which, therefore,
may adjust its cytoplasmic volume with respect to the DNA content of the nucleus
(according to the “karyoplasmic ratio” theory; Sugimoto-Shirasu and Roberts
2003). Cell expansion proceeds further through vacuolation, to adjust the size of
the vacuolar compartment in accordance with the cytoplasmic volume, and even to
increase the vacuolation index of the plant cell.
Cheniclet et al. (2005) provided evidence for such a positive correlation between
cell size and ploidy level in pericarp tissue during fruit development. However
insufficient comparison has been performed so far in other fruit species, and even in
tomato, opposing results were reported in two tomato isogenic lines differing by
fruit weight and sugar content QTLs (Bertin et al. 2003) and between large fruits of