The analysis of mutations affecting flower structure has led to the identification of some of the genes that direct flower development. Cloning of these genes has allowed the formulation of molecular models of how floral meristem and organ identity may be specified, and has shown that the distantly related flowering plants Arabidopsis thaliana and Antirrhinum majus use homologous mechanisms in floral pattern formation.

Dorsoventral asymmetry in flowers is thought to have evolved many times from a radially symmetrical ancestral condition. The first gene controlling floral asymmetry, cycloidea in Antirrhinum, has been isolated. The cycloidea gene is expressed at a very early stage in dorsal regions of floral meristems, where it affects growth rate and primordium initiation. Expression continues through to later stages in dorsal primordia to affect the asymmetry, size and cell types of petals and stamens.

Although there have been many molecular studies of morphological mutants generated in the laboratory, it is unclear how these are related to mutants in natural populations, where the constraints of natural selection and breeding structure are quite different. Here we characterize a naturally occurring mutant of Linaria vulgaris, originally described more than 250 years ago by Linnaeus, in which the fundamental symmetry of the flower is changed from bilateral to radial. We show that the mutant carries a defect in Lcyc, a homologue of the cycloidea gene which controls dorsoventral asymmetry in Antirrhinum. The Lcyc gene is extensively methylated and transcriptionally silent in the mutant. This modification is heritable and co-segregates with the mutant phenotype. Occasionally the mutant reverts phenotypically during somatic development, correlating with demethylation of Lcyc and restoration of gene expression. It is surprising that the first natural morphological mutant to be characterized should trace to methylation, given the rarity of this mutational mechanism in the laboratory. This indicates that epigenetic mutations may play a more significant role in evolution than has hitherto been suspected.

Flowering plants exhibit one of two types of inflorescence architecture: indeterminate, in which the inflorescence grows indefinitely, or determinate, in which a terminal flower is produced. The indeterminate condition is thought to have evolved from the determinate many times, independently. In two mutants in distantly related species, terminal flower 1 in Arabidopsis and centroradialis in Antirrhinum, inflorescences that are normally indeterminate are converted to a determinate architecture. The Antirrhinum gene CENTRORADIALIS (CEN) and the Arabidopsis gene TERMINAL FLOWER 1 (TFL1) were shown to be homologous, which suggests that a common mechanism underlies indeterminacy in these plants. However, unlike CEN, TFL1 is also expressed during the vegetative phase, where it delays the commitment to inflorescence development and thus affects the timing of the formation of the inflorescence meristem as well as its identity.

SummaryThe cycloidea (cyc) and teosinte branched 1 (tb1) genes code for structurally related proteins implicated in the evolution of key morphological traits. However, the biochemical function of CYC and TB1 proteins remains to be demonstrated. To address this problem, we have analysed the predicted secondary structure of regions conserved between CYC and TB1, and looked for related proteins of known function. One of the conserved regions is predicted to form a non-canonical basic-Helix-Loop-Helix (bHLH) structure. This domain is also found in two rice DNAbinding proteins, PCF1 and PCF2, where it has been shown to be involved in DNA-binding and dimerization. This indicates that the conserved domain most probably defines a new family of transcription factors, which we have termed the TCP family after its first characterised members (TB1, CYC and PCFs). Other plant proteins of unknown function also belong to this family. We have studied two of these in Arabidopsis and have shown that they are expressed in rapidly growing floral primordia. This, together with the proposed involvement of cyc and tb1 in influencing meristem growth, suggests that many members of the TCP family may affect cell division. Some of these genes may have been recruited during plant evolution to generate new morphological traits.

Although curvature of biological surfaces has been considered from mathematical and biophysical perspectives, its molecular and developmental basis is unclear. We have studied the cin mutant of Antirrhinum , which has crinkly rather than flat leaves. Leaves of cin display excess growth in marginal regions, resulting in a gradual introduction of negative curvature during development. This reflects a change in the shape and the progression of a cell-cycle arrest front moving from the leaf tip toward the base. CIN encodes a TCP protein and is expressed downstream of the arrest front. We propose that CIN promotes zero curvature (flatness) by making cells more sensitive to an arrest signal, particularly in marginal regions.

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