The mobile floral-promoting signal, florigen, is thought to consist of, in part, the FT protein named after the Arabidopsis thaliana gene FLOWERING LOCUS T. FT is transcribed and translated in leaves and its protein moves via the phloem to the shoot apical meristem where it promotes the transition from vegetative to reproductive development. In our search for a maize FT-like floral activator(s), seven Zea mays CENTRORADIALIS (ZCN) genes encoding FT homologous proteins were studied. ZCN8 stood out as the only ZCN having the requisite characteristics for possessing florigenic activity. In photoperiod sensitive tropical lines, ZCN8 transcripts were strongly upregulated in a diurnal manner under floral-inductive short days. In day-neutral temperate lines, ZCN8 mRNA level was independent of daylength and displayed only a weak cycling pattern. ZCN8 is normally expressed in leaf phloem, but ectopic expression of ZCN8 in vegetative stage shoot apices induced early flowering in transgenic plants. Silencing of ZCN8 by artificial microRNA resulted in late flowering. ZCN8 was placed downstream of indeterminate1 and upstream of delayed flowering1, two other floral activator genes. We propose a flowering model linking photoperiod sensitivity of tropical maize to diurnal regulation of ZCN8.

Most of the world's food supply is derived from cereal grains that are borne in a unique structure called the spikelet, the fundamental unit of inflorescence architecture in all grasses. branched silkless1 (bd1) is a maize mutation that alters the identity of the spikelet meristem, causing indeterminate branches to form in place of spikelets. We show that bd1 encodes a putative ERF transcription factor that is conserved in different grasses and is expressed in a distinct domain of the spikelet meristem. Its expression pattern suggests that signaling pathways regulate meristem identity from lateral domains of the spikelet meristem.

Separation of the life cycle of flowering plants into two distinct growth phases, vegetative and reproductive, is marked by the floral transition. The initial floral inductive signals are perceived in the leaves and transmitted to the shoot apex, where the vegetative shoot apical meristem is restructured into a reproductive meristem. In this study, we report cloning and characterization of the maize (Zea mays) flowering time gene delayed flowering1 (dlf1). Loss of dlf1 function results in late flowering, indicating dlf1 is required for timely promotion of the floral transition. dlf1 encodes a protein with a basic leucine zipper domain belonging to an evolutionarily conserved family. Three-dimensional protein modeling of a missense mutation within the basic domain suggests DLF1 protein functions through DNA binding. The spatial and temporal expression pattern of dlf1 indicates a threshold level of dlf1 is required in the shoot apex for proper timing of the floral transition. Double mutant analysis of dlf1 and indeterminate1 (id1), another late flowering mutation, places dlf1 downstream of id1 function and suggests dlf1 mediates floral inductive signals transmitted from leaves to the shoot apex. This study establishes an emergent framework for the genetic control of floral induction in maize and highlights the conserved topology of the floral transition network in flowering plants.

DNA methylation plays a critical role in controlling states of gene activity in most eukaryotic organisms, and it is essential for proper growth and development. Patterns of methylation are established by de novo methyltransferases and maintained by maintenance methyltransferase activities. The Dnmt3 family of de novo DNA methyltransferases has recently been characterized in animals. Here we describe DNA methyltransferase genes from both Arabidopsis and maize that show a high level of sequence similarity to Dnmt3, suggesting that they encode plant de novo methyltransferases. Relative to all known eukaryotic methyltransferases, these plant proteins contain a novel arrangement of the motifs required for DNA methyltransferase catalytic activity. The N termini of these methyltransferases contain a series of ubiquitin-associated (UBA) domains. UBA domains are found in several ubiquitin pathway proteins and in DNA repair enzymes such as Rad23, and they may be involved in ubiquitin binding. The presence of UBA domains provides a possible link between DNA methylation and ubiquitin͞proteasome pathways. Methylation of the C5 position of cytosine is the most common covalent modification of DNA in higher plants and animals. This methylation is usually associated with transcriptional gene silencing, or so-called epigenetic gene inactivation. There are several examples of epigenetic silencing in the plant kingdom, including paramutation in maize (1), PAI (2) and SUPERMAN (3) gene silencing in Arabidopsis, and transgene silencing in many plants species (4). Genetic experiments in Arabidopsis have shown that proper DNA methylation levels are required for normal development (5, 6). In animal systems, DNA methylation plays a prominent role in allele-specific gene expression that occurs during parental genomic imprinting and X chromosome inactivation (7,8). Methylation is also important in the regulation of genomic parasites such as transposable elements, retrotransposons, and retroviruses (9, 10).The presence of 5-methylcytosine in genomic DNA is the result of enzymatic activity of the C5 DNA methyltransferases, which catalyze the transfer of a methyl group from S-adenosyl-L-methionine (AdoMet). All known C5 DNA methyltransferases are characterized by the presence of several conserved motifs in the region of the protein involved in catalysis (11). This conservation suggests that cytosine methyltransferases share a common evolutionary history.There are two major types of DNA methyltransferase activities, maintenance and de novo. The methylation of hemimethylated symmetrical sequences (CpG and CpXpG) after DNA replication is maintenance methylation. This results in stable patterns of methylation that are maintained throughout development or, in many cases, between generations. Methylation that occurs at previously unmethylated cytosines is known as de novo methylation. For symmetric sites, de novo methylation need occur only once, after which methylation can be preserved by maintenance activity. However, for maintenance of methylat...