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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 Aux/IAA proteins attenuating the signalling pathway as auxin levels fall by restoring repression of the ARF transcription factors (Figure 10b). In conclusion, The SCF TIR1/AFB -mediated proteolysis of Aux/IAA proteins is clearly responsible for many of the effects of auxin, but the resulting changes in gene expression occur too slowly to account for the most rapid auxin responses, including ion fluxes across the plasma membrane (Hager et al., 2003) and mitogen- activated protein (MAP) kinase activation (Mockaitis et al., 2000) (Figure 11). There are two further sets of responses induced by auxin for whish TIR1 is unlikely to be the receptor: those for whish the receptor is shown to be extracellular and those for which other receptor can be specified. a b (Quint and Gray, 2006) Figure 10: Auxin regulation of gene expression. (a) Under basal auxin concentrations the Aux/IAA proteins heterodimerize with the ARF transcription factors, thereby repressing auxin-inducible gene expression. (b) Auxin binding to the TIR1/AFB receptors promotes the recruitment of Aux/IAA proteins to the SCF complex. Subsequent Aux/IAA ubiquitinylation and proteasome-mediated degradation results in a decline in Aux/IAA protein levels, thus de-repressing auxin-inducible gene expression. DBD, DNA-binding domain; E1, ubiquitin-activating enzyme; E2, ubiquitin-conjugating enzyme; U,ubiquitin; R, RUB,AFB1, AFB2, or AFB3. 14

ChapitreI: Bibliographic review This receptor might very likely be ABP1 (Auxin Binding Protein1) (Steffens et al., 2001) but the mechanism of ABP1 action and the identities of other components of this pathway await discovery. Furthermore, several factors, including the repressing ARFs, SAUR proteins, MAP kinase pathway components, have been implicated in auxin signalling but are presently without a home in current models. Clearly, much remains to be learned about the mechanics of auxin signalling. (Badescu and Napier., 2006) Figure 11. Early events after an auxin stimulus. The upper panel shows a trace of the growth against time for a coleoptile after addition of exogenous auxin at time 0. There is a characteristic lag of 10–15 min after which extension rate increases rapidly before falling off after 30 min. In the continuing presence of auxin, a second peak follows after 60 min and an elevated growth rate is sustained. The first 30 min of the timeline is expanded below; against this timeline times are indicated at which specific responses occur. Some of the transcripts that increase in abundance in response to auxin are indicated in the center of the figure; changes in protein activity or cellular function are shown at the base. Many responses precede changes in transcript abundance. V. HOMEOSTASIS The balance of synthesis, breakdown, conjugation, and transport is regulated rigorously to give auxin homeostasis and thus orchestrate the plant development. Clearly, changes in auxin concentration are important as plants respond to 15

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