Agriculture Reference
In-Depth Information
Because of the long and rich history of plant tissue culture (Street
1973; Sugimoto et al. 2011), the effects of environmental conditions,
such as time in culture and chemical composition of the culture environ-
ment of somatic and pluripotent cells, on both the
fidelity and the
normal execution of the somatic development program have been
more extensively studied and documented in plants (Matzke and Scheid
2007). The general principle of environment-controlled pluripotent
development also was much earlier recognized in plants with the
ubiquitous effects of balancing key hormones (i.e., auxin and cytokinin)
in the achievement of plant regeneration from somatic cell tissues (Street
1973). This was eventually realized in animals as well (Blau et al. 1983).
The genetic and epigenetic factors that control totipotency and pluri-
potency through both differentiation or dedifferentiation are not under-
stood as well in plants as in animals now, where several master
control transcription factors (TF) have been identi
ed (Chambers and
Tomlinson 2009). Mouse experiments have now determined that there
are only three types of crucial TFs, NANOG, SOX2, and OCT4 that are
needed to reprogram and establish stem cell properties (Takahashi and
Yamanaka 2006; Surani and Reik 2007). In many circumstances only
SOX2 and OCT4 are required because NANOG may be a long-lived
protein (Huangfu et al. 2008). Also, STAT3 and associated factors such
as LIF aremajor components needed tomaintain pluripotency of cultured
mouse stem cells (Sekkaï et al. 2005). Several functionally related plant
genes and their products such as those of the WUSCHEL (Sugimoto et al.
2011) family of transcription factors and others, including PLT, STM,
KNOX, andGRAS (Lawand Jacobsen2010;Miguel andMarum2011), that
appear to be required for totipotency andpluripotency have been revealed
(Elhiti et al. 2010), but appear unrelated in sequence and domain structure
to those in animals. From investigations of several species it is clear that
prefertilization factors in the egg cytoplasm and from the sperm nucleus
can start and eventually continue all necessary cascades of gene expres-
sion leading to full embryo development.
There is less consensus on the timing of the developmental epigenetic
reprogramming process in sexual reproduction in plants compared with
animals. It appears that major DNA demethylation begins at gametogen-
esis (Feng et al. 2010a; Jullien and Berger 2010; Law and Jacobsen 2010).
In eggs, prezygotic DNA demethylation appears to occur in the endo-
sperm lineage but is less certain in the embryo lineage (Hsieh et al. 2009).
Uncertainty of the resetting process also is indicated by experiments
showing that plants tolerate a high level of methyl mark plasticity and it
can require several generations to re-establish these marks. Epimark
status of the fertilized egg and early embryos in plants has been very
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