Biology Reference
In-Depth Information
and a permissive temperature in order to proceed to germination.
However, quiescent seeds can be orthodox or recalcitrant [
19
,
27
].
Orthodox seeds present higher dehydration tolerance, whereas
recalcitrant seeds are dehydration-sensitive both throughout devel-
opment and after shedding from the parent plant [
28
]. Information
on the biochemical and cellular alterations produced by dehydra-
tion in recalcitrant seeds is rare, and the bases of this sensitivity
remain obscure. Recalcitrant seeds lose their viability if stored for
any length of time, even under conditions that are normally con-
ducive to seed longevity, i.e., low moisture content and low tem-
peratures. Many tropical plants, e.g., coconut (
Cocos nucifera
L.),
rubber (
Hevea brasiliensis
Müll. Arg), and tea (
Camellia sinensis
(L.) Kuntze), have recalcitrant seeds [
28
].
In contrast, after reaching physiological maturity, seeds of many
plant species enter a state of dormancy during which they will not
germinate (or germinate very slowly). Seed dormancy is believed to
be an adaptive trait that prevents premature germination during
adverse environmental conditions. Under natural conditions, seed
survival in the soil and cycling through states of dormancy are major
ecological characters determining entry and persistence in ecosys-
tems [
29
,
30
], and seed dormancy is a major trait altered during
domestication of wild species [
31
]. Embryo dormancy is a geneti-
cally and environmentally determined developmental state imposed
following imbibition of mature seeds, in which cells are metaboli-
cally active, but growth processes are repressed [
23
].
Breaking of dormancy can either occur gradually in the dry
state (after-ripening) or be initiated by imbibition under defi ned
conditions (e.g., stratifi cation). Changes in the proteome (includ-
ing changes due to PTM) accompany both after-ripening [
7
,
8
],
and stratifi cation (Fig.
3
).
Germination sensu stricto encompasses the events beginning
with hydration of the mature dry seed and elongation of the
embryonic radicle such that it penetrates the seed coat [
19
,
23
,
32
,
33
]. Following germination there is a period of postgerminative
growth and development that precedes autotrophy [
19
]. Many
studies, including most that have used a proteomics-based strategy,
use the term “germination” when they are actually studying post-
germinative growth [
34
]. In most instances the time between seed
hydration and true germination is relatively short, and while there
are a few instances of bona fi de changes in protein abundance asso-
ciated with germination [
35
], major changes in protein profi les are
more typically associated with de novo synthesis of enzymes
involved with mobilization of reserve polymers during postgermi-
native growth [
3
,
36
,
37
]. Considering the breadth of seed physi-
ology, except in instances where the changes that occur within a
physiological stage or that accompany transition from one stage to
another are specifi cally targeted, it is critical that comparisons are
made between samples that are truly comparable!
