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Maize Genomics and Genetics 2012, Vol.3, No.3, 13
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modification that mediate gene silencing by recruiting
HDACs and heterochromatin protein 1 (Shiio and
Eisenman, 2003). DNA methylation and histone
modifications are the two interconnected processes in
epigenetic mechanisms that influence each other‟s
recruitment to the silencing complex to reinforce
differential epigenetic states (Tariq and Paszkowski,
2004; Cheung and Lau, 2005).
4 Chromatin remodeling
Chromatin structure is associated with the active/
repressed state of a gene which is directly influenced
by the DNA methylation, histone modifications and
chromatin remodeling proteins. Open state of the
chromatin makes DNA accessible to transcriptional
machinery and gene expression while gene expression
is repressed when the chromatin attains more compact
state of heterohromatin (Fransz and Jong, 2002). His-
tone modifications may impact secondary chromatin
structures through nucleosome–DNA or nucleosome–
nucleosome interactions and by neutralizing charge in
the histone N-terminal tails (Wolffe and Hayes, 1999;
Carruthers and Hansen, 2000; Wang et al., 2001;
Gilbert et al., 2007). The acetylation of histones
corresponds with „open‟ chromatin and enhanced
transcriptional activity (Strahl and Allis, 2000) and
acetylated histone tails increase the affinity of
chromatin for bromo-domain proteins (e.g. HATs) and
promote transcriptional activation (Turner, 2000).
Chromatin remodeling is mediated by the alterations
in location and structure of nucleosomes by ATP-
dependent chromatin remodeling proteins (Narlikar et
al., 2002; Singh et al., 2011) (e.g. the SWITCH2
[SWI2]/SUCROSE NON-FERMENTING2 [SNF2]
complex) and histone- modifying complexes (e.g. the
histone deacetylase complex [HDAC]) (Fransz and
Jong, 2002). Further, the repressive complex is
maintained by the heterochromatin-associated protein
HP1 that is thought to form a repressive complex by
binding to methylated H3K9 via its chromodomain
and by interacting with SUV39 (Fransz and Jong,
2002). A plant homolog of HP1, LHP1 (LIKE
HETEROCHROMATIN PROTEIN1), has been reported
in Arabidopsis (Gaudin et al., 2001).
5 Imprinted genes in maize
Genetic imprinting is seen in endosperm of flowering
plants. During fertilization one of two sperm cells
produced by the male gametophyte through meiotic
division fuses with egg cell to form seed, the other
sperm cell unites with the two central cell of same
genetic constitution giving rise to triploid endosperm
(Drews and Yadegari, 2002) that serve to nourish
embryo (Figure 1). Parental differences in DNA
methylation have been identified for the imprinted
maize genes
ZmFie1
(Hermon et al., 2007),
ZmFie2
(Gutierrez-Marcos et al., 2006),
Mez1
(Haun et al.,
2007),
Peg1
(Gutierrez-Marcos et al., 2003),
Nrp1
(Guo et al., 2003; Haun and Springer, 2008),
Meg1
(Gutierrez-Marcos et al., 2004),
Mee1
(Jahnke and
Scholten, 2009) and
VIM5
and
YUC10
(Zhang et al.,
2011) (Table 1). All the reported imprinted genes are
preferentially expressed in the endosperm and all
except
Peg1
, VIM5 and YUC10 show maternal-
specific expression (Gutierrez-Marcos, 2003). Two types
of imprinting exist in maize, allele-specific imprinting
as seen in
R
gene (Kermicle, 1970),
dzr1
gene and
zein protein (Chaudhuri and Messing, 1994; Lund et
al., 1995a), alpha-tubulin (Lund et al., 1995b) and
gene-specific imprinting as in
ZmFie1
(Danilevskaya
et al., 2003) and
Nrp1
(Guo et al., 2003). Among the
maize genes, few alleles of the
R
locus such as R-r:
standard (
R-r:std
) was first reported to be imprinted
(Kermicle, 1970). This
R-r:std
allele is responsible for
the aleurone pigmentation of maize which shows
differential expression depending on the contributing
parent (Kermicle, 1970). The paternally inherited R
allele gives mottled phenotype while maternally
inherited R allele gives solid phenotype (Kermicle,
1970). The imprinting of R allele appears to be due to
differential expression of the maternal allele in
relation to paternal allele rather than silencing of
paternal allele (Kermicle and Alleman, 1990). Alpha-
tubulin genes exhibit differential methylation depen-
ding on whether it is maternal or paternal inheritance
(Lund et al., 1995). Locus
dzr1
, a posttranscriptional
regulator of zein protein also shows allele-specific
imprinted expression (Chaudhuri and Messing, 1994).
ZmFie1
and
ZmFie2
are maize orthologs of the
Arabidopsis
FIE
gene, (Gutierrez-Marcos et al., 2006;
Raissig et al., 2011).
ZmFie1
is imprinted in the
endosperm, which shows maternal expression, and