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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 ...

CHAPITRE III. HOOKLESS1,

CHAPITRE III. HOOKLESS1, an Integrator of Multiple Signaling Pathways in Arabidopsis inhibition of the hypocotyl elongation, unfolding of the hook, stimulation of cell expansion, activation of chloroplast development and chlorophyll accumulation. In preparation of this transition, seedlings accumulate a precursor of chlorophyll (protochlorophyllde) to permit rapid assembly of photosynthetic machinery. Accumulation of this intermediate in excess can result in photooxidative damage because light absorbed by these free molecules can be dissipated as reactive oxygen or free radicals (Reinbothe et al., 1996; Op den Camp et al., 2003). Therefore, coordinate regulation of the chlorophyll biosynthetic pathway and the capacity of enzymatic conversion of protochlorophyllde to chlorophyll is particularly critical during the deetiolation process. In this work, we described for the first time a marked bleaching phenotype in hls1 mutant that had first been germinated and grown in the dark for several days before been transferred to white light. Uncontrolled over-accumulation of protochlorophyllde in etiolated seedlings was proposed as major cause of the failure to green in many pif (photochrome interacting factor 1) mutants (huq et al., 2004). This phenomenon may also operate in the case of hls1 mutant where de-etiolation leads to seedlings death. We suggest that HLS1 gene may contribute to the regulation of (Excess of light - photooxidative damage) HLS1 Glutamate Protochlorophyllide Death Light chlorophyll Greening Figure 5: Proposed view of hypothetical linkage between regulation of chlorophyll synthesis and HLS1 involvement during early photomorphogenesis. The process of chlorophyll synthesis is depicted schematicaly and the hypothetical implication of HLS1 gene in promoting greening or preventing cell death is shown. 95

CHAPITRE III. HOOKLESS1, an Integrator of Multiple Signaling Pathways in Arabidopsis chlorophyll biosynthesis or chloroplast development and hence prevent the accumulation of excess protochlorophyllde in prolonged darkness. Further experiments will be needed to test this hypothesis. This hypothesis is supported by the fact that COP3, an allelic mutant of hls1 was shown to be involved in light- regulated seedling development mediated trough the phytochrome system (hou et al., 1993). Interestingly, light signal regulation plant morphogenesis can be over ridden by metabolic signals such as the availability of glucose released from photosynthesis. For example, in Arabidopsis the availability of abundant glucose can be sensed during germination and can exert a profound influence resulting in seedlings developmental arrest (Jang and Shenn, 1997). Although the underlying mechanisms of this glucose-inducible developmental arrest are mostly unknown, a previous analysis of the gin1 mutant has revealed an antagonistic role of ethylene on sugar signalling pathway (Zhou et al., 1998). Based on the physiological characterization of hls1 mutants, we show in the present study that loss-of-function mutation of the HLS1 gene prevents glucose repression of germination, cotyledon greening and expansion and true leaf development. These data suggest that HLS1 gene may act as positive regulator of glucose-induced repression of seedling growth in light conditions. A central role of ABA in plant sugar signalling emerged from the analysis of Arabidopsis gin5 and gin6 mutants. It has been proposed that exogenous glucose increases the expression of ABA biosynthesis and signalling genes and that this glucose-specific accumulation of ABA appears to be essential for glucose signalling (Arenas-Huertero., et al., in 2000). Surprisingly, the addition of ABA in the medium of dark grown seedlings inhibit germination of wild type seeds but has not effect on hls1. This result suggests that HLS1 gene participates glucose signalling downstream of the induction of ABA (Figure 6). A molecular link between ethylene and glucose signalling has been established trough the analysis of EIN3 proteins (Yanagisawa et al in 2003). 96

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