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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 hls1 mutation and ethylene have additive effect to overcome the glucose-induced developmental arrest. hls1 Mutant is Tolerant to Elevated ABA Concentrations The characterization of Arabidopsis gin5 and gin 6 mutants uncovered that an increase in ABA levels is involved in the glucose signalling pathway leading to a decrease in gene expression and developmental arrest (Arenas-Huertero et al., 2006). We therefore tested whether ABA response is altered in hls1 mutant using seed germination as ABA sensitivity test. When grown in presence of ABA (100 nM), germination of WT seeds is strongly inhibited while hls1 mutant germinate successfully (Figure 3). However, at higher concentrations seed germination was inhibited in both WT and hls1 etiolated seedlings indicating that hls1 mutation enhances tolerance to ABA glucose whithout conferring resistance to this hormone. This result suggests that ABA may act trough HLS1 in the glucose signalling pathway. - ABA + ABA A B WT hls1 % of germination 100 80 60 40 20 0 WT hls1 Figure 3. Arabidopsis hls1 Mutant Displays Higher Tolerance to ABA- Mediated Inhibition of Seed Germination. A. Wild type and hls1 mutant seeds were kept 48h at 4°C and then germinated in the dar k 5 days in darkness in the absence or presence of 100 nM ABA. B. The Number of germinated seed giving rise to seedlings was assessed. In each experiment a large number of seeds (n>50) was used and the data presented are representative of four independent replicates. 93

CHAPITRE III. HOOKLESS1, an Integrator of Multiple Signaling Pathways in Arabidopsis hls1 Etiolated Seedlings Display Agravitropic Growth Arabidopsis plants display typical gravitropism, with roots grown toward and shoots growing away from the gravity center. (Bullen et al., 1990; Evens, 1991; Okada and Shimura, 1992). Noteworthy, this gravitropic growth pattern is altered in etiolated hls1 seedlings where both the roots and the shoots grew in all directions on agar plates (Figure 4). Interestingly, light grown hls1 mutants display normal gravitropic growth indicating that hls1 mutation alters gravitropism only in the absence of light. hls1 WT Figure 4. The Arabidopsis hls1 mutant shows altered gravitropic growth. 3day-old seedlings of wild type (right side) and hls1 (left side) grown on MS media in the dark. DISCUSSION The HOOKLESS1 gene has been shown to be essential for cell elongation and differential growth in hypocotyl of dark grown Arabidopsis seedlings. Here, we describe new phenotypes associated with the hls1 mutation that position the HLS1 gene at the crossroads of multiple signalling controlling a multitude of devlopmental growth processes including light, gravitropic, glucose and hormone resposnes. The role of HLS1 gene in integrating ethylene, auxin and light signalling has been first shown upon the identification of an auxin response factor, ARF2, as HLS1 repressor that reverses many of the developmental phenotypes observed in hls1 plants. Upon reaching the soil surface, the seedlings undergo a marked developmental transition termed de-etiolation trigged by light, involving coordinate 94

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