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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 aspects of auxin and/or cytokinin biology (Hobbie & Estelle., 1994; Catterou et al., 2002) as well as transgenic studies of plants with altered auxin or cytokinin levels (Binns et al., 1987; Eklöf., 1997). Moreover, many experiments have demonstrated the existence of synergistic, antagonistic and additive interactions between these two plant hormones, suggesting a complex web of signal interactions (Coenen and Lomax., 1997). It is clearly documented that auxin regulates cytokinin levels and vice versa. It has, for example, been observed that cytokinin overproducing tobacco had lower levels of indole-3-acetic acid (IAA), and that overproduction of IAA in tobacco leads to a reduced pool size of cytokinins (Eklöf et al., 2000). It has been argued that cytokinins can both up- and downregulate auxin levels. The increase in auxin, for example, has been demonstrated after application of exogenous cytokinin. Although the basis for these changes is not fully understood, cytokinin-induced inhibition of enzymes that conjugate free IAA into inactive IAA aspartate has been suggested as a putative mechanism. In contrast, (Eklöf et al., 2000) studies demonstrated a decrease in auxin content of transgenic plants overproducing cytokinins. These contradictory results show the complexity of the interactions between these two hormones. VII.4 Auxin and abscisic acid (ABA) The abscisic acid (ABA) is a prominent regulator of seed germination that also enables plants to respond to abiotic stresses such as drought. ABA can directly affect ion transport in guard cells to alter stomatal aperture rapidly in response to changing water availability (reviewed in Roelfsema et al., 2004). ABA and auxin have been observed to interact antagonisticaly to regulate stomatal aperture (Eckert and Kaldenhoff., 2000). Auxin serves to open the stomatal pore, whereas, ABA helps to closes the stomatal pore and reduces water loss via transpiration. The antagonistic nature of this interaction requires the precise co-ordination of ion channel activity within guard cells. These channels allow the flow of ions that decrease (auxin) or increase (ABA) the cytosolic pH and therefore effect the turgor of the guard cells (Grabov and Blatt., 1998). Aside from interactions of auxin and ABA at the level of guard cell aperture, genetic evidence from Arabidopsis indicates that these two hormones may interact to influence root 28

ChapitreI: Bibliographic review growth and seed germination. Dominant mutations in the auxin-response gene AXR2/IAA7 confer an ABA-insensitive phenotype to roots (Wilson et al., 1990; Timpte et al., 1994; Nagpal et al., 2000) and both axr1 and axr2 have weak ABA- insensitive phenotype as measured by seed germination. Furthermore, abi3 mutants that were originally identified as highly insensitive to ABA are insensitive to NPA, at the level of lateral root growth. As observed for other hormones, the ubiquitin/proteasome system is implicated in regulation of ABA-responsive transcription. Many evidence of the possible involvement of an SCF-complex in ABA-signalling was uncovered through characterisation of many proteins: ABI5 (abscisic acid insensitive5), ABI3, ABF2 (ABRE Binding Factor2), ATL ( Arabidopsis toxico para Levadura), TLP9 (TUBBY-Like Proteins)and finally XERICO (Smalle et al., 2003; Zhang et al.,2005; Kim et al.,2004; Serrano et al.,2006; Lai et al.,2004; Ko et al; 2006), but further research is needed to determine whether any integration of auxin and ABA signalling pathways is achieved though the ubiquitin/proteasome system. VII.5 Auxin and brassinosteroids (BR) Auxin response is also connected to brassinosteroids (BRs), which also interact with auxin to promote root gravitropic curvature in maize (Kim et al., 2000). Yi et al., 1999 shown that auxin and BR synergistically and antagonistically regulate expression of the ACS family of ethylene biosynthesis genes in mung bean (Vigna radiata) hypocotyl tissues. Other molecular link has been described between the auxin-response pathway and BR biosynthesis upon characterisation of the sax1 mutant (Ephritikhine et al., 1999). The sax1 mutant plant is partially restored by treatment with exogenous brassinosteroid. sax1 root growth is hypersensitive to both ABA and auxin. De Grauwe et al., 2005 described the involvement of (BRs) in auxin- and ethylene-controlled processes in the hypocotyls of both light- and dark-grown seedlings. They showed that BR biosynthesis is necessary for the formation of an exaggerated apical hook and that either application of BRs or disruption of BR synthesis alters auxin response, presumably by affecting auxin transport, eventually resulting in the disappearance of the apical hook (Figure 20). 29

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