All plant hormones that are involved in elongation growth, such as auxins, brassinosteroids (BRs), ethylene and GAs, were potential actors in SAS elongating responses (Vandenbussche et al., 2005). Interactions with the signalling pathways of these hormones have actually been established and will be discussed in this section. The role of jasmonates, another hormone class directly related with defence responses, will be discussed in the next section.
Auxins are known to regulate multiple aspects of plant growth and development. These include cotyledon/leaf expansion, hypocotyl/stem elongation and apical dominance, processes commonly associated with the SAS. Through links to the auxin system, light is able to manipulate plant growth and development in response to the frequent changes in the external environment. In general, light perception imposes a strong influence on the levels, transport and response to auxins (Halliday and Fankhauser, 2003). In the case of plant proximity perception, phytochromes control auxin biosynthesis, transport (distribution through the seedling) and the response to auxin within individual cells.
Recently, it was shown that reduction in the levels of active phyB induced by low R:FR light perception produces a rapid (within 1 h) rise in endogenous levels of free indole-3-acetic acid (IAA). This effect involves the action of TAA1 (TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS 1)/ SAV3, since sav3 mutants, that have reduced IAA levels, are unable to mount an elongation response to low R:FR light. TAA1 encodes an auxin biosyn-thetic enzyme required for the shade-induced rapid rise in the IAA levels and for full induction of SAS responses (Tao et al., 2008). Treatment of wild-type seedlings with the auxin transport inhibitor naphthylphthalamic acid (NPA) suppresses the SAS response of the hypocotyl (Steindler et al., 1999). Furthermore, shoot-derived auxin has been shown to be important for lateral root production, a response that is perturbed by treatment with low R:FR light (Bhalerao et al., 2002; Salisbury et al., 2007). These experiments led to propose that exposure of seedlings to simulated and/or canopy shade produces a reduction of basipetal auxin transport through the central cylinder of the hypocotyl tissues in favour of lateral cell layers in the same organ.
Because the lateral cell layers are less efficient in draining out auxin from the auxin source, this is likely to result in a transient increase of the auxin pool within these cell layers, enhancing cell elongation in the hypocotyl. In addition, this would result in a reduction in the amount of auxin that reaches the roots (Morelli and Ruberti, 2000, 2002). Together, these experiments involve phytochromes in auxin transport during the regulation of SAS responses. Subjecting seedlings to low R:FR light induces a change in the pattern of expression of GUS in the DR5:GUS line (that contains an artificial auxin reporter gene construct), whose expression is thought to reflect the levels of free auxin (Sabatini et al., 1999; Tao et al., 2008). Although the use of DR5: GUS as a marker of free auxin levels has been recently objected (Petersson et al., 2009), its unquestioned activity as an early auxin-responsive marker within the context of the whole seedling allows to address whether simulated shade alters the response to auxin in different organs and/or within individual cells. Indeed, DR5:GUS seedlings have enhanced GUS expression in the lower third of the hypocotyl and in the cotyledons, and reduced expression in the roots (Salisbury et al., 2007; Tao et al., 2008), indicating that, after plant proximity perception, phytochromes also control the local response to auxin.
Several of the genes of the HD-Zip class II sub-family have been implicated in shade signalling (see Section VI). As indicated, plants with elevated levels of ATHB2 display long hypocotyls, reduced cotyledon expansion and a reduced root system (Schena et al., 1993). Auxin-related aspects of the overexpression phenotype, such as lateral root number and hypocotyl length, are restored or abolished following auxin or NPA application, respectively (Steindler et al., 1999). Overexpression of HAT2 also results in auxin-related phenotypes, such as epinastic cotyledons and reduced lateral root production (Sawa et al., 2002). Epinastic cotyledons are typically displayed by auxin-overproducing plants (Boerjan et al., 1995). Finally, overexpression of ATHB4 also increased cotyledon epinasty, particularly under simulated shade conditions. This trait was suppressed when NPA was applied. In addition, plants with increased ATHB4 activity were shown to display a reduced hypocotyl response to 1||M 2,4-D (a synthetic auxin), a dose known to induce hypocotyl elongation, suggestive of a role for this factor in affecting the responsiveness of seedling hypocotyls to auxins (Sorin et al., 2009). Together, these results strongly suggest that the shade-induced expression of these HD-Zip genes may promote SAS responses by affecting auxin levels, transport and/or sensitivity. Therefore, these factors might integrate the shade signal perceived by the phyto-chromes with some aspects of auxin responsiveness in the modulation of SAS responses.
BRs are powerful growth promoters that control SAS-related traits such as hypocotyl/stem elongation and cotyledon/leaf expansion. The implication of BRs in the control of photomorphogenic development was initially suggested by the de-etiolated phenotype of mutants defective in different genes that encode enzymes that function in BR biosynthesis (e.g. DET2, CPD and DWF4) (Li and Chory, 1999). Specifically, the involvement of BRs in the regulation of the SAS was suggested because the hypocotyl of the Arabi-dopsis BR biosynthesis mutant dwarfl-101 does not elongate when exposed to canopy shade (Luccioni et al., 2002) (Table I). Similarly, the hypocotyl of Arabidopsis BR biosynthesis mutant det2 displays a reduced elongation in response to simulated shade (Fig. 7) (Table I). Another link between simulated shade perception and BRs action was proposed because enhanced expression of BAS1, which encodes a BR-inactivating enzyme, suppresses the long hypocotyl phenotype of phyB mutant seedlings, which display a phenotype reminiscent of a constitutive SAS (Neff et al., 1999). This evidence suggests the importance of an intact BR pathway for the normal d0 d2 d7
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