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Summary Differential regulation of stem cell activity in shoot meristems contributes to the wide variation in shoot architecture. 1 , 2 , 3 In most Citrus species, a thorn meristem and a dormant axillary meristem co-localize at each leaf base, offset from each other in a spiral phyllotactic pattern. We recently identified THORN IDENTITY1 (TI1) and THORN IDENTITY2 (TI2), encoding TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) transcription factors, as necessary for the termination of meristem proliferation and concomitant thorn production in Citrus. 4 However, how the dormant axillary meristem at the same leaf axil maintains stem cell activity is still unknown. The phosphatidylethanolamine-binding protein (PEBP)-type transcription factors CENTRORADIALIS (CEN) and TERMINAL FLOWER1 (TFL1) maintain inflorescence meristem indeterminacy in many plant species by antagonizing floral meristem identity regulators. 5 , 6 , 7 , 8 , 9 Here, we show that, in Citrus, Citrus CEN (CsCEN) maintains vegetative axillary meristem indeterminacy by antagonizing TI1. CsCEN is expressed in the axillary meristem, but not in the thorn meristem. Disruption of CsCEN function results in termination of the stem cell activity and conversion of dormant axillary meristems into thorns, although ectopic overexpression of CsCEN represses TI1 expression and converts thorns into dormant buds, a phenotype similar to the ti1 mutant. We further show that CsCEN interacts with Citrus FD (CsFD) to repress TI1 expression. CsCEN activity depends on the function of TI1 and TI2, as mutations in TI1 and TI2 rescue the cscen mutant phenotype. We suggest that the antagonistic roles of CsCEN and TI1 define the pattern of axillary meristem determinacy, which shapes vegetative Citrus tree shoot architecture.

The WUSCHEL (WUS) gene is necessary for the maintenance of stem cells in the shoot apical meristem. Four recent reports show that cytokinin responsive type-B ARABIDOPSIS RESPONSE REGULATORs (ARRs) directly activate WUS expression, providing a long-awaited explanation for how cytokinin influences the maintenance of the stem cell niche.

Thorns arise from axillary shoot apical meristems that proliferate for a time and then terminally differentiate into a sharp tip. Like other meristems, thorn meristems contain stem cells but, in the case of thorns, these stem cells undergo a programmed cessation of proliferative activity. Using Citrus, we characterize a gene network necessary for thorn development. We identify two Citrus genes, THORN IDENTITY1 (TI1) and THORN IDENTITY2 (TI2), encoding TCP transcription factors, as necessary for stem cell quiescence and thorn identity. Disruption of TI1 and TI2 function results in reactivation of stem cells and concomitant conversion of thorns to branches. Expression of WUSCHEL (WUS) defines the shoot stem cell niche in the apical meristems of many angiosperm species; we show that TI1 binds to the Citrus WUS promoter and negatively regulates its expression to terminate stem cell proliferation. We propose that shifts in the timing and function of components of this gene network can account for the evolution of Citrus thorn identity. Modulating this pathway can significantly alter plant architecture and could be leveraged to improve crop yields.

SummaryThe Arabidopsis petal is a simple laminar organ whose development is largely impervious to environmental effects, making it an excellent model for dissecting the regulation of cell-cycle progression and post-mitotic cell expansion that together sculpt organ form [1, 2]. Arabidopsis petals grow via basipetal waves of cell division, followed by a phase of cell expansion [3–5]. RABBIT EARS (RBE) encodes a C2H2 zinc finger transcriptional repressor and is required for petal development [6–9]. During the early phase of petal initiation, RBE regulates a microRNA164-dependent pathway that controls cell proliferation at the petal primordium boundaries [10–12]. The effects of rbe mutations on petal lamina growth suggest that RBE is also required to regulate later developmental events during petal organogenesis [6, 12]. Here, we demonstrate that, early in petal development, RBE represses the transcription of a suite of CIN-TCP genes that in turn act to inhibit the number and duration of cell divisions; the temporal alleviation of that repression results in the transition from cell division to post-mitotic cell expansion and concomitant petal maturation.

Inducible gene expression systems, in which the level or timing of activity of a gene of interest can be controlled exogenously, are an effective means to assess gene function. However, such systems have not been widely employed in crop plants. Here we show that the glucocorticoid receptor (GR)-based inducible gene expression system functions in Citrange (Citrus sinensis × C. trifoliata) plants. We generated transgenic Citrange plants containing a two component 35S::LhGR/pOp6::β-glucuronidase (GUS) system (Wielopolska et al. (2005) Plant Biotechnol. J. 3, 583), in which the synthetic transcription factor, LhGR, is glucocorticoid-inducible and can activate the pOp6 promoter driving expression of the β-glucuronidase (GUS) reporter gene. We describe a method for inducing LhGR activity using the synthetic glucocorticoid dexamethasone (DEX) in transgenic Citrange. With the advent of transgenic approaches to engineer new traits in Citrus, control of transgene activity will be essential to realize the full potential of such manipulations. The method we describe here is likely to be of general use in many Citrus cultivars to temporally control the activity of introduced transgenes.

