Abnormal flowers have been recognized for thousands of years, but only in the past decade have the mysteries of flower development begun to unfold. Among these mysteries is the differentiation of four distinct organ types (sepals, petals, stamens and carpels), each of which may be a modified leaf. A landmark accomplishment in plant developmental biology is the ABC model of flower organ identity. This simple model provides a conceptual framework for explaining how the individual and combined activities of the ABC genes produce the four organ types of the typical eudicot flower. Here we show that the activities of the B and C organ-identity genes require the activities of three closely related and functionally redundant MADS-box genes, SEPALLATA1/2/3 (SEP1/2/3). Triple mutant Arabidopsis plants lacking the activity of all three SEP genes produce flowers in which all organs develop as sepals. Thus SEP1/2/3 are a class of organ-identity genes that is required for development of petals, stamens and carpels.

The ABC model of flower organ identity is widely recognized as providing a framework for understanding the specification of flower organs in diverse plant species. Recent studies in Arabidopsis thaliana have shown that three closely related MADS-box genes, SEPALLATA1 (SEP1), SEP2 and SEP3, are required to specify petals, stamens, and carpels because these organs are converted into sepals in sep1 sep2 sep3 triple mutants. Additional studies indicate that the SEP proteins form multimeric complexes with the products of the B and C organ identity genes. Here, we characterize the SEP4 gene, which shares extensive sequence similarity to and an overlapping expression pattern with the other SEP genes. Although sep4 single mutants display a phenotype similar to that of wild-type plants, we find that floral organs are converted into leaf-like organs in sep1 sep2 sep3 sep4 quadruple mutants, indicating the involvement of all four SEP genes in the development of sepals. We also find that SEP4 contributes to the development of petals, stamens, and carpels in addition to sepals and that it plays an important role in meristem identity. These and other data demonstrate that the SEP genes play central roles in flower meristem identity and organ identity.

Changes in genes encoding transcriptional regulators can alter development and are important components of the molecular mechanisms of morphological evolution. MADS-box genes encode transcriptional regulators of diverse and important biological functions. In plants, MADS-box genes regulate flower, fruit, leaf, and root development. Recent sequencing efforts in Arabidopsis have allowed a nearly complete sampling of the MADS-box gene family from a single plant, something that was lacking in previous phylogenetic studies. To test the long-suspected parallel between the evolution of the MADS-box gene family and the evolution of plant form, a polarized gene phylogeny is necessary. MEF2 ͉ SRF ͉ homeotic genes ͉ MADS ͉ development C hanges in genes encoding transcriptional regulators may represent the most important determinants of morphological evolution in plants and animals (1), and phylogenetic analyses provide a historical framework to identify such changes. The MADS-box genes encode a eukaryotic family of transcriptional regulators involved in diverse and important biological functions, ranging from cardiac muscle development in animals to pheromone response in yeast (2). In plants, MADS-box genes encode the three floral homeotic functions predicted by the genetic ABC model of flower organ identity (3, 4). In addition, plant MADS-box genes regulate the timing of flower initiation and flower meristem identity, as well as various aspects of ovule, fruit, leaf, and root development (4, 5).Previously identified plant MADS-box genes encode proteins that share a stereotypical MIKC structure (Fig. 1), with the highly conserved DNA-binding MADS domain at the amino terminus. The moderately conserved K domain in the central portion of these proteins has been shown to be important for protein-protein interactions and likely forms a coiled-coil structure. The MADS and K domains are linked to one another by a weakly conserved I domain, whereas a poorly conserved carboxyl-terminal (C) region may function as a trans-activation domain (4). In animals and fungi, two distinct types of MADSbox genes have been identified, the SRF-like and MEF2-like classes (ref. 2; see Fig. 1).This paper provides a hypothesis on the evolutionary history of the eukaryotic MADS-box gene family. Previous studies of eukaryotic MADS-box gene evolution, which included plant and animal sequences, provided unrooted trees useful to infer the phylogenetic relationships of the MADS-box lineages (6). These previous studies suggested that at least one MADS-box gene was present in the common ancestor of plants, animals, and fungi, and that probably the duplication that gave rise to the animal MEF2-and SRF-like genes occurred after animals diverged from plants but before fungi diverged from animals (6). However, previous plant and eukaryotic studies were based on a relatively small sampling of plant MADS-box sequences for a particular species (6-9). To test whether all Arabidopsis MADSbox sequences group in a monophyletic clade distinct from all animal and fungal ...

Seasonal changes in day length influence flowering time in many plant species. In Arabidopsis, flowering is accelerated by exposure to long day (LD). Those inductive photoperiods are perceived in leaves [1] and initiate a long-distance signaling mediated by CO and FT. CO is expressed in the phloem according to a circadian rhythm [2-4]. Only under LD does CO induce FT expression as high levels of CO in the evening coincide with the external light that stabilizes CO protein [4, 5]. Subsequently, FT protein travels through the phloem to the shoot apex where, together with FD, it initiates flowering [6-12]. Despite the photoperiodic induction, a mechanism of floral repression is needed to avoid precocious flowering. We show that TEMPRANILLO genes (TEM1 and TEM2) act as novel direct FT repressors. Molecular and genetic analyses suggest that a quantitative balance between the activator CO and the repressor TEM determines FT levels. Moreover, developmental TEM downregulation marks the timing of flowering, as it shifts the CO/TEM balance in favor of CO activity, allowing FT transcript to reach the threshold level required to trigger flowering. We envision that this might be a general mechanism between long-day plants to ensure a tight regulation of flowering time.