February 17, 2017 - 12:10
TotM: AGAMOUS and the beauty of a rose
With its wonderful musical performance, its vibrant cast of characters and its gorgeous animation, Beauty and the Beast remains one of the most beloved Disney movies ever. The plot is based on a XVIIIth century fairy tale written by the french author Jeanne-Marie LePrince de Beaumont, and tells the story of a young lady that has to live with a hideous monster (The Beast) that offers her a life of luxury. The truth is, Beast is actually a cursed prince, who was condemned to its horrendous form for rejecting to help an old woman in exchange for a rose. For some reason the curse also affects all the servants of the prince who are turned into animated silverware, furniture and other miscellaneous array of common objects. To break the spell, the prince must make someone to fall in love with him, which is difficult because of his harsh and selfish personality, on top of being a grotesque monster. Even more, he must succeed in that goal before the old lady’s rose loses its last petal. Fortunately for him the rose is a garden rose instead of a wild one, otherwise the movie would have been quite short.
Roses are among the most appreciated ornamental flowers. They have been grown since ancient Greece and used as a religious or heraldic symbol in many occasions across history, traditionally associated with ideals of beauty and femininity. Without any doubt the shape of their flowers, with their endless whirl of petals is responsible for that status. However, this spectacular phenotype is actually a developmental anomaly, not unlike homeotic mutants in Drosophila with legs instead of antennae. Wild roses always have five petals and an undefined number of stamen. What is happening in garden roses is a mutation that disrupts the genetic program that leads the differentiation of the stamen and instead they develop as additional petals. And like the Beast, we have fallen in love with these flowers despite (actually thanks to) their deformed shape.
While flower developmental program can vary quite a lot between plant families, the garden rose phenotype can be replicated in Arabidopsis thaliana with a mutation in a single gene. Genetic studies of floral development mutants have led to the formulation of the ABCDE model. This model states that the different organs in the flower are originated by a combination of a small set of transcription factors that present a radial expression pattern. Expression of class A and E determines sepal identity. A, B and E produce petals. B, C and E genes produce stamens. Lastly, C and E genes produce carpels. All these genes (except APETALA2) are members of the MADS-box transcription factor family. In the absence of AGAMOUS, the only C gene in Arabidopsis, the A factors diffuse and all the organs that were supposed to be sepals will develop as petals instead. A factors are highly variable in other plant families, while the rest of the model seem to be a valid generalization.
AGAMOUS is the only C gene in Arabidopsis thaliana, presenting homology with several D class genes (AGAMOUS LIKE) that help to determine ovary differentiation. AGAMOUS presents a variable number in other species, as we can observe in our tree of the month from the Arabidopsis thaliana phylome, and all studied mutants show some degree of infertility. The repressive function of AGAMOUS that, when disrupted, leads to the supernumerary petal phenotype is not present across all plant lineages. All in all, the ABC(DE) sets a theoretical basis of floral development, helping us to understand the wonderful variability we observe in nature.
Photogram from Beauty and the Beast (1991)
Wild rose. Picture under public domain
Arabidopsis agamous mutant. Extracted from FioreDB (www.cres-t.org/cgi-bin2/show_summary.cgi?query=AG)
Schematic representation of the ABCDE model (Theißen et al, 2016)