Biology Reference
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In plants, chromatin has been studied most extensively in Arabidopsis ,
owing to its small and near-complete genome sequence, and a comprehensive
set of genomic and genetic tools. The emerging view is a subtle mix of features
conserved between plants and metazoans together with plant-specific innova-
tions (Reviewed in Feng & Jacobsen, 2011 ). In particular, plants are charac-
terized by one of the largest arrays of proteins involved in chromatin
metabolism, by DNA methylation in all sequence contexts (CG, CHG,
and CHH, where H corresponds to A, T, or C), and by an extensive diver-
sification of small RNA-based regulatory pathways ( Pontier et al., 2012 ;
reviewed in Berr, Shafiq, & Shen, 2011; He, Elling, & Deng, 2011;
Pontier et al., 2012; Simon & Meyers, 2011 ), based in part on the invention
of plant-specific RNA polymerases (Reviewed in Haag & Pikaard, 2011 ).
2.1. Chromatin components
As in other eukaryotes, canonical plant histones are typically expressed dur-
ing the S phase of the cell cycle and their incorporation is dependent on the
replication process (Reviewed in Costas, Desvoyes, & Gutierrez, 2011 ). By
contrast, expression and incorporation of histone variants occur indepen-
dently of replication. Assigning a function to the different histones is com-
plicated by the fact that plants have experienced a remarkable amplification
of the repertoire of both canonical histones and their variant isoforms, which
are often encoded by multigene families (Reviewed in Talbert et al., 2012 ).
Plant genomes contain clear homologues of the key histone chaperone com-
plexes found in other eukaryotes, including the H3-H4 chaperones
CHROMATINASSEMBLY FACTOR-1, HISTONE REGULATORY
HOMOLOG A, and ANTISILENCING FUNCTION1 ( Exner, Taranto,
Schonrock, Gruissem, & Hennig, 2006; Phelps-Durr, Thomas, Vahab, &
Timmermans, 2005; Zhu et al., 2011 ), as well as the H2A-H2B chaperones
NUCLEOSOME ASSEMBLY PROTEIN1 ( Zhu et al., 2006 ). However,
their developmental role remains, in general, poorly understood.
A striking feature of plant nucleosomes is their responsiveness to environ-
mental conditions, which can affect histone occupancy, histone variant incor-
poration, and histonemodifications (Reviewed in Zhu, Dong, & Shen, 2012 ).
This is best exemplified by the identification of theH2A.Z variant as an impor-
tant component of thermal sensing in Arabidopsis ( Kumar & Wigge, 2010 ).
Thus, increase in temperature triggers the evictionofH2A.Z in a locus-specific
manner, which results in either activation or repression of the affected loci,
suggesting thatH2A.Zoccupancy negatively regulates the accessibility of chro-
matin to both activators and repressors ( Kumar & Wigge, 2010 ).
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