Saturday, February 5, 2011

Seeds control the survival and reproductive capacity of plants

Seeds control the survival and reproductive capacity of plants and therefore occupy a critical position in the life history of higher plants. The successful establishment of the new plant, both temporally and spatially, as well as the vigour of the young seedling, is largely determined by physiological and biochemical processes that occurred earlier, i.e. during seed development. At dispersal, the quiescent mature seed, upon encountering favourable environmental conditions (that can include light of a given wavelength, sufficient water, optimal temperatures and adequate oxygen), commences germination.

Germination, sensu stricto includes those events commencing with imbibition or uptake of water by the quiescent dry seed and culminates with the elongation of the radicle (Bewley and Black, 1994; Bewley, 1997). It is accompanied by a reactivation of metabolic systems, a process that depends in part on components that were preserved during seed desiccation as well as on components that are de novo synthesised. Ultimately, there is renewed cell expansion (elongation of the radicle) and cell division as the seedling becomes established (Figure 33.1). Visible evidence of the completion of germination is usually protrusion of the radicle through the seed structures surrounding the embryo (such as the testa and endosperm, or megagametophyte).

Subsequent events, including activation of the shoot apical meristem and the mobilization of the major storage reserves are associated with the growth of the seedling. Some seeds fail to complete germination under seemingly favourable conditions, even though they are viable. These dormant seeds must be exposed to environmental cues such as periods of warm-dry conditions (after-ripening), moist chilling, or even smoke for dormancy to be terminated (Adkins et al., 1986; Egerton-Warburton, 1998).

Although generally considered an undesirable trait by the agricultural and forest industries, dormancy in nature is clearly an adaptive trait, since it improves survival by optimising the distribution of germination over time. For example, a seed that germinates in spring after remaining dormant throughout the winter has a greater chance of successful seedling establishment than that which germinates immediately after its dispersal in the fall. Seed dormancy can also lead to a distribution of germination in space (Bewley and Black, 1994). Consider those seeds whose dormancy is terminated by light, the most effective wavelengths being 660 nm (the red region of the spectrum).

This mechanism prevents these seeds from germinating and attempting seedling establishment under competitive situations (e.g. in light transmitted through green leaves, since it is poor in the red component) or when the seeds are buried at depths in the soil to which suitable light cannot penetrate. These seeds readily germinate when close to the soil surface and when unrestricted by leaf canopies. Since seedling emergence from the soil is supported solely by mobilisation of the seed's reserves, some of these mechanisms for preventing germination occur in small seeds that contain a limited amount of stored reserves (Bewley and Black, 1994).

Once a seed has initiated cell division and becomes a seedling, it passes from its most stress-resistant state to its most stress-susceptible state, during which it is highly vulnerable to environmental conditions, and there is no reversal to a resistant state. However, as will be discussed, under conditions that are not optimal for a transition from germination to seedling growth, seeds express genes that impose a transient ‘quiescence’ until those conditions become optimal. Thus, control of seed germination and growth is crucial to the survival of seeds and there are critical checkpoints at the transitions from dormancy to germination and from germination to growth.

The agricultural and forest industries rely upon seeds that exhibit high germinability and vigorous, synchronous growth after germination; hence dormancy is generally considered an undesirable trait. In agriculture, extensive breeding programmes have reduced the degree of dormancy as well as improving other traits of crops, particularly yield. In the forest industry, tree orchards have been established in order to meet the planting demands for several conifer species. Here breeding programmes are complicated by the fact that the generation time for trees are untimely long and many species are characterised by a deeply manifested dormancy. Moreover, maintaining genetic diversity is a key endeavour. Some of the problems unique to the forest industry in relation to the deep dormancy of conifer species will be discussed.

Functional genomics approaches hold promise for elucidating genes and proteins that control seed dormancy and germination. The understanding of molecular and physiological mechanisms underlying seed dormancy, particularly of angiosperms, has been accelerated through the analysis of mutants that are disrupted in their development (including dormancy inception and maintenance) as a result of a deficiency in hormone biosynthesis or response. Cloning of the genes that are defective in these mutants has opened up avenues to reduce dormancy by altering specific traits (genes) through the technology of gene transfer. The recent cloning of the genes encoding key enzymes for the metabolism of abscisic acid (ABA) (i.e. ABA 8-hydroxylase and ABA glucosyltransferase) may also lead to unique approaches for germination control. These and other biotechnological approaches for manipulating germination will be discussed.


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