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

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

Chapter 1 functionally

Chapter 1 functionally active (Pelaz et al., 2000). Moreover, the metamorphosis of leaves into petals is proved possible now, when a SEP gene is ectopically expressed together with the other A and B genes (Honma and Goto, 2001; Pelaz et al., 2001). This confirms clearly the hypothesis that a petal is a modified leaf. The SEP function was then recognised as the mysterious missing factor to the homeotic conversion of a leaf into a petal, being added to the model for flower development as the E function, designating the model as ABCDE from then on (Theissen, 2001). Despite the attention given to the ABCDE genetic regulation, little is currently known about the events downstream the ABCDE model towards the organ set up (Sablowski and Meyerowitz, 1998). The identification of the immediate targets of the transcription factors of the ABCDE model is a sine qua non for understanding the genetic mechanisms involved in floral organ formation. High throughput methodologies have been applied recently in the quest to identify genes that participate downstream the ABCDE genes in order to promote floral organ development (Scutt et al., 2003; Zik and Irish, 2003). The ABCDE model for flower development in dicot and monocots The early ABC model for flower development was assembled based upon the genetic configuration of two dicot species: Arabidopsis thaliana and Antirrhinum majus (Coen and Meyerowitz, 1991). The model proved to be very conserved among dicot species and was readily accepted by the scientific community. However, the ABC model could not be directly applied to monocot species, since there are major divergences in their floral organ anatomy and architecture that do not support the model as in its first conception. The species belonging to the Poaceae family show flowers with substantial anatomical differences in their perianth organs, with palea, lemma and lodicules, instead of sepals and petals. There is still a great debate to evolutionarily correlate these organs to those found in dicot species (Kramer and Irish, 2000; Kyozuka et al., 2000). Another monocot family with contrasting characteristics in the flowers is the Liliaceae. The most notable difference found in this family is that the flowers do not have a perianth formed by distinguishable organs, sepals and petals, but, instead, they have two whorls formed by identical organs, designated tepals. Adaptation to the ABCDE model was not difficult, since an extension of the B function to the first whorl could lead to this feature. A phenotype that corroborated with this hypothesis was 6

Introduction found in the Viridiflora tulip mutant, in which sepals were observed in the place of tepals, probably due to the loss of its B function (van Tunen and Angenent, 1993). More recently, a B functional gene from Lilium longiflorum, LMADS1 which is an AP3/DEF orthologue, was accessed by molecular analysis, showing expression activity in both perianth whorls (Tzeng and Yang, 2001). Its correspondent protein, however, was not detected in those tepals derived from the first whorl. In eudicot species, B type proteins must form heterodimers in order to be stable and functionally active (Goto and Meyerowitz, 1994; Yang et al., 2003). Results obtained for the corresponding monocot orthologues from maize suggested similar heterodimer stabilisation (Ambrose et al., 2000). Nevertheless, experiments using putative orthologues from Lilium regale confirmed that its B type proteins could indeed form heterodimers, but, surprisingly, the L. regale PI/GLO putative orthologue was also stable in vitro as homodimer, whereas the L. regale AP3/DEF putative orthologue behaved as a typical dicot counterpart, i.e. only being able to form heterodimers (Winter et al., 2002). These results are also observed in the respective orthologues from tulip (Kanno et al., 2003). It may be possible that the PI/GLO homologues in the Liliaceae family can act alone in the first whorl to induce the homeotic identity change of sepals into petals. This assumption, however, still requires further investigation using in vivo procedures. Heterologous genetic characterisation in model species Despite the differences found in the ABCDE model between dicot and monocot species and the huge flower variability among the Angiospermae species, the model shows consistency throughout evolution. As stated before, the ABCDE model were established using the model species Arabidopsis and Antirrhinum, and later, Petunia. In monocot species, such as rice and maize, many MADS-box genes have been studied, but the complexity involved in the floral anatomy of these species creates difficulties to set up a definitive and general ABCDE model for monocots so far. However, despite the dicot-monocot evolutionary divergence that happened circa 120-180 million years ago (Wolfe et al., 1989), MADS-domain proteins derived from both classes are still able to cross interact in yeast in a proper fashion (Favaro et al., 2002), indicating the importance of their functions throughout evolution. The evolutionary strength of the ABCDE model for flower development facilitates functional studies of orthologous genes, allowing using model species as 7

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