Biomedical subjects

T Beeckman

Publications and source records attributed to T Beeckman.

11 recordsLinked to original sources

PubMed2001

CKS1At overexpression in Arabidopsis thaliana inhibits growth by reducing meristem size and inhibiting cell-cycle progression.

The SUC1/CKS1 proteins associate with cyclin-dependent kinases (CDKs) and play an essential role in the regulation of the cell cycle. Recently, an Arabidopsis thaliana SUC1/CKS1 homologous gene, designated CKS1At, has been cloned. Here, overexpression of CKS1At in Arabidopsis is shown to reduce leaf size and root growth rates. Reduced root growth resulted primarily from an increase of the cell-cycle duration and a shortening of the meristem. Endoreduplication was unaffected. The increased cell-cycle duration was associated with an equal extension of both the G1 and G2 phases. This inhibition was due to the binding of CDK subunits with CDKs. The reduced growth rates in response to altered cell-cycle gene expression demonstrates a direct dependence of plant growth rates on cell-cycle regulation.

Adaptor Proteins, Signal Transducing↗

PubMed2001

The peri-cell-cycle in Arabidopsis.

The root systems of plants proliferate via de novo formed meristems originating from differentiated pericycle cells. The identity of putative signals responsible for triggering some of the pericycle cells to re-enter the cell cycle remains unknown. Here, the cell cycle regulation in the pericycle of seedling roots of Arabidopsis thaliana (L.) HEYNH: is studied shortly after germination using various strategies. Based on the detailed analysis of the promoter-beta-glucuronidase activity of four key cell cycle regulatory genes, combined with cell length measurements, microdensitometry of DNA content, and experiments with a cell cycle-blocking agent, a model is proposed for cell cycle regulation in the pericycle at the onset of lateral root initiation. The results clearly show that before the first lateral root is initiated, the pericycle consists of dissimilar cell files in respect of their cell division history. Depending on the distance behind the root tip and on position in relation to the vascular tissue, particular pericycle cells remain in the G(2) phase of the cell cycle and are apparently more susceptible to lateral root initiation than others.

Arabidopsis↗

PubMed2001

Functional analysis of cyclin-dependent kinase inhibitors of Arabidopsis.

Cyclin-dependent kinase inhibitors, such as the mammalian p27(Kip1) protein, regulate correct cell cycle progression and the integration of developmental signals with the core cell cycle machinery. These inhibitors have been described in plants, but their function remains unresolved. We have isolated seven genes from Arabidopsis that encode proteins with distant sequence homology with p27(Kip1), designated Kip-related proteins (KRPs). The KRPs were characterized by their domain organization and transcript profiles. With the exception of KRP5, all presented the same cyclin-dependent kinase binding specificity. When overproduced, KRP2 dramatically inhibited cell cycle progression in leaf primordia cells without affecting the temporal pattern of cell division and differentiation. Mature transgenic leaves were serrated and consisted of enlarged cells. Although the ploidy levels in young leaves were unaffected, endoreduplication was suppressed in older leaves. We conclude that KRP2 exerts a plant growth inhibitory activity by reducing cell proliferation in leaves, but, in contrast to its mammalian counterparts, it may not control the timing of cell cycle exit and differentiation.

Amino Acid Sequence↗

PubMed2001

Auxin transport promotes Arabidopsis lateral root initiation.

Lateral root development in Arabidopsis provides a model for the study of hormonal signals that regulate postembryonic organogenesis in higher plants. Lateral roots originate from pairs of pericycle cells, in several cell files positioned opposite the xylem pole, that initiate a series of asymmetric, transverse divisions. The auxin transport inhibitor N-1-naphthylphthalamic acid (NPA) arrests lateral root development by blocking the first transverse division(s). We investigated the basis of NPA action by using a cell-specific reporter to demonstrate that xylem pole pericycle cells retain their identity in the presence of the auxin transport inhibitor. However, NPA causes indoleacetic acid (IAA) to accumulate in the root apex while reducing levels in basal tissues critical for lateral root initiation. This pattern of IAA redistribution is consistent with NPA blocking basipetal IAA movement from the root tip. Characterization of lateral root development in the shoot meristemless1 mutant demonstrates that root basipetal and leaf acropetal auxin transport activities are required during the initiation and emergence phases, respectively, of lateral root development.

