Agriculture Reference
In-Depth Information
dormant embryo within the seed (Stone et al. 2008). Seed formation from
the fertilized egg and subsequent germination and plant development is
actually a gradual form of metamorphosis. A seed is an indeterminate
stress-tolerant developmental stage that may actually remain dormant
and persist for hundreds of years before germinating and gradually
metamorphing into an adult organism.
It is generally thought that the terrestrial lineage of plants began from
green algal ancestors that became at some point tolerant to extreme
desiccation (Oliver et al. 1998). Desiccation-tolerant species that
evolved into seed plants are now represented in the extant species
by resurrection plants (Farrant 2007) that display extreme (
5% water
by weight) desiccation tolerance in the vegetative phase as well. The
appearance of the highly tolerant, but dormant, seeds likely occurred
from ancestral resurrection-type species to achieve rapid growth dur-
ing less dry periods while allowing survival during an extreme dry
season, a version of the original defense mechanism of the spore
(Oliver et al. 2005). As an extreme growth adjustment, dormancy or
semidormancy is initiated in
<
flowering plants to resist environmental
extremes not only by the formation of seeds but also by vegetative
dormant structures (lea
ess buds) as seen in perennial plants. Epige-
netic mechanisms also appear to be involved in the control of the
activity
dormancy cycle (Cooke et al. 2012). The development pro-
gram that is executed to form dormant seeds and buds (both contain
meristems) not only allows the tolerance of extremes in temperature
and desiccation but may also evolve further into an attenuated form of
the ping
-
pong response program. This is represented in angiosperms
now by the nearly universal graduated stress response of reducing
growth rate that may represent a more sophisticated version of the
dormancy strategy (Bressan et al. 1990; Maggio et al. 2006). The
hormone abscisic acid (ABA) is considered to be the most key univer-
sal signal controlling the responses of plants to limited water, espe-
cially the reduced growth response (Fujii et al. 2009; Melcher et al.
2009). ABA was originally also called Dormin because of its key role in
dormancy. Many of the
-
first mutants in
Arabidopsis
that were identi-
fied as insensitive to ABA were screened by the decreased ability of
ABA to induce and maintain seed dormancy (Chinnusamy et al. 2008).
It is quite interesting to note that many of these mutations also
impaired the ability of stomata to close in response to ABA. Stomatal
closure, of course, is part of the overall growth reduction or plant
dormancy-like mechanism that is universally engaged in adaptation to
osmotic-based environmental stresses.
