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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 Regulatory elements of transcription and the ABCDE model for flower development. Transcription of developmental genes are under the regulation of trans and cis- regulatory elements that dictate the exact moment the genes are turned on and off in order to trigger a proper and regular developmental pattern for each species. Trans-acting elements are protein factors operating in gene promoter regions (i.e. cis-regulatory regions) in order to be associated with other factors in the RNA polymerase complex and activate transcription or, alternatively, to inhibit the transcription of a given gene. Transcription factors, such as the MADS proteins, act as trans-acting elements in eukaryotic organisms. Cis-regulatory regions are boxes present in gene promoters and other regulatory regions that modulate the spatial and temporal transcription pattern of a given gene by allowing selective interaction with transcription factors (i.e. trans-acting elements) and RNA polymerase. The CArG box is a known cis-regulatory element present in promoters of genes that interact with the MADS domain in order to assist their transcription (Pollock and Treisman, 1991). The ABCDE model is mostly composed of MADS-box transcription factors ruling as key elements to activate (or repress) specific genes in flower development that will ultimately result in formation of floral organs. Changes in the sequence of their cis-regulatory regions or trans-acting elements may result in genetic misfunction, leading to homeotic changes in the flower pattern. Lily flowers with their homeotic identity partially lost in the third whorl, as shown in Figure 1b, can be easily found. This change can be triggered by environmental conditions, suggesting a form of epigenetic regulation of homeotic genes involved in flower organ development. In the festiva phenotype, however, the homeotic change of the third whorl organs is regular and complete, suggesting a more severe cause than those triggered by stress conditions. In this way, if a specific element regulating the expression of a C functional gene is affected, in cis or in trans, its transcription may be suppressed specifically in the third whorl and not in the fourth whorl, which would lead to specific homeotic changes of stamens into petals (or tepals, in the case of Liliaceae). Keeping in mind that A and C genes have antagonistic functions, alternatively, an extension of A-type gene domain that allows its transcription in the third whorl would repress the C function and raise specific homeotic changes of stamens in petals. Important cis-regulatory regions of AG that control its spatial expression pattern were found in the second large intron of the gene (Sieburth and Meyerowitz, 86

Chapter 6 1997). Additionally, regions responsible for independent AG activation in whorls two and three were identified intragenically (Deyholos and Sieburth, 2000). The zinc- finger transcription factor SUPERMAN also has its expression whorl-regulated by cis regions in the transcribed portion of the gene (Ito et al., 2003). Trans-acting elements of the AG gene include positive and negative regulators of its expression, such as LEAFY (LFY; Weigel et al., 1992), APETALA1 (AP1; Weigel and Meyerowitz, 1993) and WUSCHEL (WUS; Lenhard et al., 2001) as positive regulators and LEUNIG (LUG; Liu and Meyerowitz, 1995; Conner and Liu, 2000), SEUSS (SEU; Franks et al., 2002), APETALA2 (AP2; Drews et al., 1991) and STERILE APETALA (SAP; Byzova et al., 1999) as negative regulators. Both hypotheses, repression of C function or activation of A function, account for early stages of development, since it is highly plausible, by the phenotype given, that the A function is active in the third whorl and the C function is absent. In situ hybridisation of festiva floral buds in early developmental stages may answer which function is (firstly) mistranscribed in order to concentrate investigation on its genetic regulation. A factor specific for the third whorl in the lily ABCDE model? The development of sexual structures located in flower and the emergence of the flowering plant species is a key event in life evolution on the Earth. Given the adaptive advantages flowers provided to plant species, it is not a surprise that the functions belonging to the ABCDE model are highly conserved in the many flowering species. Similarly, as the flowers present singular and conserved characteristics, one would expect to find evolutionary conservation also at the molecular level. Lily species show a slightly different molecular mechanism to promote floral organ development when compared to other plant families, however, the fundamental features of the model seem to remain unchanged throughout evolution. The novel festiva phenotype has never been reported before in Arabidopsis or other species and, for that, it is conceivable that some aspect of the ABCDE may have changed since the last common ancestor of lilies and mustards. Despite the overlapping nature of the ABC gene function found in Arabidopsis and Antirrhinum that gave rise to the early ABC model for flower development, it has been found that some species presented floral homeotic genes functioning in only one whorl. The petunia green petal (gp) mutant, which has a mutation in the B-type gene 87

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