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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 Introduction During germination of dicotyledonous plants, the shoot emerges from the seed with a hook-shaped structure that protects the apical meristem and first leaves as the seedling pushes through the soil. This apical hook is maintained until the seedling reaches the light, at which time, the hypocotyl straightens. The apical hook is formed by differential cell elongation on opposite sides of the hypocotyl. The growth rate in the outer (convex) side of the hypocotyls exceeds that of the inner (concave) side resulting in hypocotyl bending (Silk and Erikson., 1978). The formation and maintenance of apical hook has been studied extensively and it was suggested that the apical hook establishment is a developmental process driven by multiple hormone cross-talk where ethylene seems to play prominent role. Alterations in the response of dark-grown seedlings to ethylene, the so- called "triple response" were used to isolate a collection of ethylene-related mutants in Arabidopsis thaliana (Guzman and Ecker, 1990). Arabidopsis mutants that are defective in ethylene perception, e.g., etr1-1, ein2, ein3, etc., are not able to form a hook in response to ethylene treatment (Roman et al., 1995), whereas the Constitutive Ethylene Response mutant ctr1 develops exaggerated hook even in the absence of ethylene (Guzman and Ecker, 1990; Kieber et al., 1993). Moreover, exogenous ethylene exaggerates the apical hook (Lehman et al., 1996), confirming that the process of hook formation is at least partially dependent on ethylene. Mutants that failed to display the apical hook in the absence of ethylene, HOOKLESS (his1), exhibited reduced ethylene production. In the presence of exogenous ethylene, hypocotyl and root of etiolated his1-1 seedlings were inhibited in elongation but no apical hook was observed. HLS1 has been proposed to integrate ethylene and auxin signalling during apical hook formation of the Arabidopsis seedlings (Lehman et al., 1996). In addition, hls1 mutation alters the spatial expression pattern of two primary auxin response genes (SAUR and Aux2-11/IAA4). In 2004, Li et al. pinpoint definitely HLS1 as a key integrator of auxin and ethylene pathway by identifying hookless suppressors as an Auxin Response Factor (ARF2). On the other hand, auxin treatment or inhibition of auxin transport disrupts apical hook formation (Lehman et al., 1996). Consistent with this results, some 89

CHAPITRE III. HOOKLESS1, an Integrator of Multiple Signaling Pathways in Arabidopsis auxin mutants such as axr1 (Lincoln et al., 1990) hls3 (king et al., 1995) and yucca (Zhao et al., 2001) lack normal apical hook. An additional level of complexity to the role of HLS1 was added when the requirement for gibberellins in hook maintenance and hook exaggeration upon ethylene treatment was demonstrated (Achard et al., 2003, Vriezen et al., 2004). The role of brassinosteroids (BRs) in hook development is longstanding, as BR mutants such as cbb1, det2 and cpd, which are defective in the synthesis of BRs, are constitutively photomorphogenic and thus lack the characteristic hook (Chory et al., 1991, Kauschmann et al., 1996, Szekeres et al., 1996). More recently, it was shown that HLS1 acts on the auxin-ethylene interaction rather than at the level of BRs (De Grauwe et al., 2005). In addition to ethylene and auxin, light is another critical regulator of apical hook development. Opposite from apical hook formation and maintenance, light of various wavelengths causes rapid apical hook opening (Liscum and Hangarter, 1993; Rubenstein, 1971), which is part of the photomorphogenesis process. All constitutive photomorphogenic mutants (cop/det/fus) completely lack apical hooks (Chory and Peto, 1990; Hou et al., 1993; Kwok et al., 1996). Ethylene and light were shown to affect differential cell growth by modulating the auxin-response factor (ARF2) in a HLS1-dependent manner (Li et al., 2004). Recently, Ohto et al. (2006) showed that hls1 mutant is hypersensitive to sucrose in terms of the expression of sugar responsive genes in mature leaves. Furthemore, IAA repression of some sugar-induced gene expression was less pronounced in hls1 than in the wild type, thus suggesting that the negative effect of auxin on sugar signalling may be mediated by HLS1. We report here the discovery of new phenotypes associated with hls1 mutation indicating that the HLS1 gene is in the cross-road of multiple signalling pathways including ethylene, auxin, ABA, light and sugar. RESULTS hls1 Mutant Shows De-etiolated Phenotype During the course of examination of hls1 mutant, we found that it commonly shows an interesting phenotype related to greening upon light exposure of 90

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