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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 have been isolated as gain-of-function alleles that stabilize the proteins and confer auxin-resistant phenotypes. Domains III and IV are involved in homodimerization and heterodimerization with other Aux/IAA proteins and with Auxin Reponse Factors (ARFs) (Kim et al., 1997; Ulmasov et al., 1999; Hardtke et al., 2004) (Figure 7). Gain-of-function mutations in several Aux/IAA genes, including axr5/iaa1 (Yang et al.,2004), shy2/iaa3 (Tian and Reed., 1999), shy1/iaa6 (Kim et al., 1996; Reed., 2001), axr2/iaa7 (Timpte et al., 1994; Nagpal et al., 2000), bdl/iaa12 (Hamann et al., 2002), solitary root slr/iaa14 (Fukaki et al., 2002), axr3/iaa17 (Rouse et al., 1998), iaa18 (Reed., 2001), msg2/iaa19 (Tatematsu et al., 2004), and iaa28 (Rogg et al., 2001), have pleiotropic effects on plant growth. All of these primary mutations in Aux/IAA genes were found in highly conserved domain II, which is responsible for protein degradation. The mutations stabilize the proteins resulting in gain of function phenotype. IV.2.b. Auxin-responsive- elemen Aux/RE and isolation of Auxin Response Factor (ARF) Many genes with auxin-induced expression, including most SAUR, GH3 and Aux/IAA genes, share a common sequence in their upstream regulatory regions, TGTCTC or variants, first identified from the promoter region of the pea Ps- IAA4/5 gene (Ballas et al., 1993). Regions including this sequence, known as the Auxin-Responsive Element, or AuxRE, confer auxin-induced gene expression in synthetic constructs (Ulmasov et al., 1995, 1997b). More recently, genome-wide profiling experiments have revealed a wealth of auxin-induced genes (Sawa et al., 2002; Pufky et al., 2003; Cluis et al., 2004; Himanen et al., 2004), many of which contain AuxREs in putative regulatory regions (Pufky et al., 2003; Nemhauser et al., 2004). The identification of the AuxRE sequence led to the isolation of the Arabidopsis Auxin Response Factor1 (ARF1) gene (Ulmasov., 1997a) and subsequent genetic, genomic, and molecular studies have identified 23 ARF genes in Arabidopsis (Liscum and Reed., 2002). In addition to a conserved amino-terminal (N-terminal) domain that mediates AuxRE binding (Tiwari et al., 2003), most ARF transcription factors also contain carboxyl-terminal 12

ChapitreI: Bibliographic review (C-terminal) dimerization elements (domains III and IV). In between is a variable middle region (MR) that generally is either glutamine (Q)-rich or serine (S)-rich. This difference is apparently a major determinant of ARF function, with S-rich ARFs acting as transcriptional repressors and Q-rich ARFs as transcriptional activators in protoplast transfection assays (Tiwari et al., 2003). AuxRE binding domain Figure 9. Diagram of Aux/IAA and ARF protein with conserved domains. Aux/IAA genes share four conserved domains including the C-terminal domains III and IV that are found in most ARF proteins. These domains mediate Aux/IAA–ARF heterodimerization. Glutamine-rich ARFs such as ARF5 and ARF7, when mutated, they often give remarkable phenotypes; for example, those seen in monopteros (ARF5) (rootless) and those seen in non phototropic hypocotyl4 (ARF7) (unable to bend towards light) (Hardtke et al., 2004). There is emerging evidence that ARFs that lack a glutamine-rich middle region function act as transcriptional repressors (Tiwari et al., 2003). IV.2.c.Transcriptional control I II III IV Aux/IAA III IV ARF The effects of auxin are thought to depend on its concentration, with high and low doses eliciting different responses. At basal auxin levels, Aux/IAAs are relatively stable, homodimerize and heterodimerize with ARFs that can bind to Aux/RE in the promoters of auxin-responsive genes. The ARF-bound Aux/IAA proteins block transcription from auxin-responsive promoters by controlling the amount of free ARF transcription factors to the promoters (Figure 10a). An increase in auxin levels causes the proteasome mediated degradation of Aux/IAAs, which in turn allows for a gradually increasing number of functionally active ARF proteins and the transcriptional activation of auxin regulated genes. The Aux/IAA genes themselves are auxin-inducible. This might represent a negative feedback loop that ensures a transient response, with the nascent 13

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