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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 On the other hand, auxin, which stimulates cell expansion to promote hypocotyl elongation, is also unequally distributed in the apical hook (Schwark and Schierle., 1992). Inhibition of auxin transport disrupts formation of the apical hook, suggesting that auxin asymmetry or an asymmetry in the perception or response of cells to auxin may exist in the hypocotyls (Figure 27). Consistent with these observations, auxin mutants such as axr1 (Lincoln et al., 1990), hls3/rooty (King et al., 1995) and yucca (Zhao et al., 2001) also lack normal apical hooks. Figure 26. Effects of IAA and NPA on apical hook development. (Park et al., 2006) In 1996, Echer’s group identified an Arabidpsis mutant that showed no differential growth in the apical region of the hypocothy (hookless1). This gene has been proposed to be a key regulator that integrates ethylene and auxin signalling pathways during apical hook formation of Arabidopsis seedlings (Lehman et al., 1996). Plants that lack HOOKLESS (HLS1) are unable to maintain an apical hook despite normal responses to ethylene in other tissues, whereas transgenic plants that overexpress HLS1 develop an exaggerated differential growth (hook curvature) in the absence of ethylene (Lehman et al., 1996) (Figure 27). - ethylene + ethylene Wt hls1 Wt hls1 (Lehman et al., 1996) Figure 27. Morphology of wild-type and hookless1 dark-grown Arabidopsis seedlings. Seeds were grown either in air or 10 ml ethylene per liter of air in the dark. 36

ChapitreI: Bibliographic review Thus, HLS1 is required for hook formation and is sufficient to induce enhanced hook curvature in the absence of exogenous ethylene. In addition the absence of HLS1 leads to abnormal auxin-regulated gene expression in the cotyledons and apical region of the hypocotyl. In 2004, Li et al showed that the supressors of hls1 were identified as mutations in Auxin Reponse Factor 2 (ARF2). Exposure to light decreased HLS1 protein level and evoked a concomitant increase in ARF2 accumulation. These studies demonstrate that both ethylene and light signals affect differential cell growth by acting through HLS1 to modulate the auxin response factor, pinpointing HLS1 as a key integrator of the signalling pathways that control hypocotyls binding (Figure 28). (Li et al., 2004) Figure 28: Model for integration of ethylene, auxin, and light signalling in differential growth of the seedling hypocotyl. An asymmetric auxin distribution in hook tissues is proposed to cause differential auxin responses in the region, resulting in asymmetric cell elongation of the hypocotyl and formation of the apical hook structure. Ethylene enhances apical hook bending through activation of HLS1 transcription. One of the roles for HLS1 is to inhibit the function of the auxin response factor ARF2, a negative regulator of the differential auxin response in apical hook, leading to enhanced differential growth and exaggerated hook curvature. In contrast to ethylene, light disrupts the differential auxin responses in hook tissues by decreasing HLS1 abundance. Subsequently, Fine Mapping of hss1 (hookless supressors) Mutations ARF2 protein levels increase and the hook opens. These findings provide another molecular link that connects ethylene and light signalling to auxin-mediated differential cell elongation process. 37

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