Arabidopsis↗

PubMed2000

Developmental expression of the Arabidopsis thaliana CycA2;1 gene.

The associations of cyclins with highly conserved cyclin-dependent kinases are key events in the regulation of cell cycle progression. The spatio-temporal expression of an Arabidopsis thaliana (L.) Heynh. mitotic cyclin, Arath;CycA2;1, was studied by histochemical beta-glucuronidase (GUS) analysis and in-situ hybridizations. The CycA2,1] promoter was active in the egg apparatus before fertilization. During embryogenesis, CycA2;1:gus expression was found in the embryo and the developing endosperm. Throughout plant development, CycA2;1 transcripts were found in both dividing and non-dividing cells, indicating that the expression of this cyclin is not a limiting factor for cell division. In the pericycle and stelar parenchyma, CycA2;1 transcripts were located at the xylem poles, a position that can be correlated with competence for lateral root formation. In addition, CycA2;1:gus expression was upregulated in roots by auxins and in the shoot apex by cytokinins. Transcription of CycA2;1 was shown by reverse transcription-polymerase chain reaction to be strongly induced by sucrose in A. thaliana cell suspensions.

Arabidopsis↗

PubMed2000

Expression of cell cycle regulatory genes and morphological alterations in response to salt stress in Arabidopsis thaliana.

Hyperosmotic stress severely affects plant growth and development. To examine the effect of salt stress on cell cycle activity in Arabidopsis thaliana (L.) Heynh., the transcriptional regulation of a cyclin-dependent kinase, CDC2aAt, and two mitotic cyclins, Arath;CycB1;1 and Arath;CycA2;1, was studied by using the beta-glucuronidase (gus) reporter gene. Moreover, the mRNA abundance of these cell cycle genes as well as CDC2bAt were monitored during salt stress. Upon NaCl treatment, the promoter activities and transcript levels of all cell cycle genes diminished initially in the shoot apex and were subsequently induced during salt-stress adaptation. Additionally, the promoter activities of CDC2aAt and CycA2;1 decreased in the vascular cylinder of the root in correlation with reduced lateral root formation. In the root tips, a regression of CDC2aAt, CycA2;1, and CycB1;1:gus expression was observed, concomitant with a shrinkage of the root meristem and inhibition of root growth. Our data indicate that salt stress interferes with cell cycle regulation at the transcriptional level, resulting in an adaptive growth response.

Arabidopsis↗

PubMed2000

Increased leakiness of the tetracycline-inducible Triple-Op promoter in dividing cells renders it unsuitable for high inducible levels of a dominant negative CDC2aAt gene.

A tetracycline-inducible promoter system was used to generate transgenic tobacco plants that confer inducible expression of the wild type or a dominant negative allele of the gene coding for the cyclin-dependent kinase (CDK) of Arabidopsis thaliana CDC2aAt. Although the total extractable CDK activity was doubled, the induced expression of the wild-type CDC2aAt did not correlate with any change of the cell cycle kinetics. An increase of CDK activity upon CDC2aAt expression was only seen in dividing cell populations, demonstrating that CDC2aAt expression itself is not sufficient to induce CDK activation. Induced expression of the dominant negative CDC2aAt.N146 correlated with a reduction of CDK activity to 66% of the level found in non-induced cells. This decrease was not sufficient to block cell division. The isolation of plants showing only low inducible levels of CDC2aAt.N146 suggests that a counterselection against strong inducible lines had occurred. Accordingly, Triple-Op promoter activity was found in dividing cells in the absence of tetracycline.

Arabidopsis↗

PubMed2000

ABI3 affects plastid differentiation in dark-grown Arabidopsis seedlings.

