68 W.-H. Shen
mechanisms of control for the specific demand of histone chaperon activ-
ity during the S-phase. Although it is generally accepted that euchromatin is
replicated in the early S-phase and heterochromatin in the late S-phase, the
mechanism of this time-controlled process is still poorly studied, particularly
in plants.
5
Conclusions and Perspectives
The powerful genetic approach, particularly inArabidopsis,allowsanexpo-
nentially increased number of genes to be examined for their role in plant
growth. This likely will continue and cover the other uncharacterized genes
potentially involved in the G1/S transition and the Rb-E2F pathway, as well as
in DNA and chromatin replication. Many studies are starting to uncover es-
sential functions of the G1/S transition in highly regulated cell division and
cell differentiation in plant development. However, molecular and functional
links among different regulators or pathways at the G1/S transition remain in
many cases elusive. Furthermore, because of functional redundancy, cellular
or developmental compensation as well as indirect effects, many complex as-
pects uncovered by the genetic approach need to be refined for mechanistic
understanding. These different issues appear challenging for future research.
AcknowledgementsI thank Dr. Jean Mollier for helpful comments, and apologize to col-
leagues whose work I cited only obliquely through references to other articles because of
space limitation.
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