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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 insensitive to the auxin 2,4-dichlorophenoxyacetic acid (2,4-D) (Bennett et al., 1996). The AUX1 is an integral membrane protein, a member of the amino acid- proton cotransporter superfamily. Arabidopsis and other plant species tend to have small gene families of AUX1-like proteins (Schnabel and Frugoli, 2004). In addition, a member of the aromatic and neutral amino acid transporter family (AtANT1) has been found to carry IAA in heterologous expression studies using yeast (Chen et al., 2001) and seems likely to contribute to overall auxin influx. (Perrot-Rechenmann and Napier., 2005) Figure 3. The dynamics of auxin transport. Auxin is imported into the cell (at the top) through influx carriers, notably AUX1 and the less specific ANT1. Influx is cotransport with protons. Inside the cell, IAA dissociates to the IAA- anion and is subject to conjugation as well as export through an efflux complex. Conjugates (and possibly free IAA) are compartmentalized through carrier proteins like MRP5. The most important component of the efflux complex is one or more PIN proteins and it seems likely that the kinase PID and phosphatase RCN1 and BIG are all involved in the regulation of PIN activity. The PIN proteins are rapidly recycled away from the plasma membrane to the plant endosomal system. New PIN proteins are secreted through a GNOMregulated ER–Golgi– vesicle pathway. Additionally, MDR1 may regulate the PIN complex and transport IAA directly. The twisted protein (TWD1) interacts with a cytosolic domain of MDR1 and also with PGP1, which in turn interacts with APM1. III.2 The efflux complex and the importance of vesicle cycling There are tree classes of transmenbrane protein that have been involved in polar auxin transport. The PINs (a plant-specific transporter family), the PGP/MDRs (Multi-Drug Resistant-like transporters) and the KUPs (Potassium 6

ChapitreI: Bibliographic review transporter-like) (Vicente-Agullo et al., 2004). The relationship between these classes is unclear; however, mutants in many of the genes encoding these proteins result in auxin transport defects. Both the PINs and the MDRs appear to be able to transport auxin directly and at least partially independently of each other (Geisler et al., 2005; Petraek, et al., 2006). The PINs are the best characterised in planta and show an excellent correlation between PIN localisation to a particular cell face, and the direction of auxin transport (Winiewska et al., 2006). PIN targeting appears to be a highly dynamic process with continuous cycling of the PINs between the cell surface and an intracellular compartment, a process dependent on ARF-GEF proteins such as GNOM (Figure3). IV. Auxin perception, receptor and signalling IV.1 .Perception and receptors: TIR1 the heart of auxin-signalling Receptor proteins are key components of signalling systems and their discovery is fundamental to both the understanding and the exploitation of hormonal regulation. In 2005, a receptor of auxin was identified as the F-box protein TIR1 (Transport Inhibitor Response 1). The TIR1 gene was first identified in a genetic screen for Arabidopsis plants tolerant to auxin transport inhibitors (NPA). However, soon after it was shown that TIR1 was involved in auxin action. TIR1 is a component of a cellular protein complex known as SCF TIR1 (Skp1/Cullin/F-box) (Dharmasiri et al., 2005; Kepinski et al., 2005) involved in ubiquitin-mediated protein degradation (Ruegger et al., 1998). The substrates for TIR1, Aux/IAA repressors, are recruited to the receptor in an auxin-dependent manner and, after binding to TIR1, are degraded. Identification of the TIR1 receptor suggested that auxin perception and the signalling pathway to auxin- regulated gene expression was direct and simple, but it left various questions. 7

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