Biology Reference
In-Depth Information
1. INTRODUCTION
Spatial and temporal patterns of gene expression are central to develop-
mental programs in multicellular organisms. In plants, development and
growth are continuous processes that occurmostly postembryonically. Follow-
ing seed germination, two primary stem cell populations that are established
during embryogenesis, the root and shoot meristems, generate the different
organs throughout plant life. In the absence of cellular migration, due to the
presence of a rigid surrounding wall, organ patterning mainly results from
the precise coordination of cell division, expansion, and differentiation.
Plants are characterized by a remarkable phenotypic plasticity that meets
the constraints of a sessile lifestyle. They have developed several strategies to
perceive environmental changes, integrate them with endogenous cues, and
adjust developmental pathways accordingly. Indeed, during their life cycle,
plants undergo several major developmental transitions, many of which are
determined by the environment. Chromatin-based regulation of transcrip-
tional patterns, in conjunction with a large repertoire of transcription factors,
likely underpins most of this developmental flexibility. Here, we review our
current knowledge on chromatin organization and dynamics in plants, with
a special emphasis on the role of chromatin-based mechanisms in regulating
key aspects of
Arabidopsis
development.
2. PLANT EPIGENOMES: COMPONENTS AND
ORGANIZATION
Packaging of DNA into chromatin is pivotal for the regulation of
genome activity in eukaryotes, and plants are no exception. The basic unit
of chromatin is the nucleosome, which is composed of 147 bp of DNA
wrapped around a protein octamer composed of two molecules each of
the core histones H2A, H2B, H3, and H4. Histone H1 is further associated
with linker DNA, which contributes to higher levels of chromatin organi-
zation. The functional properties, positioning, and occupancy of nucleo-
somes can be modulated in several ways. Thus, covalent modifications of
core histones, incorporation of histone variants, and other factors, such as
chromatin-modifying and remodeling enzymes, DNA methylation, and
small RNAs, all contribute to defining dynamic chromatin states that mod-
ulate access to DNA and confer distinct transcriptional outcomes (Reviewed
in
Berger, 2007; Jenuwein & Allis, 2001; Strahl & Allis, 2000
).
