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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 Introduction The economical importance of lily species (Lilium spp.) lies in their attractive flowers. Lily flowers already present a huge phenotypic variation in their colours, patterns, shapes and sizes. Even though, novel and aesthetically interesting phenotypes are welcome in the market, since novelty is a highly priced commodity in the ornamental business. Homeotic changes in floral organs are easily noticed and could give rise to novel varieties. Double flowers are the result of homeotic mutations of stamens into petals and, frequently, of carpels into a new flower. This phenotype is found in a range of species, including lily. Other phenotypes can also be found in lily, such as partial homeotic changes of stamens into petal-stamen hybrid organs or a complete change of stamen into petal but keeping the carpel identity unchanged (Figure 1). Many of the homeotic changes observed are the result of environmental stresses, such as high temperatures, being characterized as epigenetic events that do not segregate according to the Mendelian laws (Figure 1b). Other homeotic identity changes, however, are genetic alterations originated from loss-of-function of important genes that will segregate the alterations in future generations, if their sexual functions are not compromised in the mutant, as it is in complete double lilies (Figure 1d). The ABCDE model for flower development was set up based primarily on dicot model species, such as Arabidopsis, Antirrhinum and Petunia (Coen and Meyerowitz, 1991; Colombo et al., 1995). It explains the conversion of a vegetative meristem into reproductive structures as the act of specific genetic regulatory functions working in an overlapping manner. This model can be applied to lily flower development and can explain the formation of double lilies. However, we found a novel phenotype in lily with a complete transformation of stamens into tepals, but keeping the wild-type identity of carpels, which seems not readily explained by the current ABCDE model. Here, we present lily flower development in the light of an adapted ABCDE model. We also introduce homeotic floral phenotypes and suggest hypotheses on the molecular genetic mechanisms that would bring them up. 80

A C B D Chapter 6 Figure 1. Floral phenotypes in Lilium spp. (A) Wild-type flower, with six tepals distributed in the two outer whorls, six stamens and a carpel in the inner whorls. (B) Wild-type stamen (left) next to a hybrid tepal-stamen organ (right), arisen due to physiological stress induced by heat. (C) festiva phenotype with complete homeotic change of every stamen to a tepal and showing a normal carpel development. (D) Double flower showing complete replacement of stamens by tepals and of its carpel by a new flower in a reiterated manner. The modified ABCDE model for lily flower development In flowers from Lilium species the two outermost whorls are composed of similar petal-like organs designated tepals. Stamens are distributed in two whorls (designated here as just one whorl, the third, for the sake of simplicity when comparing 81

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