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PubMed1998

Expression patterns of GASA genes in Arabidopsis thaliana: the GASA4 gene is up-regulated by gibberellins in meristematic regions.

The GASA gene family previously identified in Arabidopsis belongs to a wide-spread class of genes found in mono- and dicotyledonous plants, all structurally related to the original GA-regulated GAST1 gene from tomato. They encode small peptides (97 to 112 residues) of unknown function sharing a 60 amino acid conserved C-terminal domain comprising twelve conserved cysteine residues which define a pattern not related to other known cysteine-rich motifs. Northern blot hybridization analysis revealed sequential expression of three genes during flowering, silique development and seed germination. GASA4 transcripts were detected in flower buds. GASA1 transcripts markedly accumulated in siliques, about five days after pollination, and correlated with the peak of GA biosynthesis at this stage of silique development. GASA3 transcripts accumulated at the end of the maturation stage of the silique, and transcripts were still present in dry seeds but degraded rapidly during imbibition. In addition, the GASA4 gene was again actively transcribed after germination and this expression was shown to be dependent on the presence of GAs in GA-deficient mutants. Immunoblot analysis confirmed the presence of the GASA4 gene product in flower buds, seedlings and roots. We focused on the GASA4 gene and characterized its expression. The upstream region (-890 to +128) was fused to the GUS reporter gene. GASA4/GUS expression was detected in transgenic Arabidopsis primarily in all meristematic regions, including vegetative, inflorescence and floral meristems, as well as primary and lateral root tips. In a GA-deficient background (ga1-3), GUS activity in the vegetative meristem was detected only in the presence of supplied GA. In root and flower meristems, basal GUS activity was slightly enhanced by exogenous GA. Interestingly, GA strongly inhibit GUS activity in expanding cotyledons and leaves in ga1-3 mutants supplied with exogenous GAs, as well as in the wild type. The GA-dependent meristem-specific expression pattern suggests that the GASA4 protein plays a role in dividing cells rather than in elongating cells.

Amino Acid Sequence↗

PubMed2000

Developmental control of cell division patterns in the shoot apex.

The shoot apical meristem is a group of rapidly dividing cells that generate all aerial parts of the plant. It is a highly organised structure, which can be divided into functionally distinct domains, characterised by specific proliferation rates of the individual cells. Genetic studies have enabled the identification of regulators of meristem function. These factors are involved in the formation and maintenance of the meristem, as well as in the formation of the primordia. Somehow, they must also govern cell proliferation rates within the shoot apex. Possible links between meristem regulators and the cell cycle machinery will be discussed. In order to analyse the role of cell proliferation in development, cell cycle gene expression has been perturbed using transgenic approaches and mutation. The effect of these alterations on growth and development at the shoot apex will be presented. Together, these studies give a first insight into the regulatory networks controlling the cell cycle and into the significance of cell proliferation in plant development.

Cell Division↗

PubMed2003

Regulation of phyllotaxis by polar auxin transport.

The regular arrangement of leaves around a plant's stem, called phyllotaxis, has for centuries attracted the attention of philosophers, mathematicians and natural scientists; however, to date, studies of phyllotaxis have been largely theoretical. Leaves and flowers are formed from the shoot apical meristem, triggered by the plant hormone auxin. Auxin is transported through plant tissues by specific cellular influx and efflux carrier proteins. Here we show that proteins involved in auxin transport regulate phyllotaxis. Our data indicate that auxin is transported upwards into the meristem through the epidermis and the outermost meristem cell layer. Existing leaf primordia act as sinks, redistributing auxin and creating its heterogeneous distribution in the meristem. Auxin accumulation occurs only at certain minimal distances from existing primordia, defining the position of future primordia. This model for phyllotaxis accounts for its reiterative nature, as well as its regularity and stability.

Arabidopsis↗

PubMed2004

The role of barren stalk1 in the architecture of maize.

The architecture of higher plants is established through the activity of lateral meristems--small groups of stem cells formed during vegetative and reproductive development. Lateral meristems generate branches and inflorescence structures, which define the overall form of a plant, and are largely responsible for the evolution of different plant architectures. Here, we report the isolation of the barren stalk1 gene, which encodes a non-canonical basic helix-loop-helix protein required for the initiation of all aerial lateral meristems in maize. barren stalk1 represents one of the earliest genes involved in the patterning of maize inflorescences, and, together with the teosinte branched1 gene, it regulates vegetative lateral meristem development. The architecture of maize has been a major target of selection for early agriculturalists and modern farmers, because it influences harvesting, breeding strategies and mechanization. By sampling nucleotide diversity in the barren stalk1 region, we show that two haplotypes entered the maize gene pool from its wild progenitor, teosinte, and that only one was incorporated throughout modern inbreds, suggesting that barren stalk1 was selected for agronomic purposes.

