Zygomorphic flowers, with bilateral (dorsoventral) symmetry, are considered to have evolved several times independently in flowering plants. In Antirrhinum majus, floral dorsoventral symmetry depends on the activity of two TCP-box genes, CYCLOIDEA (CYC) and DICHOTOMA (DICH). To examine whether the same molecular mechanism of floral asymmetry operates in the distantly related Rosid clade of eudicots, in which asymmetric flowers are thought to have evolved independently, we investigated the function of a CYC homologue LjCYC2 in a papilionoid legume, Lotus japonicus. We showed a role for LjCYC2 in establishing dorsal identity by altering its expression in transgenic plants and analyzing its mutant allele squared standard 1 (squ1). Furthermore, we identified a lateralizing factor, Keeled wings in Lotus 1 (Kew1), which plays a key role in the control of lateral petal identity, and found LjCYC2 interacted with Kew1, resulting in a double mutant that bore all petals with ventralized identity to some extents. Thus, we demonstrate that CYC homologues have been independently recruited as determinants of petal identities along the dorsoventral axis in two distant lineages of flowering plants, suggesting a common molecular origin for the mechanisms controlling floral zygomorphy.dorsoventral axis ͉ floral development ͉ keeled wings in Lotus ͉ LjCYC2 ͉ squared standard

Floral zygomorphy (flowers with bilateral symmetry) has multiple origins and typically manifests two kinds of asymmetries, dorsoventral (DV) and organ internal (IN) asymmetries in floral and organ planes, respectively, revealing the underlying key regulators in plant genomes that generate and superimpose various mechanisms to build up complexity and different floral forms during plant development. In this study, we investigate the loci affecting these asymmetries during the development of floral zygomorphy in pea (Pisum sativum L.). Two genes, LOBED STANDARD 1 (LST1) and KEELED WINGS (K), were cloned that encode TCP transcription factors and have divergent functions to constitute the DV asymmetry. A previously undescribed regulator, SYMMETRIC PETALS 1 (SYP1), has been isolated as controlling IN asymmetry. Genetic analysis demonstrates that DV and IN asymmetries could be controlled independently by the two kinds of regulators in pea, and their interactions help to specify the type of zygomorphy. Based on the genetic analysis in pea, we suggest that variation in both the functions and interactions of these regulators could give rise to the wide spectrum of floral symmetries among legume species and other flowering plants.symmetric petal ͉ dorsoventral asymmetry ͉ organ internal asymmetry ͉ KEELED WINGS ͉ LOBED STANDARD 1 F lower development in higher plants gives rise to an enormous variation of flower morphology and immense aesthetic diversification in nature. An important aspect for divergent floral developments is the establishment of floral symmetries, where a few distinct basic forms could be distinguished (1, 2): the monosymmetry (zygomorphy, with one symmetric plane), polysymmetry (actinomorphy, with several symmetric planes), and left-right asymmetry (with no symmetric plane). Among these, zygomorphy is considered the more specialized form and has been the most under investigation for its origin and underlying mechanisms.Fabaceae (legumes) is one of the largest families in angiosperm, with a range of floral symmetric forms, and its success is thought to be coupled with its predominant zygomorphic flowers (3, 4). Most zygomorphic flowers are found in the subfamily Papilionoideae (5, 6), which attracted the attention of researchers since the end of the 18th century (7). Darwin (8) demonstrated the role of this type of zygomorphy in pollination biology, and the special floral shape of papilionoid legumes was an important factor in Mendel's groundbreaking work on the laws of genetic inheritance in the 1850s. Pea flowers, like most zygomorphic flowers, possess prominent corolla with three petal types, which are arranged along a dorsoventral (DV) axis, and manifest two types of asymmetries: DV asymmetry in the floral plane and organ internal (IN) asymmetry in the floral organ plane (Fig. 1a). It is well documented that DV asymmetry in papilionoid legumes commences in the floral meristem when the asymmetric development of floral organ primordia occurs (Fig. 2a) (5, 6). However, IN asymmetry is variable among pet...