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Flower development of Lilium longiflorum - The Lilium information ...

Flower development of Lilium longiflorum - The Lilium information ...

Floral homeotic mutants

Floral homeotic mutants of Lilium to dicot model species). A carpel is regarded as the innermost whorl of lily flowers. In our efforts to study lily floral development at the molecular level, we also attempted to isolate a gene with an A function. In order to generate a homologous probe for cDNA library screening, we tried amplifying a homologous fragment using degenerated primers conceived from AP1 monocot sequences (Theissen et al., 2000). Contrary to our successes in amplifying a putative B (PI-like, data not shown) and of E-type gene fragment (SEP3-like), we were unable to amplify a putative A-type fragment. Therefore, this function still remains a mystery in lily species, since no orthologues with an A function have been reported by others as well. The presence of tepals in the first whorl of lily flowers can be explained by the extension of the B and E functional domains towards the outermost whorl. If so, the loss-of-function of a B functional gene would lead to homeotic conversions of tepals in both outer whorls into sepal-like organs. This phenomenon was indeed observed in tulips, which belongs to the Liliaceae family, corroborating the modified ABCDE model in this plant family (van Tunen and Angenent, 1993). In this way, the B function presents a special character in Liliaceae flower development and its candidate genes have already been object to scientific study. The first gene belonging to the ABCDE model from Lilium to be functionally investigated was a B-type orthologue of APETALA3 (AP3) designated LMADS1 (Tzeng and Yang, 2001). Strikingly, instead of showing a similar expression level of this gene in tepals from both whorls, LMADS1 showed a stronger transcription activity in the second whorl organs and no protein was detected in the first whorl. This result raised uncertainties on the developmental identity of organs from the first and second whorls. Another deviating feature found in B functional genetics of Liliaceae species is the facultative homodimerization of PISTILLATA (PI) orthologues from Lilium regale LRGLOA and LRGLOB whereas B type proteins from dicot species show obligate heterodimerization with other B type proteins (Winter et al., 2002). The same phenomenon was observed in Tulipa gesneriana, with the obligate heterodimerization of the B types TGDEFA and TGDEFB, but with the facultative homodimerization of the B type TGGLO (Kanno et al., 2003). These results support the dislocated B function in Liliaceae species, since the absence of AP3 orthologues in the first whorl organs may be functionally circumvented by the alternative PI-like protein homodimerization. We isolated and studied the molecular and functional characteristics of a putative C functional gene from Lilium longiflorum, LLAG1, described in the second chapter of this thesis. Functional analysis was carried out in the heterologous species 82

Chapter 6 Arabidopsis, giving the expected homeotic changes, including a complete homeotic conversion of petals into stamens. A putative D functional gene from Lilium longiflorum was also recently characterized (Tzeng et al., 2002). This function is responsible for ovule development in dicot species. Interestingly, when overexpressed in Arabidopsis, the lily D-type gene did not induce ectopic ovule formation, but instead, produced floral homeotic mutations similar to what a C-type gene would do, such as AGAMOUS. The more recently characterized function in the ABCDE model, the E type, was also analysed in lily during our studies (chapter four). We were able to isolate and perform a molecular characterization of a putative E functional homologue from Lilium longiflorum, LLSEP3. Overexpression of this gene in Arabidopsis showed early flowering phenotype and no homeotic mutations, as observed when the SEP3 is overexpressed in Arabidopsis. Additional studies must be carried out to establish the function of LLSEP3, such as its protein-protein interactions and capability of inducing leaf conversion into petal when overexpressed together with A and B genes. However, its sequence identities and ability of inducing early flowering are good indications that LLSEP3 has indeed the E function in lily. The C function and the double lily The molecular elucidation of the mechanisms involved in the development of double flowers was started in Arabidopsis, in which a T-DNA insertion in a certain gene produced its loss of function, enabling to trace the responsible gene for the phenotype (Yanofsky et al., 1990). The Arabidopsis double flower phenotype showed conversion of stamens into petals, and the replacement of the carpel by a new flower, in a reiterated fashion. This phenotype was named AGAMOUS (AG) (from the Greek: a = not, and gamous = fecundate; due to its lack of sexual organs). The gene was recognised as a transcription factor and, together with three other genes, established the MADS-box gene family, which currently encloses more than 450 genes from many species (for a list see http://www.mpiz-koeln.mpg.de/mads/madslist.html). In the agamous mutant, the homeotic conversion of stamens into petals is explained by the ABCDE model by the antagonism between A and C functions (Coen and Meyerowitz, 1991). If there is loss of C function, the A function takes over its domain, and vice versa. Stamens, which development is dictated by B+C+E functions, in the agamous phenotype are comprehensively replaced in the third whorl by petals, which is the result of A+B+E functions. Additionally, in this mutant, the flower becomes indeterminate, with its fourth whorl being replaced by a new flower, in a 83

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