Amino Acid Sequence↗

PubMed1998

Transcription pattern of a FIM homologue in Impatiens during floral development and reversion.

Flowering and reversion in Impatiens are characterised by gradual transitions of organ identity and constitute a unique system for the molecular and physiological study of floral organogenesis. The authors have isolated an Impatiens homologue of the FIM gene of Antirrhinum (UFO in Arabidopsis), Imp-FIM, and analysed its expression in three states of the terminal meristem: vegetative, floral, and reverted. In floral meristems, Imp-FIM transcription is associated with petal identity, as in Antirrhinum and Arabidopsis, but this is achieved through a novel transcription pattern, characterised by a high level of transcript within petal primordia. This novel transcription pattern could contribute to the more diffuse boundaries between organ types in Impatiens. In vegetative meristems, Imp-FIM is expressed in the axils of leaf primordia which are arranged in a spiral. A similar pattern is observed in reverted meristems in which leaf primordia are initiated in a whorled arrangement. This result indicates that the maintenance of floral phyllotaxis is not associated with a specific pattern of Imp-FIM transcription. Transcription of Imp-FIM in a non-reverting line is no different from that in the reverting line. Therefore, the lack of floral commitment in the reverting line does not seem to be responsible for Imp-FIM transcription within petals. The novel transcription pattern in petals, together with features of Impatiens that are reminiscent of fim and ufo mutant phenotypes suggest an evolutionary divergence for Imp-FIM regulation in this species.

Amino Acid Sequence↗

PubMed2005

Cell-to-cell movement of GFP during embryogenesis and early seedling development in Arabidopsis.

There is increasing evidence for intercellular trafficking of macromolecules through plasmodesmata (PD) during plant development. Here we study the ability of PD to traffic proteins during embryogenesis and early seedling development in Arabidopsis. Transgenic lines that induce GFP expression only in meristems, MSG (meristem-specific GFP), were used to monitor GFP movement. Cell-to-cell movement of different-sized GFP reporters reveals that embryos and young seedlings traffic proteins at least 54 kDa in size. Although 27-kDa soluble GFP (1 x sGFP) freely moves between cells throughout the entire embryo during all stages analyzed, 2 x sGFP movement becomes more restricted as development proceeds. After germination, cells near the apical meristem in seedlings show a higher size exclusion limit (SEL), whereas the SEL becomes more restricted as surrounding tissues develop identities. Although 1 x sGFP moves throughout leaf primordia, as the leaf develops only the basal part of leaf petioles, main vascular tissues, and leaf veins (not blades) allow 1 x sGFP movement. Although previous studies showed that embryos allow movement of small symplastic tracers (0.5 kDa), the present data demonstrate that the embryo constitutes a single symplast that allows transport of macromolecules as well. Even 2 x sGFP moves from its site of expression at the apical meristem in embryos and seedlings, yet the extent of movement is more limited than 1 x sGFP. Thus, PD have distinct SELs in different subregions of the embryo and seedling. These studies support the general concept that PD in younger tissues are more dilated and less restrictive than PD in older (nonvascular) tissues.

Arabidopsis↗

PubMed2003

Maintenance of stem cell populations in plants.

Flowering plants have the unique ability to produce new organs continuously, for hundreds of years in some species, from stem cell populations maintained at their actively growing tips. The shoot tip is called the shoot apical meristem, and it acts as a self-renewing source of undifferentiated, pluripotent stem cells whose descendents become incorporated into organ and tissue primordia and acquire different fates. Stem cell maintenance is an active process, requiring constant communication between different regions of the shoot apical meristem to coordinate loss of stem cells from the meristem through differentiation with their replacement through cell division. Stem cell research in model plant systems is facilitated by the fact that mutants with altered meristem cell identity or accumulation are viable, allowing dissection of stem cell behavior by using genetic, molecular, and biochemical methods. Such studies have determined that in the model plant Arabidopsis thaliana stem cell maintenance information flows via a signal transduction pathway that is established during embryogenesis and maintained throughout the life cycle. Signaling through this pathway results in the generation of a spatial feedback loop, involving both positive and negative interactions, that maintains stem cell homeostasis. Stem cell activity during reproductive development is terminated by a temporal feedback loop involving both stem cell maintenance genes and a phase-specific flower patterning gene. Our current investigations provide additional insights into the molecular mechanisms that regulate stem cell activity in higher plants.

