Molecular and Genetic Mechanisms of Floral Control

In the last 15 years, knowledge of the molecular and genetic mechanisms that underlie floral induction, floral patterning, and floral organ identity has exploded. Elucidation of basic mechanisms has derived primarily from work in three dicot species: Antirrhinum majus, Arabidopsis thaliana, and Petunia hybrida. Although Antirrhinum and petunia have contributed fundamental breakthroughs to our understanding of flower development, it is from Arabidopsis that the most detailed and comprehensive picture of the molecular mechanisms underlying flower development has been obtained. In this review, I will outline the present state of knowledge, focusing on molecular and genetic mechanisms revealed in work on Arabidopsis, specifically in three areas: the integration of floral induction signals by a small group of floral integrators, the activation of the floral organ identity genes by the floral meristem identity genes, and interactions among the floral organ identity genes, particularly the A and C class genes. By choosing to focus on progress in Arabidopsis, I do not mean to suggest that work in other species is unimportant or uninformative. To the contrary, without studies in Antirrhinum and petunia, our knowledge and the broad impact of what has been learned would clearly be less. One of the satisfying things about the field of flower development is the applicability of the floral patterning mechanisms to a wide range of plant species; such a conclusion only comes from careful analysis in multiple distantly related species. The general pattern in the field has been that molecular and genetic mechanisms, based on work in model species, serve as the basis for work in other species, many of which are of economic importance. Ultimately, the goal is to use information discovered in the model plants to engineer economically important plants for human and ecological benefit. The first unifying principle in the flower development field is the ABC model. This model, initially proposed in the early 1990s based on genetic experiments in Antirrhinum and Arabidopsis, is striking in its simplicity and is applicable to a wide range of angiosperm species, both dicots and monocots, including economically important grass species such as rice and maize (Bowman et al., 1991; Coen and Meyerowitz, 1991; Ambrose et al., 2000; Fornara et al., 2003). The Arabidopsis flower, like most angiosperm flowers, consists of four organ types that are arranged in a series of concentric rings or whorls. From outside to inside, the flower consists of sepals in whorl 1, petals in whorl 2, stamens in whorl 3, and carpels in whorl 4. The ABC model postulates that three activities, A, B, and C, specify floral organ identity in a combinatorial manner. Specifically, A alone specifies sepals, A+B specifies petals, B+C specifies stamens, and C alone specifies carpels. A second major aspect is that A and C classes are mutually repressive. In the absence of A, C activity is present throughout the flower. Likewise, in the absence of C, A activity is present throughout the flower. Throughout the 1990s, the ABC genes from a wide range of species, and molecular studies molecular experiments the major of the ABC model by and Meyerowitz, and and The second major unifying principle the of the et al., et al., are present in a wide range of and plant species and 2000; et al., In many is and to specify a meristem as floral and In in is a of the of floral et al., and is a of the floral organ identity ABC genes and Meyerowitz, et al., et al., et al., of are in flower development be four that in a in to both and the plant from to this is by a group of genes. signals from the are and to the activation of a small group of meristem identity genes that specify floral the meristem identity genes the floral organ identity genes in of the flower. the floral organ identity genes genes that specify the types and that the four floral The genes on four major and in the are in genes in A and or of the and by and 2000; et al., and 2003). 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