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PubMed1999

Transcriptional activation of APETALA1 by LEAFY.

Plants produce new appendages reiteratively from groups of stem cells called shoot apical meristems. LEAFY (LFY) and APETALA1 (AP1) are pivotal for the switch to the reproductive phase, where instead of leaves the shoot apical meristem produces flowers. Use of steroid-inducible activation of LFY demonstrated that early expression of AP1 is a result of transcriptional induction by LFY. This AP1 induction is independent of protein synthesis and occurs specifically in the tissues and at the developmental stage in which floral fate is assumed. Later expression of AP1 appears to be only indirectly affected by LFY.

Arabidopsis↗

PubMed1999

Activation of a floral homeotic gene in Arabidopsis.

The patterned expression of floral homeotic genes in Arabidopsis depends on the earlier action of meristem-identity genes such as LEAFY, which encodes a transcription factor that determines whether a meristem will generate flowers instead of leaves and shoots. The LEAFY protein, which is expressed throughout the flower, participates in the activation of homeotic genes, which are expressed in specific regions of the flower. Analysis of a LEAFY-responsive enhancer in the homeotic gene AGAMOUS indicates that direct interaction of LEAFY with this enhancer is required for its activity in plants. Thus, LEAFY is a direct upstream regulator of floral homeotic genes.

AGAMOUS Protein, Arabidopsis↗

PubMed2000

Dependence of stem cell fate in Arabidopsis on a feedback loop regulated by CLV3 activity.

The fate of stem cells in plant meristems is governed by directional signaling systems that are regulated by negative feedback. In Arabidopsis thaliana, the CLAVATA (CLV) genes encode the essential components of a negative, stem cell-restricting pathway. We used transgenic plants overexpressing CLV3 to show that meristem cell accumulation and fate depends directly on the level of CLV3 activity and that CLV3 signaling occurs exclusively through a CLV1/CLV2 receptor kinase complex. We also demonstrate that the CLV pathway acts by repressing the activity of the transcription factor WUSCHEL, an element of the positive, stem cell-promoting pathway.

Arabidopsis↗

PubMed2003

VARICOSE, a WD-domain protein, is required for leaf blade development.

To gain insight into the processes controlling leaf development, we characterized an Arabidopsis mutant, varicose (vcs), with leaf and shoot apical meristem defects. The vcs phenotype is temperature dependent; low temperature growth largely suppressed defects, whereas high growth temperatures resulted in severe leaf and meristem defects. VCS encodes a putative WD-domain containing protein, suggesting a function involving protein-protein interactions. Temperature shift experiments indicated that VCS is required throughout leaf development, but normal secondary vein patterning required low temperature early in leaf development. The low-temperature vcs phenotype is enhanced in axr1-3 vcs double mutants and in vcs mutants grown in the presence of polar auxin transport inhibitors, however, vcs has apparently normal auxin responses. Taken together, these observations suggest a role for VCS in leaf blade formation.

Amino Acid Sequence↗

PubMed2004

The Arabidopsis thaliana SNF2 homolog AtBRM controls shoot development and flowering.

Chromatin remodeling is essential for the reprogramming of transcription associated with development and cell differentiation. The SWI/SNF complex was the first chromatin remodeling complex characterized in yeast and Drosophila. In this work we have characterized an Arabidopsis thaliana homolog of Brahma, the ATPase of the Drosophila SWI/SNF complex. As its Drosophila counterpart, Arabidopsis thaliana BRAHMA (AtBRM) is a nuclear protein present in a high molecular mass complex. Furthermore, the N terminus of AtBRM interacts, in the two-hybrid system, with CHB4 (AtSWI3C), an Arabidopsis homolog of the yeast SWI/SNF complex subunit SWI3. The AtBRM gene is primarily expressed in meristems, organ primordia and tissues with active cell division. Silencing of the expression of the AtBRM gene by RNA interference demonstrated that AtBRM is required for vegetative and reproductive development. The AtBRM silenced plants exhibited a reduction in overall plant size with small and curled leafs, as well as a reduction in the size of the inflorescence meristem. In the absence of AtBRM, Arabidopsis flowers have small petals and stamens, immature anthers, homeotic transformations and reduced fertility. The AtBRM silenced plants flower earlier than wild-type plants both under inductive and non-inductive photoperiods. Furthermore, levels of CO, FT and SOC1 transcripts were up-regulated under non-inductive conditions suggesting that AtBRM is a repressor of the photoperiod-dependent flowering pathway.