Arabidopsis↗

PubMed1999

The Lateral suppressor (Ls) gene of tomato encodes a new member of the VHIID protein family.

The ability of the shoot apical meristem to multiply and distribute its meristematic potential through the formation of axillary meristems is essential for the diversity of forms and growth habits of higher plants. In the lateral suppressor mutant of tomato the initiation of axillary meristems is prevented, thus offering the unique opportunity to study the molecular mechanisms underlying this important function of the shoot apical meristem. We report here the isolation of the Lateral suppressor gene by positional cloning and show that the mutant phenotype is caused by a complete loss of function of a new member of the VHIID family of plant regulatory proteins.

Amino Acid Sequence↗

PubMed2003

Winter bud content according to position in 3-year-old branching systems of 'Granny Smith' apple.

An investigation was made of the number of preformed organs in winter buds of 3-year-old reiterated complexes of the 'Granny Smith' cultivar. Winter bud content was studied with respect to bud position: terminal buds were compared on both long shoots and spurs according to branching order and shoot age, while axillary buds were compared between three zones (distal, median and proximal) along 1-year-old annual shoots in order 1. The percentage of winter buds that differentiated into inflorescences was determined and the flowers in each bud were counted for each bud category. The other organ categories considered were scales and leaf primordia. The results confirmed that a certain number of organs must be initiated before floral differentiation occurred. The minimum limit was estimated at about 15 organs on average, including scales. Total number of lateral organs formed was shown to vary with both bud position and meristem age, increasing from newly formed meristems to 1- and 2-year-old meristems on different shoot types. These differences in bud organogenesis depending on bud position, were consistent with the morphogenetic gradients observed in apple tree architecture. Axillary buds did not contain more than 15 organs on average and this low organogenetic activity of the meristems was related to a low number of flowers per bud. In contrast, the other bud categories contained more than 15 differentiated organs on average and a trade-off was observed between leaf and flower primordia. The ratio between the number of leaf and flower primordia per bud varied with shoot type. When the terminal buds on long shoots and spurs were compared, those on long shoots showed more flowers and a higher ratio of leaf to flower primordia.

Cell Differentiation↗

PubMed2002

In situ hybridization of the MADS-box gene POTM1 during potato floral development.

The potato MADS-box gene POTM1 is a member of the SQUA-like family of plant MADS-box genes. The amino acid sequence of POTM1 most closely matches the sequence of PFG (Petunia Flowering Gene), a petunia MADS-box gene involved in the transition from vegetative to reproductive development. To examine the likely role of POTM1 in potato reproductive tissue, in situ hybridizations were performed on sections of the potato shoot apex at various stages during the transition to flowering. Scanning electron micrographs provide a reference for each stage to illustrate the progression from vegetative meristem to inflorescence meristem and floral organs. POTM1 mRNA accumulates in vegetative, inflorescence and floral meristems. Transcripts are present in petal primordia during the later stages of petal development and they accumulate later during floral development inside the stamens and carpels, but not in carpel walls. This widespread pattern of mRNA accumulation is unique among the SQUA-like genes and indicates that POTM1 is active during a variety of stages of development, and may function in regulating cell determination in transitional phases of both vegetative and floral meristems.

In Situ Hybridization↗

PubMed1999

The PERIANTHIA gene encodes a bZIP protein involved in the determination of floral organ number in Arabidopsis thaliana.

Mutations in the PERIANTHIA (PAN) gene of Arabidopsis thaliana specifically transform flowers from tetramerous to largely pentamerous, which is a characteristic of flowers of ancestral plants. We have cloned the PAN gene and here we show that it encodes a member of the basic region/leucine zipper class of transcription factors. Immunohistochemical analysis shows that the encoded protein is present in the apical meristem, the floral meristem, each whorl of organ primordia, and in ovule primordia during wild-type flower development. PAN expression occurs independently of genes affecting floral meristem identity, floral meristem size, or floral organ number. The near absence of a phenotype in transgenic plants overexpressing PAN and the contrast between the broad expression of PAN and the specificity of its mutant phenotype suggest that its activity may be regulated post-translationally or by the presence of partner proteins. Based on these results and on data reported previously, we propose models for the role of PAN in the evolution of flower pattern in the mustard family.

