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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 (Stepanova et al., 2007) Figure 23. Schematic representation of the mechanistic model of ethylene–auxin crosstalk in roots of etiolated Arabidopsis seedlings. The model assumes existence of at least three different types of molecular interactions between ethylene and auxin. A subset of ethylene responses (left side of the panel) is dependent on auxin levels. In this case, the role of auxin is restricted to promoting (or attenuating) the ethylene effect. By contrast, the auxin-mediated responses correspond to those changes in gene expression that are directly triggered by auxin, but in this case, by an ethylene-induced auxin activity. Finally, those ethylene effects that are not affected by the levels of auxin are classified as auxin independent, with some of these changes being independently stimulated by auxin. Equivalent interactions can be defined among auxin responses (right side of the panel). 1996; Raz and Ecker., 1999). Bending (180°) of the embryonic stem (hypocotyl) at its apex just below two seed leaves (cotyledons), results in the formation of an apical hook. The structure performs an important biological function, as it places the cotyledons below the hook region so that the meristematic primordial can be protected from damage during penetration of soil (Harpham et al., 1991). The apical hook is formed by differential cell elongation on opposite sides of the hypocotyl, in which the rates of cell elongation on outside of the hook are modulated differently than of cells inside the hook (Silk and Erickson., 1978). In the hook region, the growth rate of the outer (convex) side of the hypocotyl exceeds that of the inner (concave) side, resulting in hypocotyl bending (Figure 24). (Park et al., 2006) Figure 24. Apical hook development during early stages of seedling germination. Wild-type seeds were germinated in air and photographs were taken after 24, 26, 28, 30, 32, 34 and 36 hours. 34

ChapitreI: Bibliographic review Ethylene and auxin play a major role in this differential cell elongation process (Schwark and Bopp., 1993; Schwark and Schierle., 1992). In dark-grown seedlings, ethylene exposure enhances apical hook curvature, causing cotyledons to form a 270° bend relative to the hypo cotyls (Ecker., 1995) (Figure 25). Moreover, mutants that are defective in ethylene perception, e.g., etr1-1, ein2, ein3, etc., do not form exaggerated hooks in response to ethylene treatment (Park et al.,2006) Figure 25. Ethylene enhances exagerated hook in etiolated seedlings. Seeds were germinated in darkness in air (left panel) or ethylene (lower panel); photographs were taken of 64- , 66-, 68- and 70-hour-old seedlings. During photography, the ethylene-grown seedlings were transferred to AT plates containing 10 mM ACC. (Roman et al., 1995), whereas the constitutive ethylene responsive mutant ctr1 develops exaggerated hooks even in the absence of ethylene (Guzman and Ecker 1990; Kieber et al., 1993). The precursor to ethylene, ACC, is asymmetrically localized in cells of the apical hook in bean (Schwark and Bopp., 1993). Expression of the gene encoding ACO, the terminal enzyme in ethylene biosynthesis and its enzyme activity have also been found to be higher on the concave side of the apical hook than on the convex side of pea hook (Figure26) (Peck et al., 1998). Another ACO gene, AtACO2, is predominantly expressed on the convex side of the hook (Raz and Ecker., 1999). Outer side Inner side (Peck et al., 1998) Figure 26. Localization of Ps-ACO1 mRNA in the apical hook of etiolated pea seedlings. Apical hooks were isolated from air-grown etiolated 5- to 6-day-old seedlings. The apex (a) is towards the right side and the stem (s) is on the left. At left are dark-field images, and at right are bright-field images.Hybridization was performed with a 35S-labeled antisense strand of Ps-ACO1. 35

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