Adenosine Triphosphatases↗

PubMed2006

Chromatin assembly factor CAF-1 is required for cellular differentiation during plant development.

Chromatin assembly factor CAF-1 facilitates the formation of nucleosomes on newly replicated DNA in vitro. However, the role of CAF-1 in development is poorly understood because mutants are not available in most multicellular model organisms. Biochemical evidence suggests that FASCIATA1, FASCIATA2 and MSI1 form CAF-1 in Arabidopsis thaliana. Because fasciata mutants are viable, CAF-1 is not essential for cell division in plants. Arabidopsis CAF-1 mutants have defects in shoot apical meristems; in addition, CAF-1 is required to establish seedling architecture, leaf size and trichome differentiation. CAF-1 is needed to restrict branching of trichomes on rosette leaves. Increased trichome branching in CAF-1 mutants is not strictly correlated with increased nuclear DNA content. In addition, fas2 glabra3 double mutants show an additive genetic interaction, demonstrating that CAF-1 acts genetically parallel to the GLABRA3-containing, endoreduplication-coupled trichome branching pathway. However, CAF-1 is often needed to restrict endoreduplication, because seedlings of most CAF-1 mutants have increased ploidy. Notably, in the Landsberg erecta background, loss of CAF-1 does not affect ploidy, demonstrating that loss of CAF-1 can be compensated in some Arabidopsis accessions. These results reveal that the functions of FAS1, FAS2 and MSI1 are not restricted to meristems, but are also needed to control genome replication at multiple steps of development.

Animals↗

PubMed1996

The maize mutant narrow sheath fails to establish leaf margin identity in a meristematic domain.

The maize mutant narrow sheath (ns) displays a leaf shape and plant stature phenotype that suggests the preprimordial deletion of a leaf domain. The ns mutant phenotype is inherited as a recessive, duplicate-factor trait, conditioned upon homozygosity for each of the two unlinked mutations narrow sheath-1 (ns1) and narrow sheath-2 (ns2). Mutant leaves are missing a large domain including the leaf margin, and mutant internodes are shortened on the marginal side of the stem. This domain deletion extends from the internode to beyond the longitudinal mid-length of the blade, and corresponds to an alteration in the organization of a specific region of the shoot apical meristem. The premargin region of mutant founder cells fail to down-regulate expression of Knox genes, markers of nonleaf meristematic identity. Our results indicate that leaf domains may acquire identity in the meristem itself, and that the subdivision of preprimordial developmental fields into differential domains is a common feature of both plant and animal organogenesis.

Animals↗

PubMed1997

LEAFY expression and flower initiation in Arabidopsis.

During the initial vegetative phase, the Arabidopsis shoot meristem produces leaves with associated lateral shoots at its flanks, while the later reproductive phase is characterized by the formation of flowers. The LEAFY gene is an important element of the transition from the vegetative to the reproductive phase, as LEAFY is both necessary and sufficient for the initiation of individual flowers. We have analyzed in detail the expression of LEAFY during the plant life cycle, and found that LEAFY is extensively expressed during the vegetative phase. In long days, Arabidopsis plants flower soon after germination, and this is paralleled by rapid upregulation of LEAFY. In short days, Arabidopsis plants flower several weeks later than in long days, but LEAFY expression increases gradually before flowering commences. Application of the plant hormone gibberellin, which hastens flowering in short days, enhances the gradual change in LEAFY expression observed in short days. Changes in LEAFY expression before the transition to flowering suggest that the time point of this transition is at least partly controlled by the levels of LEAFY activity that are prevalent at a given time of the life cycle. This assumption is borne out by the finding that increasing the copy number of endogenous LEAFY reduces the number of leaves produced before the first flower is formed. Thus, LEAFY combines properties of flowering-time and flower-meristem-identity genes, indicating that LEAFY is a direct link between the global process of floral induction and the regional events associated with the initiation of individual flowers.

Arabidopsis↗

PubMed1998

STYLOSA and FISTULATA: regulatory components of the homeotic control of Antirrhinum floral organogenesis.