Amino Acid Sequence↗

PubMed2002

The KNAT2 homeodomain protein interacts with ethylene and cytokinin signaling.

Using a transgenic line that overexpresses a fusion of the KNAT2 (KNOTTED-like Arabidopsis) homeodomain protein and the hormone-binding domain of the glucocorticoid receptor (GR), we have investigated the possible relations between KNAT2 and various hormones. Upon activation of the KNAT2-GR fusion, we observed a delayed senescence of the leaves and a higher rate of shoot initiation, two processes that are also induced by cytokinins and inhibited by ethylene. Furthermore, the activation of the KNAT2-GR fusion induced lobing of the leaves. This feature was partially suppressed by treatment with the ethylene precursor 1-aminocyclopropane-1-carboxylic acid, or by the constitutive ethylene response ctr1 mutation. Conversely, some phenotypic traits of the ctr1 mutant were suppressed by the activation of the KNAT2-GR fusion. These data suggest that KNAT2 acts synergistically with cytokinins and antagonistically with ethylene. In the shoot apical meristem, the KNAT2 gene is expressed in the L3 layer and the rib zone. 1-Aminocyclopropane-1-carboxylic acid treatment restricted the KNAT2 expression domain in the shoot apical meristem and reduced the number of cells in the L3. The latter effect was suppressed by the activation of the KNAT2-GR construct. Conversely, the KNAT2 gene expression domain was enlarged in the ethylene-resistant etr1-1 mutant or in response to cytokinin treatment. These data suggest that ethylene and cytokinins act antagonistically in the meristem via KNAT2 to regulate the meristem activity.

Arabidopsis↗

PubMed2004

Cell cycle modulation in the response of the primary root of Arabidopsis to salt stress.

Salt stress inhibits plant growth and development. We investigated the importance of cell cycle regulation in mediating the primary root growth response of Arabidopsis to salt stress. When seedlings were transferred to media with increasing concentrations of NaCl, root growth rate was progressively reduced. At day 3 after transfer of seedlings to growth medium containing 0.5% NaCl the primary roots grew at a constant rate well below that prior to the transfer, whereas those transferred to control medium kept accelerating. Kinematic analysis revealed that the growth reduction of the stressed roots was due to a decrease in cell production and a smaller mature cell length. Surprisingly, average cell cycle duration was not affected. Hence, the reduced cell production was due to a smaller number of dividing cells, i.e. a meristem size reduction. To analyze the mechanism of meristem size adaptation prior to day 3, we investigated the short-term cell cycle events following transfer to saline medium. Directly after transfer cyclin-dependent kinase (CDK) activity and CYCB1;2 promoter activity were transiently reduced. Because protein levels of both CDKA;1 and CDKB1;1 were not affected, the temporary inhibition of mitotic activity that allows adaptation to the stress condition is most likely mediated by posttranslational control of CDK activity. Thus, the adaptation to salt stress involves two phases: first, a rapid transient inhibition of the cell cycle that results in fewer cells remaining in the meristem. When the meristem reaches the appropriate size for the given conditions, cell cycle duration returns to its default.

Adaptation, Physiological↗

PubMed2000

Pericycle cell proliferation and lateral root initiation in Arabidopsis.

In contrast with other cells generated by the root apical meristem in Arabidopsis, pericycle cells adjacent to the protoxylem poles of the vascular cylinder continue to cycle without interruption during passage through the elongation and differentiation zones. However, only some of the dividing pericycle cells are committed to the asymmetric, formative divisions that give rise to lateral root primordia (LRPs). This was demonstrated by direct observation and mapping of mitotic figures, cell-length measurements, and the histochemical analysis of a cyclin-GUS fusion protein in pericycle cells. The estimated duration of a pericycle cell cycle in the root apical meristem was similar to the interval between cell displacement from the meristem and the initiation of LRP formation. Developmentally controlled LRP initiation occurs early, 3 to 8 mm from the root tip. Thus the first growth control point in lateral root formation is defined by the initiation of primordia in stochastic patterns by cells passing through the elongation and young differentiation zones, up to where lateral roots begin to emerge from the primary root. Therefore, the first growth control point is not restricted to a narrow developmental window. We propose that late LRP initiation is developmentally unrelated to the root apical meristem and is operated by a second growth control point that can be activated by environmental cues. The observation that pericycle cells divide and lateral root primordia form without intervening mitotic quiescence suggests that lateral organ formation in roots and shoots might not be as fundamentally different as previously thought.

