Biology Reference
In-Depth Information
4.1. Chromatin changes at the core of reproductive
development transitions
Plant reproductive development thus follows a series of developmental tran-
sitions: (1) from the soma to the precursor cells of the male and female gam-
etes, equivalent to the animals' primordial germ cells; (2) from the haploid
spores to the gametes; and (3) from the gametes to the embryo upon fertil-
ization (
Fig. 6.2
). These transitions entail global epigenomic changes that
affect chromatin in a cell-specific manner and result in the establishment
of germ cell and zygote-specific chromatin states (
Autran et al., 2011;
Calarco et al., 2012; Ibarra et al., 2012; Ingouff et al., 2010; Pillot et al.,
2010
). Recent progress is starting to shed light on the importance of these
chromatin states for gamete and seed development, and also for the repro-
gramming of genome activity.
The first developmental transition during plant reproduction is the spec-
ification of germ cells from the somatic tissues. In the ovules, this transition is
governed at least partially by small RNA-dependent pathways acting onDNA
methylation in the precursors of the germ cells, the archespore (
Garcia-
Aguilar, Michaud, Leblanc, & Grimanelli, 2010; Olmedo-Monfil et al.,
2010; Singh et al., 2011
). Defective alleles of
ARGONAUTE9
(
AGO9
)
in
Arabidopsis
produce ovules with multiple germ cells indicating that this
RNAi-associated function likely contributes to repress germ cell specification
in the ovule to a single archespore. AGO104 deficiency in maize turns meiosis
into a mitosis-like division at a high frequency, indicating a role in repressing
somatic (mitotic) fate in the germ cells. AGO104 and AGO9 protein accu-
mulation is restricted to sporophytic cells that surround the gametic cells,
whichmight indicate non-cell-autonomous spreading of a mobile silencing
signal from the somatic cells. AGO9 and AGO104 bind heterochromatic
24-nt siRNA and are involved in the establishment or maintenance of
involving movement of the small RNAs from the soma, has been proposed during germ
cell specification but may also act at later stages of gamete development (not indicated).
In the mature gametophytes, small RNAs originating from companion cells target
gametic chromatin in the egg and the sperm cells. The gametes are characterized by
distinct epigenomes indicated by different colors. The epigenetic states of the paternally
(♂) and maternally (♀) inherited genomes following fertilization are indicated sepa-
rately. The extent of epigenome reprogramming in the embryo remains a matter of
debate, but the picture assumes a phase of reprogramming before the first division
of the zygote, resulting in the reestablishment of a mostly
“
somatic
”
genome where
parental origins have been largely erased.