The identity and developmental pattern of the four organ types constituting the flower is governed by three developmental functions, A, B and C, which are defined by homeotic genes and established in two adjacent whorls. In this report we morphologically and genetically characterise mutants of two genes, STYLOSA (STY) and FISTULATA (FIS) which control floral homeotic meristem- and organ-identity genes and developmental events in all floral whorls. The morphology of the reproductive organs in the first and second whorls of sty fis double mutant flowers indicate that the two genes are part of the mechanism to prevent ectopic expression of the C-function in the perianth of wild-type flowers. This is verified by the detection of the expansion of the expression domain of the class C gene PLENA (PLE) towards the perianth. Interestingly, in the second whorl of sty and fis mutants, spatial differences in stamenoid features and in the pattern of ectopic expression of the PLE gene were observed. This suggests that, with respect to the negative control of PLE, petals are composed of two regions, a lateral and a central one. Mutation in ple is epistatic to most of the sty/fis-related homeotic defects. PLE, however, is not the primary target of STY/FIS control, because dramatic reduction of expression of FIMBRIATA, meristem identity genes (FLORICAULA and SQUAMOSA) and of class B organ identity genes (GLOBOSA) occur before changes in the PLE expression pattern. We propose that STY/FIS are hierarchically high-ranking genes that control cadastral component(s) of the A-function. SQUAMOSA as a potential target of this control is discussed. Retarded growth of second whorl organs, subdivision of third whorl primordia and the failure to initiate them in sty/fis mutants may be mediated by the FIMBRIATA gene.

Gene Expression Regulation, Developmental↗

PubMed1998

Control of floral homeotic gene expression and organ morphogenesis in Antirrhinum.

The development of reproductive organs in Antirrhinum depends on the expression of an organ identity gene, plena, in the central domain of the floral meristem. To investigate the mechanism by which plena is regulated, we have characterised three mutants in which the pattern of plena expression is altered. In polypetala mutants, expression of plena is greatly reduced, resulting in a proliferation of petals in place of reproductive organs. In addition, polypetala mutants exhibit an altered pattern of floral organ initiation, quite unlike that seen in loss-of-function plena mutants. This suggests that polypetala normally has two roles in flower development: regulation of plena and control of organ primordia formation. In fistulata mutants, plena is ectopically expressed in the distal domain of petal primordia, resulting in the production of anther-like tissue in place of petal lobes. Flowers of fistulata mutants also show a reduced rate of petal lobe growth, even in a plena mutant background. This implies that fistulata normally has two roles in the distal domain of petal primordia: inhibition of plena expression and promotion of lobe growth. A weak allele of the floral meristem identity gene, floricaula, greatly enhances the effect of fistulata on plena expression, showing that floricaula also plays a role in repression of plena in outer whorls. Taken together, these results show that genes involved in plena regulation have additional roles in the formation of organs, perhaps reflecting underlying mechanisms for coupling homeotic gene expression to morphogenesis.

Base Sequence↗

PubMed1999

Refined analysis of early symbiotic steps of the Rhizobium-Medicago interaction in relationship with microtubular cytoskeleton rearrangements.

In situ immunolocalization of tubulin revealed that important rearrangements occur during all the early symbiotic steps in the Medicago/R. meliloti symbiotic interaction. Microtubular cytoskeleton (MtC) reorganizations were observed in inner tissues, first in the pericycle and then in the inner cortex where the nodule primordium forms. Subsequently, major MtC changes occurred in outer tissues, associated with root hair activation and curling, the formation of preinfection threads (PITs) and the initiation and the growth of an infection network. From the observed sequence of MtC changes, we propose a model which aims to better define, at the histological level, the timing of the early symbiotic stages. This model suggests the existence of two opposite gradients of cell differentiation controlling respectively the formation of division centers in the inner cortex and plant preparation for infection. It implies that (i) MtC rearrangements occur in pericycle and inner cortex earlier than in the root hair, (ii) the infection process proceeds prior to the formation of the nodule meristem, (iii) the initial primordium prefigures the future zone II of the mature nodule and (iv) the nodule meristem derives from the nodule primordium. Finally, our data also strongly suggest that in alfalfa PIT differentiation, a stage essential for successful infection, requires complementary signaling additional to Nod factors.