Arabidopsis↗

PubMed2005

CORONA, a member of the class III homeodomain leucine zipper gene family in Arabidopsis, regulates stem cell specification and organogenesis.

Organogenesis at the shoot meristem requires a delicate balance between stem cell specification and differentiation. In Arabidopsis thaliana, WUSCHEL (WUS) is a key factor promoting stem cell identity, whereas the CLAVATA (CLV1, CLV2, and CLV3) loci appear to promote differentiation by repressing WUS expression. In a screen for mutations modifying clv1 mutants, we have identified a novel regulator of meristem development we term CORONA (CNA). Whereas cna single mutant plants exhibit subtle defects in meristem development, clv cna double mutants develop massively enlarged apices that display early loss of organogenesis, misexpression of WUS and CLV3, and eventual differentiation of the entire apex. The CNA gene was isolated by positional cloning and found to encode a class III homeodomain Leu zipper protein. A missense mutation resulting in the dominant-negative cna-1 allele was identified in a conserved domain of unknown function, and a likely null allele was shown to display a similar but weaker phenotype. CNA is expressed in developing vascular tissue, diffusely through shoot and flower meristems, and within developing stamens and carpels. Our analysis of WUS expression in wild-type, clv, and clv cna plants revealed that, contrary to current models, WUS is neither necessary nor sufficient for stem cell specification and that neither WUS nor CLV3 is a marker for stem cell identity. We propose that CNA functions in parallel to the CLV loci to promote organ formation.

Alleles↗

PubMed1996

Temperature-sensitive mutations that arrest Arabidopsis shoot development.

To identify genes involved in meristem function we have designed a screen for temperature-sensitive mutations that cause a conditional arrest of early shoot development in Arabidopsis. We describe the characterization of three mutations, arrested development (add) 1, 2 and 3. At the restrictive temperature the add1 and add2 mutations disrupt apical meristem function as assayed by leaf initiation. Furthermore, add1 and add2 plants exhibit defects in leaf morphogenesis following upshift from permissive to restrictive temperature. This result suggests that proximity to a functional meristem is required for the completion of normal leaf morphogenesis. The add3 mutation does not have a dramatic effect on the production of leaves by the apical meristem; however, add3 prevents the expansion of leaf blades at high temperature. Thus, in this mutant the temperature-dependent arrest of epicotyl development is due to a failure of normal leaf development rather than new leaf initiation. While all add mutants have a reduced rate of root growth in comparison to wild-type plants, the mutants do not display a temperature-dependent arrest of root development. All add mutants display some developmental defects at low temperature, suggesting that these mutations affect genes involved in inherently temperature-sensitive developmental processes.

Arabidopsis↗

PubMed1996

Pathways for inflorescence and floral induction in Antirrhinum.

The presentation of flowers on a modified stem, the inflorescence, requires the integration of several aspects of meristem behaviour. In Antirrhinum, the inflorescence can be distinguished by its flowers, hairy stem, modified leaves, short internodes and spiral phyllotaxy. We show, by a combination of physiological, genetical and morphological analysis, that the various aspects of the inflorescence are controlled by three pathways. The first pathway, depends on expression of the floricaula gene, and is rapidly and discretely induced by exposure to long daylength. Activation of this pathway occurs in very young axillary meristems, resulting in a floral identity. In addition, the length of subtending leaves and hairiness of the stem are partially modified. The second pathway affects leaf size, internode length, and stem hairiness, but does not confer floral meristem identity. This pathway is induced by long daylength, but not as rapidly or discretely as the floricaula-dependent pathway. The third pathway controls the switch in phyllotaxy from decussate to spiral and is activated independently of daylength. The coordination of these three programmes ensures that apical and axillary meristem behaviour is integrated.

Base Sequence↗

PubMed1996

An altered body plan is conferred on Arabidopsis plants carrying dominant alleles of two genes.

In this paper, we describe a late-flowering ecotype of Arabidopsis, Sy-0, in which the axillary meristems maintain a prolonged vegetative phase, even though the primary shoot apical meristem has already converted to reproductive development. This novel heterochronic shift in the development of axillary meristems results in the formation of aerial rosettes of leaves at the nodes of the primary shoot axis. We present evidence that the aerial-rosette phenotype arises due to the interaction between dominant alleles of two genes: ART, aerial rosette gene (on chromosome 5) and EAR, enhancer of aerial rosette (on chromosome 4): EAR has been tentatively identified as a new allele of the FRI locus. The possible role of these two genes in the conversion of shoot apical meristems to reproductive development is discussed.

Alleles↗