Trends in Plant Science
OpinionTo Grow or not to Grow?
Section snippets
Seed Dormancy
Seed dormancy is classically defined as an intrinsic block to the germination of seeds under otherwise optimal conditions (adequate moisture, temperature, etc.) [1]. Dormancy in seeds is an adaptive trait that enables plants to move through time and space 2, 3, while defining their entry into ecosystems. Dormancy control also has a central role in plant agriculture with regards to crop establishment and weed management strategies [4]. Dormant seeds maintain an embryonic cell identity [5] and
A Simplified Developmental Framework
Following the completion of seed development, seeds receive a continuous stream of information from the environment, which is used to time their decision to terminate dormancy and commence germination (Figure 1). Following the perception of these signals, downstream signalling events integrate these multiple inputs, a process that concludes with a single output: the flipping of a developmental fate switch initiating the embryonic to vegetative cellular phase transition. Only following the
Perception
Seeds use multiple cues from the environment to terminate dormancy and decide where and when to establish a new plant. Primary signals used by arabidopsis seeds to guide their timing to germinate include light [7], temperature 8, 9, 10, 11, nutrients [12], smoke [13], and likely other signals, to provide positional and seasonal information. The perception of light in arabidopsis 14, 15 is a necessary requirement for the completion of germination in nondormant seeds, and likely not a primary
Downstream Signalling and Signal Integration
Proteins associated with the primary receptors and sensors of the environment transduce perceived external information into signals within plant cells. A range of genetic loci and diverse biochemical pathways [17] has been reported to be involved in these downstream signalling processes, and have been reviewed previously 3, 18.
Central to the integration of environmental signals in seeds are the endogenous signalling molecules abscisic acid (ABA) and gibberellins (GA), which promote dormancy and
Developmental Fate Switch
The output of downstream signalling and integration in seeds is completed with the flipping of a developmental fate switch, which leads to the concurrent termination of dormancy and initiation of germination. Fate switches have been described in diverse biological systems [29], including plants 30, 31. Delineation between each downstream signalling and signal integration cannot be clearly established within this developmental framework due to the current lack of understanding of the nature of
Flipping of the Switch
Understanding when a fate switch has been flipped is key to the delineation of developmental events. After the flipping of the switch in seeds, an irreversible commitment to a cellular phase transition from the embryonic to vegetative state is initiated [5]. Associated with this, two important and measureable changes occur that signify the flipping of the switch in seeds.
The first is the loss of seed storage potential, or the length of time a seed remains viable in dry storage [61]. The loss of
Discrete Induction of Growth
Dormant seeds represent a nongrowing stage in the life cycle of plants, where cell wall-modifying genes associated with the promotion of cell expansion are not expressed [59]. Following the flipping of the developmental fate switch, there is a discrete induction of cell wall-modifying gene expression, which promotes the expansion of cells within the embryo. This induced growth-promoting gene expression occurs downstream of genetic targets induced following the flipping of the switch, which
Spatial Control of Seed Behaviour
Most studies of seed dormancy have focused on whole-seed responses, and the spatial distribution of the events underlying this developmental transition is beginning to emerge 60, 66. The arabidopsis seed comprises an embryo and surrounding endosperm and testa. This species exhibits coat dormancy [1], whereby the removal of the surrounding tissues leads to the growth of the embryo. The role of the endosperm in the control of coat dormancy, and informing the decision whether to germinate, has
Concluding Remarks
The developmental framework proposed here seeks to assist in the delineation of events underlying the seed to seedling transition, particularly those events that occur before and after the flipping of a developmental fate switch. This will hopefully assist in the further exploitation of seeds as a developmental system and avoid confounding interpretation of data within this clarified context. Systems and modelling-based approaches represent a promising avenue to untangle the complex trait of
Acknowledgements
I am grateful to Bill Finch-Savage for helpful comments on the manuscript. G.W.B. was supported by BBSRC grants BB/L010232/1, BB/J017604/1 and BB/N009754/1, and by a Birmingham Research Fellowship.
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