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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 tomato shoot apex have shown that for leaves and flowers, auxin application determines primordium development in the radial dimension (Reinhardt et al., 2000). The cellular and subcellular distributions of the efflux proteins PIN and AUX1 in the surface layers of cells towards the tip of the shoot apical meristem were shown to correspond to the subsequent differentiation of leaf primordial (Reinhardt et al., 2003). VI.2.b Root initiation and gravitropism Auxin is transported into distinct tissues of the root drives lateral primordium development (Blilou et al., 2005; Casimiro et al., 2001). Specific PIN proteins then contribute to root patterning, particularly AtPIN4 (Friml et al., 2002), although the AUX1 protein is again an essential contributor. The center of auxin concentration in roots is at the root tip, just behind the cap (Bhalerao et al., 2002). From here both AUX1 and PIN proteins work to produce defined gradients along the root epidermal layers stretching back into the elongation zone (Ottenschlager et al., 2003). These control not just lateral rooting but also gravitropism (Friml et al., 2002; Müller et al., 1998). VI.3. Apical dominance and branching In addition to controlling the initiation of leaf primordia at shoot apices it has been recognized for many years that auxin transported back from the shoot apex controls the outgrowth of side branches from axillary buds (Figure 13). Gardeners and horticulturalists make use of this apical dominance to control plant structure by pruning. By removal of the stem apex, the source of auxin is removed and its inhibitory action on axillary buds is released, leading to side branch outgrowth and bushier plants. Consistent with these observations, classic experiments by Thimann and Skoog showed that if auxin is applied to the pruned apical stump, bud outgrowth is inhibited (Davies, 1995). Apical dominance is active in roots as well as stems. Root tip-derived auxin certainly inhibits lateral root initiation close to the apex (Casimiro et al., 2001). 18

ChapitreI: Bibliographic review (Teale et al., 2006) Figure 13. The developmental processes that are controlled by auxin flux. a Lateral root. PINs conduct auxin from the centre of the root (stele) to the new root tip (auxin is indicated in green and auxin transport is indicated by red arrows), and then away again through the epidermis. b Embryo. Auxin is taken to the very young embryo by PIN7 (left). At a later stage (right), the auxin flux is reversed as PIN1, PIN4 and PIN7 conduct auxin out of the embryo. Transport by PIN1, PIN4 and PIN7 is indicated by blue, green and red arrows, in corresponding order. c Shoot apical meristem. Auxin is redirected towards the site of new leaf formation (primordial P1 and P2 and the incipient primordium) in the epidermal layer. The shoot apex is indicated in blue. d Leaves. Auxin mediates vascular tissue development and patterning in the developing leaf through non-polar PIN1. The arrows indicate sites of auxin production and the redcircles indicate auxin accumulation. e. Main root. PINs determine the flux of auxin towards the root tip in the centre of the root, and back again in the epidermis. This movement forms the basis of the root’s ability to respond quickly to gravity. VI.4.Tropisms Plants respond to different signals as light and gravity by differential growth. Auxin gradients, generated by polar auxin transport, have been implicated. This hypothesis is supported by the experiments of Cholodny and Went (Davis., 1995). In stems, greater auxin concentrations enhance elongation growth. In roots auxin inhibits growth. Auxin measurements have shown that the side of stems expanding more rapidly was found to contain a little more IAA (Harrison and Pickard., 1989). Recent data on the stimulus-induced redistribution of specific members of the PIN family in roots (e.g., PIN3), data from auxin measurements, 19

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