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PubMed1996

Cell fate in the development of the Arabidopsis flower.

The Arabidopsis flower consists of four concentric whorls of organs. The first (outermost) whorl consists of four sepals and the fourth (innermost) whorl is made up of two carpels. Cell fate in the first and fourth whorls was studied using X-ray-induced yellow ch-42 sectors. Sector boundaries were found to be non-random around the two whorls and four generalizations relating the marked and unmarked tissues were deduced. In the sepal and carpel whorls the smallest sectors of marked and unmarked tissue were found to be one half of a sepal and one half of a carpel, respectively. A detailed frequency-distance map of the floral primordium was made and found to be a ridge with the fourth whorl carpels at the summit and the first whorl transverse sepal pair at the base. Consideration of: the rate of loss of chimerism in the inflorescence meristem, the frequency-distance across the flower and the frequency-distance between successive flowers, was used to produce an abstract model of the inflorescence meristem.

Arabidopsis↗

PubMed1997

Differential expression of a CAK (cdc2-activating kinase)-like protein kinase, cyclins and cdc2 genes from rice during the cell cycle and in response to gibberellin.

Progress through the eukaryotic cell cycle is regulated by cyclin-dependent cdc2 protein kinases. In rice (Oryza sativa L), two cdc2 protein kinases, cdc2Os-1 and cdc2Os-2, and two cyclins, cycOs1 and cycOs2, have been described. In this study, we report on the cell-cycle phase-specific expression of these genes. Using partially synchronized suspension cells from rice, we found that cdc2Os-1 was expressed constitutively throughout the cell cycle. The cdc2Os-2 transcript level was elevated in G1 and S phase. The cycOs1 and cycOs2 transcripts increased steadily through G2 and dropped off rapidly in mitosis as is typical for mitotic cyclins. We hypothesize that the cdc2Os-2 gene product acts in G1/S and that the growth-promoting hormone gibberellin (GA) that induces expression of cdc2Os-2, cycOs1 and cycOs2 in the intercalary meristem of deepwater rice internodes accelerates G1/S phase progression through increased expression of cdc2Os-2 and G2/M phase progression through increased expression of the mitotic cyclins cycOs1 and cycOs2. The R2 gene from rice has 55% sequence identity to the cdc2-activating kinase (CAK) family of protein kinases which have been shown to phosphorylate and thereby activate cdc2 protein kinases in animals and yeast. In partially synchronized suspension cells, R2 mRNA levels were elevated in G1 and S phase. In GA-treated rice internodes, R2 transcript levels were elevated in the meristem and part of the elongation zone. These results are consistent with a role for R2 in regulating G1/S phase progression.

Amino Acid Sequence↗

PubMed1997

Conditional transgenic expression of the ipt gene indicates a function for cytokinins in paracrine signaling in whole tobacco plants.

This study investigated whether an increased production of the plant hormone cytokinin in roots, the main site of its synthesis and putative signaling organ, can influence developmental events, such as growth of axillary shoot meristems or leaf senescence, in the plant shoot. To this end, transgenic tobacco plants (Nicotiana tabacum L.) were generated that conditionally overproduce cytokinins. These plants harbour the ipt gene under the transcriptional control of a modified 35S promoter that is repressed in plants with high titers of tetracycline repressor protein. De-repression of transcription led to a rapid more than 50-fold increase of hormone concentration. The time course of changes in the steady-state levels of 16 different cytokinin metabolites, as a consequence of IPT enzyme activity, was monitored in different plant tissues. Zeatin riboside was the first and most dramatically increased product; zeatin, dihydrozeatin and glucosides accumulated later. The consequences of enhanced cytokinin synthesis remained mainly restricted to the site of hormone production. For example, de-repression of ipt gene transcription in lateral buds caused the growth of single buds only at the site of tetracycline application. In reciprocal grafts of transgenic plants with wild-type plants, no biological cytokinin effects, i.e. growth of lateral shoot meristems or sequential leaf senescence, were observed in the non-transgenic plant part. Also, the increase in steady-state levels of cytokinins remained restricted mainly to the transgenic part, despite a specific increase of the zeatin riboside concentration in the transpiration stream. These results question the role of cytokinins as a long-range root-to-shoot signal in correlative control of apical dominance and sequential leaf senescence of tobacco, and support the assumption that this hormone is relevant to paracrine signaling.

Aging↗

PubMed1997

ACL5: an Arabidopsis gene required for internodal elongation after flowering.

