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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 VII.7 Other gene bridge auxin response with other stimulus VII.7.a Auxin and light-dependant growth Darwin originally proposed that light and a transmissible signal (later discovered to be auxin) interact to cause phototropic curvature in plants (Darwin, 1880). The recent adoption of a genetic approach in Arabidopsis has significantly advanced our understanding of how phototropic and auxin-signalling pathways interact. Phototropic defects have been described for mutations disrupting several auxin transport and signal transduction components. For example, reverse genetic studies on the auxin efflux carrier gene At-PIN3 have uncovered a phototropic defect (Friml and Palme, 2002). Moreover the nph4 mutant was originally identified by its reduced phototropic response (Liscum and Briggs., 1996). The hypocotyl growth of NPH4 mutant is resistant to the auxins IAA, 2,4-D and 1-NAA (Ruegger et al., 1997; Watahiki and Yamamoto., 1997; Harper et al., 2000), suggesting that the NPH4 protein plays a central role in auxin-mediated differential growth. The NPH4 gene has been cloned and found to encode the auxin response factor, ARF7 (Harper et al., 2000). Recently (Salisbury et al., 2007) show that phytochrome regulate emergence of lateral roots at least partially by manipulating auxin distribution within the seedling. Thus, shoot-localised phytochrome is able to act over long distances, through manipulation of auxin to regulate root development. These results represent a new link between phytochrome and auxin. VII.7.b Auxin and photomorphogenesis Many genetic and biochemical experiments indicate that auxin is closely associated with photomorphogenesis (Jones et al., 1991; Behringer and Davies., 1992; Boerjan et al., 1995; Kraepiel et al., 1995; Gil et al., 2001). Several evidences suggest that members of the GH3 family are involved in phytochrome signalling. FIN219, a member of the GH3 family, is involved in phyA signalling (Hseih et al., 2000) and WES1, an Arabidopsis GH3 Gene, encoding an auxin- 38

ChapitreI: Bibliographic review conjugating enzyme, mediates phytochrome B-regulated light signals in hypocotyl growth (Park et al., 2007b). The Aux/IAA proteins provide an example of the proposed link between auxin signalling and light. Mutations in Arabidopsis Aux/IAA genes such as AXR2/IAA7, AXR3/IAA17 and SHY2/IAA3 induce photomorphogenic characteristics in dark- grown seedlings (Kim et al.,1996; Reed et al., 1998; Nagpal et al., 2000), suggesting that light may normally regulate these genes or proteins to induce morphological responses. Furthermore, Aux/IAA proteins from Arabidopsis and pea are phosphorylated by phyA in vitro. Together, these results suggest that phytochrome-dependent phosphorylation of Aux/IAA proteins may provide a molecular mechanism for integrating light and auxin signalling in plant development (Colon-Carmona et al., 2000; Tian and Reed, 2001). Additional evidence for a link between light and IAA signalling comes from the characterisation of the constitutive photomorphogenic Arabidopsis mutant long hypocotyls 5 (hy5). This gene, which encodes a bZIP transcription factor, acts as a positive regulator of photomorphogenesis (Osterlund et al., 2000; Oyama et al., 1997). Loss-of-function hy5 mutants exhibit an auxin-related phenotype and overexpressing AXR2/IAA7 in this plants can partially rescued these phenotypes (Cluis et al., 2004). Conclusion Auxin is a critical phytohormone. Complex and redundant regulation of IAA abundance, transport and response allow an intricate system of auxin utilization that achieves a variety of purposes in plant development. As a result, the study of auxin biology is making an impact on our understanding of a variety of processes, from regulated protein degradation to signal transduction cascades, from organelle biogenesis to plant morphogenesis. Despite prodigious historical and ongoing auxin research, many of the most fundamental original questions remain incompletely answered. The discovery of TIR1/AFB F-box proteins that function as a auxin receptor is a surprising development that fills in a crutial piece of the auxin puzzle and might well serve our understanding of the interaction of this key hormones with other signalling pathway (Figure 29). 39

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