The Arabidopsis ABSCISIC ACID-INSENSITIVE3 (ABI3) protein has been identified previously as a crucial regulator of late seed development. Here, we show that dark-grown abi3 plants, or abi3 plants returned to the dark after germination in the light, developed and maintained an etioplast with a prominent prolamellar body at developmental stages in which the wild type did not. Overexpression of ABI3 led to the preservation of the plastid ultrastructure that was present at the onset of darkness. These observations suggest that ABI3 plays a role in plastid differentiation pathways in vegetative tissues. Furthermore, the analysis of deetiolated (det1) abi3 double mutants revealed that DET1 and ABI3 impinge on a multitude of common processes. During seed maturation, ABI3 required DET1 to achieve its full expression. Mature det1 abi3 seeds were found to be in a highly germinative state, indicating that germination is controlled by both DET1 and ABI3. During plastid differentiation in leaves of dark-grown plants, DET1 is required for the action of ABI3 as it is during seed development. Together, the results suggest that ABI3 is at least partly regulated by light.

Arabidopsis↗

PubMed2000

Embedding thin plant specimens for oriented sectioning.

Small plant structures such as small primary roots, filamentous mosses and algae are difficult to orient for sectioning since they become wavy and curl during embedding. A method is described for embedding and orienting tiny plant specimens in a glycol methacrylate resin using self-constructed flat molds. Prior to sectioning, small samples can be oriented in both the longitudinal and the transverse plane. As several samples can be sectioned simultaneously, time-consuming trimming of the blocks is reduced substantially. The efficiency of this technique has been demonstrated using the tiny roots of the model plant Arabidopsis thaliana (L.) Heynh.

Acrylic Resins↗

PubMed1998

Accumulation pattern of IgG antibodies and Fab fragments in transgenic Arabidopsis thaliana plants.

For the further optimization of antibody expression in plants, it is essential to determine the final accumulation sites of plant-made antibodies. Previously, we have shown that, upon secretion, IgG antibodies and Fab fragments can be detected in the intercellular spaces of leaf mesophyll cells of transgenic Arabidopsis thaliana plants. However, immunofluorescence microscopy showed that this is probably not their final accumulation site. In leaves, IgG and Fab fragments accumulate also at the interior side of the epidermal cell layers and in xylem vessels. These accumulation sites correspond with the leaf regions where water of the transpiration stream is entering a space impermeable to the proteins or where water is evaporating. In roots, plant-made Fab fragments accumulate in intercellular spaces of cortex cells, in the cytoplasm of pericycle and, to a lesser extent, endodermis cells, and in cells of the vascular cylinder. In other words, antibody accumulation occurs at the sites where water passes on its radial pathway towards and within the vascular bundle. Taken together, our results suggest that, upon secretion of plant-made antibodies or Fab fragments, a large proportion of these proteins are transported in the apoplast of A. thaliana, possibly by the water flow in the transpiration stream.

Animals↗

PubMed1995

Superroot, a recessive mutation in Arabidopsis, confers auxin overproduction.

We have isolated seven allelic recessive Arabidopsis mutants, designated superroot (sur1-1 to sur1-7), displaying several abnormalities reminiscent of auxin effects. These characteristics include small and epinastic cotyledons, an elongated hypocotyl in which the connection between the stele and cortical and epidermal cells disintegrates, the development of excess adventitious and lateral roots, a reduced number of leaves, and the absence of an inflorescence. When germinated in the dark, sur1 mutants did not develop the apical hook characteristic of etiolated seedlings. We were able to phenocopy the Sur1- phenotype by supplying auxin to wild-type seedlings, to propagate sur1 explants on phytohormone-deficient medium, and to regenerate shoots from these explants by the addition of cytokinins alone to the culture medium. Analysis by gas chromatography coupled to mass spectrometry indicated increased levels of both free and conjugated indole-3-acetic acid. sur1 was crossed to the mutant axr2 and the altered-auxin response mutant ctr1. The phenotype of both double mutants was additive. The sur1 gene was mapped on chromosome 2 at 0.5 centimorgans from the gene encoding phytochrome B.

Arabidopsis↗