The control of seed germination is a very sophisticated
process which requires the concerted action of and interaction
between diverse phytohormones (Kucera et al.
2005). In seed tissues, the best investigated are the antagonistic
actions of abscisic acid (ABA) and gibberellins
(GA) on dormancy and germination. Dormancy is an
intrinsic seed state that can be described as the inability of
an intact, viable seed to complete germination under
favorable conditions. It is a mechanism of the plant to
adapt the timing of germination to the surrounding environmental
conditions to prevent germination during seasons
when the conditions for the subsequent seedling
establishment and plant growth would be unfavorable.
Different seed tissues, such as the testa, endosperm and
embryo can contribute to dormancy; it is a very complex
trait determined by genetic factors and environmental cues,
which has been reviewed in detail by Finch-Savage and
Leubner-Metzger (2006) and Holdsworth et al. (2008a).
Hormonally, the dormant seed state is induced and maintained
by ABA and released by GA. This ABA–GA
antagonism is also evident in a broader view on plant
dormancy, in which it controls the sprouting of tuber and
tree buds (Rentzsch et al. 2011; Rohde et al. 2007). ABA
signaling components are already present in the non-vascular
bryophytes including liverworts, which represent the
most basal members of the extant land plant lineage. ABA
is considered as a major stress hormone, and its accumulation
is especially connected to dehydration-related processes
in plants (Hauser et al. 2011). The conservation of
ABA signaling across diverse plant taxa from basal land
plants to core eudicots suggests that ABA and its roles
related to stress tolerance, especially to drought stress and
dehydration, are some of the key evolutionary steps and a
fundamental process for the adaptation to air/land conditions
(Takezawa et al. 2011). Desiccation also occurs
during seed maturation of orthodox seeds, which occurs in
concert with increased ABA contents and often also with
the induction of dormancy (Kucera et al. 2005). In that
context, ABA functions as a positive regulator of physiological
dormancy, the most widespread seed dormancy
class among gymnosperm and angiosperm species (Finch-
Savage and Leubner-Metzger 2006; Nambara et al. 2010).