our conditions). Petiole elongation is responsible for most of the increase in longitudinal expansion (length) of both cotyledons and primary leaves (Fig. 2B). This is a trait very responsive to low R:FR in leaves formed in all stages of plant development (Djakovic-Petrovic et al., 2007; Lorrain et al., 2008; Tao et al., 2008).
Although there are some contrasting results, some authors have also reported that the increase in petiole length in response to simulated shade is usually associated with a reduction in the leaf blade area at later stages of Arabidopsis development (Franklin, 2008; Robson et al., 1993; Tao et al., 2008). Similar results were observed for EOD-FR treatments (Devlin et al., 1996; Johnson et al., 1994). In other species, plants that grow for extended periods under these light conditions also display long stems and an upward reorientation of leaves (leaf hyponasty) (Franklin, 2008). In a rosette plant such as Arabidopsis, however, internodes do not normally become apparent until the bolting of the floral (or cauline) stem, which occurs at the onset of flowering. In the presence of a plant proximity signal, enhancement of cau-line stem elongation can be observed, but it is usually a relatively weak response (Botto and Smith, 2002). Leaf hyponasty in combination with stem elongation serves to elevate leaves within the canopy, a response that is likely to enhance light-foraging capacity in dense stands and enable plants to overtop competing vegetation (Ballare, 1999; Mullen et al., 2006; Pierik et al., 2003, 2004b). In the case of rosette plants, such as Arabidopsis, it can also be seen as a physical element of the plant to lean over surrounding plants and help stabilize its taller stature due to the SAS.
In Arabidopsis, responses to low R:FR light also include metabolic changes, such as the reduction in leaf chlorophyll and carotenoid content in the seedling (Roig-Villanova et al., 2007). In addition, low R:FR light increases apical dominance, leading to reduced branching (Smith and White-lam, 1997). In the long term, and once the plant is competent to flower, a persistent reduction in the R:FR of the incident light conditions accelerates flowering (Halliday et al., 1994), a response associated with reduced seed set, truncated fruit development and, as indicated before, a severe reduction in the germination rate of the seeds produced (Dechaine et al., 2009; Smith and Whitelam, 1997).
Different plant species display varied SAS responses as a result of its adaptation to their particular environments. For instance, plant with inter-nodes, such as Chenopodium, cowpea (Vigna sinensis), white mustard (Sina-pis alba) or tobacco, display a very rapid (within minutes) increase in the elongation growth rate of stems after initiating the simulated shade treatment (Casal and Smith, 1988; Garcia-Martinez et al., 1987; Morgan et al., 1980; Smith, 1982). These alterations are accompanied by changes similar to those described for Arabidopsis, such as reduced chlorophyll content, hypo-nastic leaves, increase in apical dominance (leading to reduced branching in dicots and tillering in grasses) and acceleration of flowering (Casal et al., 1990; Kebrom and Brutnell, 2007; Smith and Whitelam, 1997). Growing under low R:FR light can also affect positively or negatively other plant responses: it has been shown to increase freezing tolerance in Arabidopsis (Franklin and Whitelam, 2007) and to down-regulate defence responses in tobacco (Izaguirre et al., 2006).
Overall, the SAS responses involve a global change in the development and metabolism of the plant, directed towards the increase in elongation, flowering induction and a reduced seed production to enhance the probability of a viable next generation of plants in more advantageous light conditions. However, for crop species the SAS could lead to decreased yields if plants invest resources on vegetative growth at the expense of reproductive development. It is generally believed that SAS in crop plants has probably been refined to maximize yield under limited light environments (Kebrom and Brutnell, 2007; Smith, 1982). Fundamental understanding of SAS response pathways might help to further guide breeding programmes towards the creation of varieties that respond to new demands of modern cultivation, such as the use of low-input non-food feedstocks for biofuels or the development of varieties better adapted to a warmer world (Kebrom and Brutnell, 2007; Sawers et al., 2005).
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