In rosette plants, the formation of initial flowers is closely linked to the lengthening of internodes (bolting). In order to clarify the molecular basis of bolting, mutants with reduced lengths of internodes were screened. This paper presents the identification and characterization of recessive mutations in ACAULIS5 (ACL5), a gene required for internodal growth in Arabidopsis thaliana. Unlike previously described mutants with reduced size of organs, the acl5 mutant has a severe defect that is restricted to the process of cell elongation after transition to the reproductive stage and shows no phenotype before floral induction. The results of RNA blot hybridizations showed that the acl5 mutation causes a striking reduction in the transcript levels of genes encoding the tonoplast intrinsic protein (gamma-TIP) and the endoxyloglucan transferase (EXGT-A1), both of which have recently been suggested to be important for cell elongation. Furthermore, our morphological study indicates that the mutation also causes proliferative arrest of the apical inflorescence meristem. These results strongly suggest that, during the reproductive phase, the wild-type ACL5 gene product has a critical function not only in the control of elongation growth of organs but also in the continued maintenance of the proliferative activity of flower-producing meristems.

Arabidopsis↗

PubMed2000

Mutations in the WUSCHEL gene of Arabidopsis thaliana result in the development of shoots without juvenile leaves.

The vegetative growth of Arabidopsis thaliana can be divided into two phases. The transition from the juvenile (early) phase to the adult (later) phase is associated with changes in several morphological features of leaves, such as the shape of leaf blades, the number of trichomes and patterns of venation. In a screening of mutants with altered morphological identities of leaves, we found one which we named juvenile leafless and misshapen shoot apical meristem (jam). The mutation represented a new allele of the WUSCHEL (WUS) gene, and, in its presence, plants produced no juvenile leaves. Analysis of the morphology of mutant plants revealed that all the rosette leaves had characteristics of adult leaves. The formation of the first rosette leaf in the wus(jam) mutant was markedly delayed, and occurred at the almost same time as formation of the third or fourth leaf in wild-type plants. In the wild-type, these leaves correspond to the first adult leaves. Analysis by RT-PCR showed that transcripts of WUS accumulated in shoot apices and roots, but not in cotyledons and leaves. The present results suggest that the WUS gene controls the morphological traits of rosette leaves either directly or indirectly. In view of the predicted function of the WUS gene, namely maintenance of stem cells within the shoot apical meristem, we suggest that the lack of juvenile leaves in the mutant might have been caused by interruption of leaf initiation during the juvenile phase or by halting of an entire process of formation of juvenile leaves.

Arabidopsis↗

PubMed2000

Cytokinin and gibberellin activate SaMADS A, a gene apparently involved in regulation of the floral transition in Sinapis alba.

In plants of Sinapis alba induced to flower by one long day, the MADS box gene, SaMADS A, is expressed initially in the central corpus (L3 cells) of the shoot apical meristem (SAM), about 1.5-2 days before initiation of the first floral meristem. We have combined a physiological approach by testing the effects of three putative floral signals on SaMADS A expression in the SAM of S. alba plants with a transgenic approach using Arabidopsis thaliana plants. A single application of a low dose of a cytokinin or a gibberellin to the apex of vegetative S. alba plants is capable of mimicking perfectly the initial effect of the long day on SaMADS A transcription. A treatment combining the two hormones causes the same activation but seems to enhance the level of SaMADS A expression. A sucrose application to the apex of vegetative plants is, on the contrary, unable to activate SaMADS A expression. None of these chemicals, alone or combined, is capable of causing the floral shift at the SAM. Since the constitutive expression of SaMADS A leads to precocious flowering in A. thaliana and antisense expression of a fragment of the A. thaliana homologue AGL20 leads to a delay in flowering time, these results are consistent with SaMADS A activation being an intermediate event in a cytokinin- and/or gibberellin-triggered signal transduction pathway that is involved in the regulation of floral transition in S. alba.

Agrobacterium tumefaciens↗

PubMed2002

Overexpression of RCN1 and RCN2, rice TERMINAL FLOWER 1/CENTRORADIALIS homologs, confers delay of phase transition and altered panicle morphology in rice.