Cell Differentiation↗

PubMed2000

NARROW SHEATH1 functions from two meristematic foci during founder-cell recruitment in maize leaf development.

The narrow sheath duplicate genes (ns1 and ns2) perform redundant functions during maize leaf development. Plants homozygous for mutations in both ns genes fail to develop wild-type leaf tissue in a lateral domain that includes the leaf margin. Previous studies indicated that the NS gene product(s) functions during recruitment of leaf founder-cells in a lateral, meristematic domain that contributes to leaf margin development. A mosaic analysis was performed in which the ns1-O mutation was exposed in hemizygous, clonal sectors in a genetic background already homozygous for ns2-O. Analyses of mutant, sectored plants demonstrate that NS1 function is required in L2-derived tissue layers for development of the narrow sheath leaf domain. NS1 function is not required for development of the central region of maize leaves. Furthermore, the presence of the non-mutant ns1 gene outside the narrow sheath domain cannot compensate for the absence of the non-mutant gene within the narrow sheath domain. NS1 acts non-cell autonomously within the narrow sheath-margin domain and directs recruitment of marginal, leaf founder cells from two discrete foci in the maize meristem. Loss of NS1 function during later stages of leaf development results in no phenotypic consequences. These data support our model for NS function during founder-cell recruitment in the maize meristem.

Chromosome Mapping↗

PubMed2001

The Handlebars gene is required with Phantastica for dorsoventral asymmetry of organs and for stem cell activity in Antirrhinum.

In angiosperms, individual lateral organs and whole flowers may develop asymmetrically along their dorsoventral axes. Dorsoventral asymmetry of Antirrhinum leaves requires activity of the Phantastica gene and other factors acting redundantly with it. We describe the effects of a mutation in the Handlebars gene, identified as an enhancer of the phantastica mutant phenotype. Genetic analysis suggests that Handlebars functions redundantly with Phantastica to promote dorsal fate in lateral organs and to maintain activity of stem cells within shoot apical meristems. Handlebars appears dispensable in vegetative development but is needed for asymmetry of petals along the dorsoventral axis of the flower as a whole. This suggests that common mechanisms may control dorsoventral asymmetry in lateral organ primordia and in floral meristems.

Cotyledon↗

PubMed2002

SCHIZORIZA controls an asymmetric cell division and restricts epidermal identity in the Arabidopsis root.

The primary root of Arabidopsis has a simple cellular organisation. The fixed radial cell pattern results from stereotypical cell divisions that occur in the meristem. Here we describe the characterisation of schizoriza (scz), a mutant with defective radial patterning. In scz mutants, the subepidermal layer (ground tissue) develops root hairs. Root hairs normally only form on epidermal cells of wild-type plants. Moreover, extra periclinal divisions (new wall parallel to surface of the root) occur in the scz root resulting in the formation of supernumerary layers in the ground tissue. Both scarecrow (scr) and short root (shr) suppress the extra periclinal divisions characteristic of scz mutant roots. This results in the formation of a single layered ground tissue in the double mutants. Cells of this layer develop root hairs, indicating that mis-specification of the ground tissue in scz mutants is uncoupled to the cell division defect. This suggests that during the development of the ground tissue SCZ has two distinct roles: (1) it acts as a suppressor of epidermal fate in the ground tissue, and (2) it is required to repress periclinal divisions in the meristem. It may act in the same pathway as SCR and SHR.

Arabidopsis↗

PubMed2004

Molecular characterization of the cytoplasmic interacting protein of the receptor kinase IRK expressed in the inflorescence and root apices of Arabidopsis.

Meristem maintenance and differentiation is regulated by intercellular communication through receptor-like kinases (RLKs) in plants, but the underlying molecular mechanisms of RLK signaling remain largely unknown. A cytoplasmic interactor for inflorescence and root apices receptor-like kinase (IRK), which is a typical meristematic RLK with leucine-rich repeats in Arabidopsis, was identified using a yeast two-hybrid assay and named IRK-interacting protein (IRKI). IRKI is a novel but highly conserved protein found in higher plants. The interaction between IRK and IRKI was confirmed by an in vitro pull-down assay and supported by their simultaneous expression in actively dividing cells in meristems. In the root tip, IRKI expression and localization visualized by green fluorescence protein (GFP) were observed in the quiescent center, initial cells, and immature stele cells. IRKI expression was expanded by exogenous auxin treatment and repressed by inhibitor treatment of polar auxin transport.

