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Functional characterization of tomato Sl-IAA3 and Sl-hls genes. Role ...

Functional characterization of tomato Sl-IAA3 and Sl-hls genes. Role ...

ChapitreI: Bibliographic

ChapitreI: Bibliographic review Figure 17. Auxin interacts positively and negatively with other plant hormones throughout the plant’s life cycle. Some of the effects are shown. Aux, Auxin; CK, Cytokinin; ABA, abscissic acid; GA, Gibberllin. VII.1 Auxin and gibberellin (GA) The activities of gibberellin and auxin overlap with respect to the regulation of cell expansion and tissue differentiation. Auxin affects GA signalling as well as GA biosynthesis (Figure 18). In Arabidopsis, GA stimulation of root elongation has been shown to require auxin. GA-induced root elongation was inhibited by the removal of the shoot apex that is a major auxin source and this effect was reversed by auxin application. Moreover, application of the auxin-transport inhibitor NPA or mutation in the auxin efflux regulator AtPIN1 suppressed the effect of GA on root elongation. In addition to its requirement for GA signaling in the root, auxin also affects GA production in the stem by positively regulating the expression of GA biosynthetic genes (Nemhauser et al., 2006). Decapitation of pea (Pisum sativum) and tobacco (Nicotiana tabacum) shoot apices reduced the level of active GAs in the stems and this effect was reversed by auxin application (Ross et al., 2000; Wolbang and Ross., 2001). As the interactions between auxin and GA involve components from the GA biosynthetic and response pathways, we briefly introduce a few relevant players in these pathways: The first few steps of the GA synthesis, from trans- geranylgeranyl diphosphate to GA12-aldehyde, are common to all species. The final steps to produce active GAs are species specific but in most cases require activity of the GA 20-oxidase (GA20ox) and GA3ox enzymes. In contrast, the enzyme GA2ox antagonizes GA activity by deactivating GAs. The level of endogenous active GA is governed by feedback regulation, where active GAs suppress the expression of the GA20ox and GA3ox genes and promote the expression of the GA2ox gene (Figure18) (Lange and Lange., 2006; Razem et al., 2006). Auxin was shown to induce the expression of the GA biosynthetic gene GA20ox in tobacco and Arabidopsis (O’Neill and Ross, 2002; Frigerio et al., 2006). This effect of auxin was shown to transducer via the degradation of auxin signaling suppressors Aux/IAA proteins (Teale et al., 2006) and the resulting activation of the transcription factor AUXIN RESPONSE FACTOR7 (ARF7). 24

ChapitreI: Bibliographic review (Weiss and Ori., 2007) Figure 18. Network of interactions between auxin and GA. Auxin promotes GA responses by destabilizing DELLA and by promoting the expression of GA biosynthetic genes. Interactions mediated by changes in protein activity or stability are in gray and those mediated by gene expression are in black. Numbers in parentheses indicate the respective reference as follows: 1, Frigerio et al., 2006; 2, Fu and Harberd., 2003; 3, O’Neill and Ross., 2002; 4, Ross et al., 2000; 5, Wolbang and Ross., 2001. Moreover, loss of the auxin receptor TIR1, suppressed auxin regulation of GA biosynthetic gene expression (Frigerio et al., 2006). Therefore, auxin positively interacts with GA either at the biosynthesis level or by promoting DELLA degradation. DELLA proteins are the most characterized GA signalling components. DELLA proteins belong to the GRAS family of transcriptional regulators and act as suppressors of GA signalling. The interaction between DELLA domain of the DELLA proteins with GA receptor (GA INSENSITIVE DWARF1 (GID1)) stimulates binding of the DELLA proteins to an SCF E3 ubiquitin ligase via specific F-box proteins (GID2), leading to polyubiquitination and degradation of the DELLA protein by the 26S proteosome (Sasaki et al., 2003; Dill et al., 2004; Griffiths et al., 2007). In conclusion, Auxin and GA perception use a unique SCF-based proteolysis mechanism that takes advantage of F-box selectivity or target proteins. With 25

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