TERMINAL FLOWER 1 (TFL1)/CENTRORADIALIS (CEN)-like genes play important roles in determining plant architecture, mainly by controlling the timing of phase transition. To investigate the possibility of similar mechanisms operating in the control of inflorescence architecture in rice, we analysed the functions of RCN1 and RCN2, rice TFL1/CEN homologs. Constitutive overexpression of RCN1 or RCN2 in Arabidopsis caused a late-flowering and highly branching phenotype, indicating that they possess conserved biochemical functions as TFL1. In 35S::RCN1 and 35S::RCN2 transgenic rice plants, the delay of transition to the reproductive phase was observed. The transgenic rice plants exhibited a more branched, denser panicle morphology. Detailed observation of the panicle structure revealed that the phase change from the branch shoot to the floral meristem state was also delayed, leading to the generation of higher-order panicle branches. These results suggest rice has a pathway that can respond to the overexpressed TFL1/CEN-like functions, and the molecular mechanisms controlling the phase transition of meristems are conserved between grass and dicot species, at least to some extent.

Arabidopsis↗

PubMed2003

Interactions between gene activity and cell layers during floral development.

The DEFICIENS (DEF) gene is required for establishing petal and stamen identity in Antirrhinum and is expressed in all three layers of the floral meristem in whorls 2 and 3. Expression of DEF in a subset of meristem layers gives rise to organs with characteristic shapes and cell types, reflecting altered patterns and levels of DEF gene activity. To determine how the contributions of layers and gene activity interact, we exploited a DEF allele which carries a transposon insertion in the MADS box region to generate periclinal chimeras expressing alleles with different activities. By comparing the phenotype, development and expression patterns of these chimeras we show that expression of DEF in L1 makes a major contribution to morphology in whorl 2, irrespective of the allele. By contrast L1 expression is largely unable to rescue whorl 3, possibly because of a non-autonomous inhibitor of DEF activity in this whorl.

Alleles↗

PubMed2006

Crosstalk between auxin, cytokinins, and sugars in the plant cell cycle.

Plant meristems are utilization sinks, in which cell division activity governs sink strength. However, the molecular mechanisms by which cell division activity and sink strength are adjusted to a plant's developmental program in its environmental setting are not well understood. Mitogenic hormonal as well as metabolic signals drive and modulate the cell cycle, but a coherent idea of how this is accomplished, is still missing. Auxin and cytokinins are known as endogenous mitogens whose concentrations and timing, however, can be externally affected. Although the sites and mechanisms of signal interaction in cell cycle control have not yet been unravelled, crosstalk of sugar and phytohormone signals could be localized to several biochemical levels. At the expression level of cell cycle control genes, like cyclins, Cdks, and others, synergistic but also antagonistic interactions could be demonstrated. Another level of crosstalk is that of signal generation or modulation. Cytokinins affect the activity of extracellular invertases and hexose-uptake carriers and thus impinge on an intracellular sugar signal. With tobacco BY-2 cells, a coordinated control of cell cycle activity at both regulatory levels could be shown. Comparison of the results obtained with the root cell-representing BY-2 cells with literature data from shoot tissues or green cell cultures of Arabidopsis and Chenopodium suggests opposed and tissue-specific regulatory patterns of mitogenic signals and signal crosstalk in root and shoot meristems.

Arabidopsis↗

PubMed2006

An auxin-driven polarized transport model for phyllotaxis.

Recent studies show that plant organ positioning may be mediated by localized concentrations of the plant hormone auxin. Auxin patterning in the shoot apical meristem is in turn brought about by the subcellular polar distribution of the putative auxin efflux mediator, PIN1. However, the question of what signals determine PIN1 polarization and how this gives rise to regular patterns of auxin concentration remains unknown. Here we address these questions by using mathematical modeling combined with confocal imaging. We propose a model that is based on the assumption that auxin influences the polarization of its own efflux within the meristem epidermis. We show that such a model is sufficient to create regular spatial patterns of auxin concentration on systems with static and dynamic cellular connectivities, the latter governed by a mechanical model. We also optimize parameter values for the PIN1 dynamics by using a detailed auxin transport model, for which parameter values are taken from experimental estimates, together with a template consisting of cell and wall compartments as well as PIN1 concentrations quantitatively extracted from confocal data. The model shows how polarized transport can drive the formation of regular patterns.

Arabidopsis↗

PubMed2006

The tomato FT ortholog triggers systemic signals that regulate growth and flowering and substitute for diverse environmental stimuli.