Amino Acid Sequence↗

PubMed2005

Meristematic activity of the Endodermis and the Pericycle in the primary thickening in monocotyledons: considerations on the "PTM".

This paper proposes a new interpretation for primary thickening in monocotyledons. The anatomy of the vegetative organs of the following species was examined: Cephalostemon riedelianus (Rapataceae), Cyperus papyrus (Cyperaceae), Lagenocarpus rigidus, L. junciformis (Cyperaceae), Echinodorus paniculatus (Alismataceae) and Zingiber officinale (Zingiberaceae). The endodermis with meristematic activity was observed in the root of all the species, in the stem of Cyperus, Cephalostemum and Lagenocarpus rigidus, and in the leaf trace of Cyperus and leaf of Echinodorus. Considering the continuity of tissues through the root, stem and leaf, the authors conclude that in the stem the pericycle remains active throughout the life of the plant as the generator of the vascular tissue. The "Primary Thickening Meristem" is in fact the pericycle plus the endodermis and its derivatives (or only the pericycle). Close to the stem apex, the assemblage of seems to be a unique meristem, giving rise to the inner cortex and vascular tissues.

Endoderm↗

PubMed2006

Colleters in Caryocar brasiliense (Caryocaraceae) ontogenesis, ultrastructure and secretion.

Colleters are widely distributed and have been reported in several taxa. However, the only report of colleters in the Caryocaraceae family is in Caryocar brasiliense. This study purports to describe the distribution, structure and ultrastructure of colleters in C. brasiliense. Vegetative shoot apices with stipules in different stages of development were collected, fixed, and processed by light, transmission and scanning electron microscopy. Colleters occur on the stipule adaxial face and are differentiated on younger stipules, remaining functional until senescence. Stipules are deciduous, falling before the beginning of leaf expansion. Colleter secretion is abundant, filling all the space delimited by the stipule. Colleter origin is mixed, involving the protoderm and ground meristem. In this species, colleters are finger-shaped, consisting of a column of non-secretory central cells and a secretory epithelium whose cells are arranged in palisades. Secretory epithelium cells show dense cytoplasm, rough endoplasmic reticulum (RER) segments, free ribosomes, mitochondria, and abundant well developed dictyosomes. The colleters central cell shows a flocculated cytoplasm and developed vacuole. These cells have a small lobated nucleus with an inconspicuous nucleolus. Mitochondria and RER stand out in the organelles. There is evidence of granulocrine secretion and colleter involvement with protection of the apical meristem and leaves in the initial stages of development.

Magnoliopsida↗

PubMed2002

Influence of summer sowing dates, N fertilization and irrigation on autumn VSP accumulation and dynamics of spring regrowth in alfalfa (Medicago sativa L.).

Herbage yield of alfalfa (Medicago sativa L.) depends on forage management or environmental conditions that change C and N resource acquisition, and endogenous plants factors such as root organic reserves and number of active meristems. The aim of this work is to study the influence of two sowing dates in summer (12 July or 9 August), N fertilization (0 or 100 kg ha(-1)) and/or irrigation applied during the first year of alfalfa establishment on (i) the accumulation of N organic reserves (soluble proteins and more specifically vegetative storage protein) in taproots during autumn, (ii) the number of crown axillary meristems present at the end of winter and (iii) the dynamics of spring shoot growth. Delaying the sowing date for one month reduced root growth and root N storage, especially vegetative storage proteins (VSP) during autumn. Irrespective of sowing dates, N fertilization did not affect root biomass, number of crown buds, total root N, root soluble protein or VSP concentrations. By contrast, water deficiency during alfalfa establishment in the early summer reduced both root growth and N reserve accumulation. When spring growth resumed, there is a significant linear relationship between leaf area development and soluble protein and VSP concentrations in taproots, and also the number of crown buds. The results showed that an early sowing date and adequate water status during the summer allowed alfalfa plants to accumulate N reserves by increasing taproot mass and soluble protein concentrations, especially VSPs. This resulted in rapid shoot regrowth rates the following spring.

Carbon↗