de la structure à la croissance cellulaire - Université Bordeaux 1

120 M. Bourdon et al.
transition in plant cells is exerted through the retinoblastoma-related protein (RBR)
pathway (de Jager and Murray 1999; Gutierrez et al. 2002), where the hypophosphorylated
form of RBR binds to the E2F-DP dimeric transcription factor, thereby
repressing the E2F-responsive genes required for the commitment to the S-phase,
such as those encoding the pre-RC components. Interestingly, the ectopic expressions
of either CDC6 (Castellano et al. 2001) or CDT1 (Castellano et al. 2004), or E2Fa
with its dimerization partner DPa (De Veylder et al. 2002) are sufficient to trigger
extra endocycles, as well as the inhibition of RBR function, consistent with the
inhibitory function of RBR on E2F (Park et al. 2005). It is noteworthy that the
activation of DNA replication in all these transgenic plants results in extra endocycles
and also causes prolonged cell proliferation activity. These data illustrate the conservation
of molecular controls between the endocycle and the canonical cell cycle, but
more importantly the importance of regulating the CDK activity, including the
pivotal role of MIF at the onset of endoreduplication. It was thus proposed as a
quantitative model that the amount of CDK activity controls the differentiation – and
obviously the endoreduplication – status of a cell (De Veylder et al. 2007).
5.2.3 Regulation of CDK Activity in Endoreduplication During
Fruit Development
Among the potential mechanisms regulating the CDK activities in endoreduplicating
tissues, three distinct mechanisms are proposed to be affecting the components
of the CDK/CYC complexes at the posttranslational level: (1) the WEE1
kinase regulates negatively the CDK activity by phosphorylation of residue Tyr15
before the commitment to mitosis as to ensure that DNA replication and repair on
damaged DNA have been completed; (2) the active CDKA/CYCD complexes may
be inhibited by specific CDK inhibitors, termed ICK/KRPs (for Interactor of cyclindependent
kinase/Kip-related protein); (3) loss of CDK activity occurs upon the
proteolytic destruction of the cyclin subunits via the ubiquitin proteasome pathway,
involving the activation of the anaphase-promoting complex (APC) through its
association with the CCS52.
Since endoreduplication plays such an important part during fruit development
(Chevalier 2007), the relative contribution of these different control mechanisms on
CDK activity has been addressed in tomato.
Role of WEE1
Homologues to WEE1 have been isolated from various plant species (Shimotohno
and Umeda 2007). While functional analyses performed in Schizosaccharomyces
pombe indicated that expression of the maize or Arabidopsis gene led to the inhibition
of cell division and significant cell enlargement (Sun et al. 1999; Sorrell et al. 2002),
Arabidopsis knock-out mutants for WEE1 grow perfectly well under nonstress conditions
(De Schutter et al. 2007). Neither cell division nor endoreduplication was
affected in these mutants, thus indicating that WEE1 is not rate-limiting for cell cycle