The systemic model for floral induction, dubbed florigen, was conceived in photoperiod-sensitive plants but implies, in its ultimate form, a graft-transmissible signal that, although activated by different stimuli in different flowering systems, is common to all plants. We show that SFT (SINGLE-FLOWER TRUSS), the tomato ortholog of FLOWERING LOCUS T (FT), induces flowering in day-neutral tomato and tobacco plants and is encoded by SFT. sft tomato mutant plants are late-flowering, with altered architecture and flower morphology. SFT-dependent graft-transmissible signals complement all developmental defects in sft plants and substitute for long-day stimuli in Arabidopsis, short-day stimuli in Maryland Mammoth tobacco, and light-dose requirements in tomato uniflora mutant plants. The absence of donor SFT RNA from flowering receptor shoots and the localization of the protein in leaf nuclei implicate florigen-like messages in tomato as a downstream pathway triggered by cell-autonomous SFT RNA transcripts. Flowering in tomato is synonymous with termination of the shoot apical meristems, and systemic SFT messages attenuate the growth of apical meristems before and independent of floral production. Floral enhancement by systemic SFT signals is therefore one pleiotropic effect of FT orthologs.

Cell Nucleus↗

PubMed2002

Selective interaction of plant homeodomain proteins mediates high DNA-binding affinity.

Understanding molecular mechanisms that control cell fate in the shoot apical meristem is a fundamental question in plant development. Genetic and molecular studies demonstrate that maize KNOTTED1 (KN1) of the TALE (3-aa acid loop extension) class of homeodomain (HD) proteins is involved in shoot apical meristem function. We show that KN1 interacts with knotted interacting protein (KIP), a BEL1-like TALE HD protein. Interaction between KN1 and KIP is mediated by conserved domains in the N termini of both proteins. The KN1 DNA-binding sequence, TGACAG(G/C)T, was biochemically identified, and in vitro DNA-binding assays show that individually KN1 and the HD of KIP bind specifically to this motif with low affinity. The KN1-KIP complex, however, binds specifically to this DNA-binding motif with high affinity, indicating that the association of KN1 and KIP may function in transcriptional regulation.

Amino Acid Motifs↗

PubMed2004

PLASTOCHRON1, a timekeeper of leaf initiation in rice, encodes cytochrome P450.

During postembryonic development of higher plants, the shoot apical meristem produces lateral organs in a regular spacing (phyllotaxy) and a regular timing (plastochron). Molecular analysis of mutants associated with phyllotaxy and plastochron would greatly increase understanding of the developmental mechanism of plant architecture because phyllotaxy and plastochron are fundamental regulators of plant architecture. pla1 of rice is not only a plastochron mutant showing rapid leaf initiation without affecting phyllotaxy, but also a heterochronic mutant showing ectopic shoot formation in the reproductive phase. Thus, pla1 provides a tool for analyzing the molecular basis of temporal regulation in leaf development. In this work, we isolated the PLA1 gene by map-based cloning. The identified PLA1 gene encodes a cytochrome P450, CYP78A11, which potentially catalyzes substances controlling plant development. PLA1 is expressed in developing leaf primordia, bracts of the panicle, and elongating internodes, but not in the shoot apical meristem. The expression pattern and mutant phenotype suggest that the PLA1 gene acting in developing leaf primordia affects the timing of successive leaf initiation and the termination of vegetative growth.

Amino Acid Sequence↗

PubMed1997

The Arabidopsis KNOLLE protein is a cytokinesis-specific syntaxin.

In higher plant cytokinesis, plasma membrane and cell wall originate by vesicle fusion in the plane of cell division. The Arabidopsis KNOLLE gene, which is required for cytokinesis, encodes a protein related to vesicle-docking syntaxins. We have raised specific rabbit antiserum against purified recombinant KNOLLE protein to show biochemically and by immunoelectron microscopy that KNOLLE protein is membrane associated. Using immunofluorescence microscopy, KNOLLE protein was found to be specifically expressed during mitosis and, unlike the plasma membrane H+-ATPase, to localize to the plane of division during cytokinesis. Arabidopsis dynamin-like protein ADL1 accumulates at the plane of cell plate formation in knolle mutant cells as in wild-type cells, suggesting that cytokinetic vesicle traffic is not affected. Furthermore, electron microscopic analysis indicates that vesicle fusion is impaired. KNOLLE protein was detected in mitotically dividing cells of various parts of the developing plant, including seedling root, inflorescence meristem, floral meristems and ovules, and the cellularizing endosperm, but not during cytokinesis after the male second meiotic division. Thus, KNOLLE is the first syntaxin-like protein that appears to be involved specifically in cytokinetic vesicle fusion.