Organogenesis entails the regulation of cell division, cell expansion, cell and tissue type differentiation, and patterning of the organ as a whole. Petals are ideally suited to dissecting these processes. Petals are dispensable for growth and reproduction, enabling varied manipulations to be carried out with ease. In Arabidopsis, petals have a simple laminar structure with a small number of cell types, facilitating the analysis of organogenesis. This review summarizes recent studies that have illuminated some of the complex interplay between the genetic pathways controlling petal specification, growth and differentiation in Arabidopsis. These advances, coupled with the advantages of using petals as a model experimental system, provide an excellent platform to investigate the underlying mechanisms driving plant organogenesis.

Summary Orchids serve as useful model plants for the discovery and study of genes involved in novel processes in floral development because of their highly modified flowers. In this study three different FRUITFULL ( FUL )-like MADS-box genes, DthyrFL1 , -2 , and -3 have been isolated from the orchid Dendrobium thyrsiflorum . Sequence alignment indicates that the entire sequence of exon 6 is missing in DthyrFL 3 and that a frame shift could explain the missing FUL -like motif found in the sequence of both DthyrFL1 and -2 . Phylogenetic analysis of the APETALA1/FRUITFULL lineage shows that FUL -like sequences from monocots all fall within one major clade and that subsequent gene duplication within this group is specific to monocots. At least two major duplication events have occurred: one before the split of the Poaceae and another within the Poaceae. Quantitative real-time RT-PCR analysis of the DthyrFL genes shows that they are expressed at different levels during inflorescence development but also transcribed in ovules and at very low levels in roots and leaves. These results suggest that the three FUL -like members are involved in floral development in D. thyrsiflorum .

The genomic approach to understanding how evolution has generated the extraordinary diversity of flowers is to assemble a floral EST database for several missing-link taxa and then use gene phylogenies and expression data to identify genes that are important in flower evolution. However, such a genomic approach is likely to miss important genes that are not members of gene families that control flower development in Arabidopsis , and can overlook genes that are not expressed, or are weakly expressed, at their site of action. Therefore we propose complementary genetic approaches in which a few phylogenetically well distributed species are developed as model systems and floral differentiation among closely related species is studied using functional approaches.

It is clear that cell-cell signaling is critical for the development of both root and shoot structures. Recently, several of the key gene products required for intercellular signaling have been defined, and the developmental processes regulated by cell-cell interactions are beginning to be elucidated. Surprisingly, these results suggest that the mechanisms by which plant cells communicate with each other may be quite distinct from those used in animal systems.

The angiosperms, or flowering plants, are the most abundant plant group on the earth today. They have evolved sophisticated reproductive structures, the flowers, which have long attracted interest from gardeners, poets, and artists from around the world. Scholars, too, have focused on studying flowers, and floral morphology is one of the most common characters used in the classification of the angiosperms. This is largely because differences in floral forms are so dramatic, and yet flowers from a particular species are remarkably consistent in their development and final morphology. Flowers arise from florally determined shoot apical meristems. Meristems are small populations of stem cells, which divide in a stereotypical fashion to produce both daughter cells which go on to form organs, as well as daughters to replenish the meristematic population. During vegetative development, cells on the flanks of the shoot apical meristem proliferate to form the vegetative structures, the leaves with their associated axillary buds. Once a plant has been induced to flower, some or all of the meristems can become committed to forming a flower. The floral organs arise sequentially on the flanks of the floral meristem: first the sepals, followed by the petals, stamens, and finally carpels (Fig. 1). The sepals protect and enclose the bud, while the petals in many species serve to attract pollinators. The sepals and petals surround the stamens, which produce the pollen, and the carpels, which contain the ovules. This review will focus on two related issues: p

The Arabidopsis flower is composed of four concentric whorls of organs: the sepals, petals, stamens and carpels. The development of this pattern depends on two general processes: that of establishing a florally determined meristem, and that of establishing particular organ identities. Mutational analyses have been instrumental in defining classes of genes that participate in each of these processes. The floral meristem identity genes are required to specify a florally determined meristem, and also appear to be required in part to activate the floral homeotic genes. The floral homeotic genes function in discrete regions of the florally determined meristem to specify the different organ types. The molecular analysis of these genes is beginning to shed light on the details of their regulation and function. The challenge that remains is to understand how the action of these genes in the dividing meristematic cell population can be coordinated and maintained to result in the differentiation of the complex array of cell and tissue types that make up the Arabidopsis flower.

The controlled ablation of cells is an effective technique with which to study and manipulate plant development. Traditional microsurgical techniques have been used to remove plant cells with varying degrees of success, but modern methods using either lasers or genetic ablation are proving to be easier to control. Lasers can be used to remove accessible single cells or small groups of cells, and genetic ablation — employing a tissue-specific promoter to control the expression of a cytotoxic gene — is effective for killing a specific set of cells. Ablation techniques have been instrumental in investigating cell—cell interactions, as well as for more applied research, such as on the control of fertility and disease resistance.

Organs are formed throughout a plant's lifetime by the continued proliferation of cells in the apical meristems. Cell divisions within the meristem are fairly regular and give rise to predictable portions of the plant body. However, mosaic analyses have demonstrated that cell fates are not strictly dependent on lineage. Interactions between cells must therefore play a role in defining the final differentiated pattern. Some of the gene products that may be involved in these cell signalling processes are beginning to be defined.