Animals↗

PubMed2006

Plant reproductive allocation predicts herbivore dynamics across spatial and temporal scales.

Life-history theory suggests that iteroparous plants should be flexible in their allocation of resources toward growth and reproduction. Such plasticity could have consequences for herbivores that prefer or specialize on vegetative versus reproductive structures. To test this prediction, we studied the response of the cactus bug (Narnia pallidicornis) to meristem allocation by tree cholla cactus (Opuntia imbricata). We evaluated the explanatory power of demographic models that incorporated variation in cactus relative reproductive effort (RRE; the proportion of meristems allocated toward reproduction). Field data provided strong support for a single model that defined herbivore fecundity as a time-varying, increasing function of host RRE. High-RRE plants were predicted to support larger insect populations, and this effect was strongest late in the season. Independent field data provided strong support for these qualitative predictions and suggested that plant allocation effects extend across temporal and spatial scales. Specifically, late-season insect abundance was positively associated with interannual changes in cactus RRE over 3 years. Spatial variation in insect abundance was correlated with variation in RRE among five cactus populations across New Mexico. We conclude that plant allocation can be a critical component of resource quality for insect herbivores and, thus, an important mechanism underlying variation in herbivore abundance across time and space.

Animals↗

PubMed2003

Cell division and morphological changes in the shoot apex of Arabidopsis thaliana during floral transition.

Eight-week-old vegetative plants of Arabidopsis thaliana, ecotype Columbia, were induced to flower by a single long day (LD). In this experimental system, it is known that the last component of the floral stimulus moves from the leaves to the apex 24-36 h after the start of the LD, and the first floral meristem is initiated by the shoot apical meristem (SAM) at 44-56 h (Corbesier et al., 1996, The Plant Journal 9: 947-952). Here we show that the rate of cell division is increased at floral transition in all SAM parts but not in the sub-apical pith cells. Mitotic activity starts to increase 24 h after the start of the LD and is two- to three-fold higher at peak times than that in non-induced plants. This activation is followed by the start of SAM enlargement at 44 h, SAM doming at 48 h, and the elongation of apical internodes (bolting) at 52 h.

Arabidopsis↗

PubMed2002

Cell numbers and leaf development in Arabidopsis: a functional analysis of the STRUWWELPETER gene.

The struwwelpeter (swp) mutant in Arabidopsis shows reduced cell numbers in all aerial organs. In certain cases, this defect is partially compensated by an increase in final cell size. Although the mutation does not affect cell cycle duration in the young primordia, it does influence the window of cell proliferation, as cell number is reduced during the very early stages of primordium initiation and a precocious arrest of cell proliferation occurs. In addition, the mutation also perturbs the shoot apical meristem (SAM), which becomes gradually disorganized. SWP encodes a protein with similarities to subunits of the Mediator complex, required for RNA polymerase II recruitment at target promoters in response to specific activators. To gain further insight into its function, we overexpressed the gene under the control of a constitutive promoter. This interfered again with the moment of cell cycle arrest in the young leaf. Our results suggest that the levels of SWP, besides their role in pattern formation at the meristem, play an important role in defining the duration of cell proliferation.

Amino Acid Sequence↗

PubMed2005

Flower primordium formation at the Arabidopsis shoot apex: quantitative analysis of surface geometry and growth.

Geometry changes, especially surface expansion, accompanying flower primordium formation are investigated at the reproductive shoot apex of Arabidopsis with the aid of a non-invasive replica method and a 3-D reconstruction algorithm. The observed changes are characteristic enough to differentiate the early development of flower primordium in Arabidopsis into distinct stages. Primordium formation starts from the fast and anisotropic growth at the periphery of the shoot apical meristem, with the maximum extension in the meridional direction. Surprisingly, the primordium first becomes a shallow crease, and it is only later that this shape changes into a bulge. The bulge is formed from the shallow crease due to slower and less anisotropic growth than at the onset of primordium formation. It is proposed that the shallow crease is the first axil, i.e. the axil of a putative rudimentary bract subtending the flower primordium proper, while the flower primordium proper is the bulge formed at the bottom of this axil. At the adaxial side of the bulge, the second axil (a narrow and deep crease) is formed setting the boundary between the flower primordium proper and the shoot apical meristem. Surface growth, leading to the formation of the second axil, is slow and anisotropic. This is similar to the previously described growth pattern at the boundary of the leaf primordium in Anagallis.

Algorithms↗