SEARCH · PubMed Health
Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Recent studies have identified a complex intercellular communication network which maintains the balance of indeterminate and determinate cells at the plant apical meristem. The widespread presence of homologous regulatory genes indicates that 'stemness' arose before the evolutionary split between plants and animals.
We have investigated the appearance and reorganization of tubulin-containing arrays induced by colchicine in the root meristem of wheat Triticum aestivum, using immunostaining and electron microscopy. Colchicine caused depolymerization of microtubules and formation of tubulin cortical strands composed of filamentous material only in C-mitotic cells. After prolonged exposure to the drug, both interphase and C-mitotic cells acquired needle-type bundles, arranged as different crystalloids and/or macrotubules. The unmodified tyrosinated form of alpha-tubulin was detected within microtubules in control cells, but was not found within cortical strands. It was identified, however, within needle-type bundles. The modified acetylated form of alpha-tubulin, which was absent in control cells, was detected within needle-type bundles. Thus, cortical strands were transitory arrays, transformed into needle-type bundles during prolonged exposure to colchicine. Cortical strands appeared in a cell cycle-dependent manner, whereas needle-type bundles were cell cycle stable arrays. The diverse morphological organization, intracellular distribution and stability of tubulin-containing arrays may be associated with heterogeneity of alpha-tubulin isoforms. We assume that non-microtubular arrays substitute for microtubules in conditions where normal tubulin polymerization is inhibited.
Two genes cloned from Eucalyptus globulus, Eucalyptus LeaFy (ELF1 and ELF2), have sequence homology to the floral meristem identity genes LEAFY from Arabidopsis and FLORICAULA from Antirrhinum. ELF1 is expressed in the developing eucalypt floral organs in a pattern similar to LEAFY while ELF2 appears to be a pseudo gene. ELF1 is expressed strongly in the early floral primordium and then successively in the primordia of sepals, petals, stamens and carpels. It is also expressed in the leaf primordia and young leaves and adult and juvenile trees. The ELF1 promoter coupled to a GUS reporter gene directs expression in transgenic Arabidopsis in a temporal and tissue-specific pattern similar to an equivalent Arabidopsis LEAFY promoter construct. Strong expression is seen in young flower buds and then later in sepals and petals. No expression was seen in rosette leaves or roots of flowering plants or in any non-flowering plants grown under long days. Furthermore, ectopic expression of the ELF1 gene in transgenic Arabidopsis causes the premature conversion of shoots into flowers, as does an equivalent 35S-LFY construct. These data suggest that ELF1 plays a similar role to LFY in flower development and that the basic mechanisms involved in flower initiation and development in Eucalyptus are similar to those in Arabidopsis.
Membrane intrinsic proteins facilitate movement of small molecules often times functioning as water channels. We have identified two genes from rice which encode proteins with characteristic features of plasma membrane intrinsic proteins (PIP). They possess six membrane-spanning domains, an NPA repeat, overall high sequence homologies and characteristic C- and N-terminal hallmark motifs which allowed assignment of OsPIP1a to the PIP1 subfamily and of OsPIP2a to the PIP2 subfamily. OsPIP1a and OsPIP2a showed similar but not identical expression patterns. The two genes were expressed at higher levels in seedlings than in adult plants and expression in the primary root was regulated by light. In internodes of deepwater rice plants which were induced to grow rapidly by submergence, transcript levels were slightly induced in the intercalary meristem (IM) and slightly reduced in the elongation zone (EZ) after 18 h. In internodes of GA-induced excised stem sections transcript levels transiently declined in the IM and EZ after 1 h and subsequently recovered to elevated levels after 18 h. GA also induced OsPIP expression in non-growing tissue after 18 h. In the IM of submergence-induced stem sections transcript levels remained constitutive. The different growth-promoting treatments showed no direct correlation between growth rate and OsPIP gene expression in dividing or expanding cells. In fact, treatment of excised stem sections with ABA or drought stress induced similar changes in OsPIP expression in the growing zone during the first 6 h as GA did. We conclude that regulation of OsPIP1a and OsPIP2a expression is not primarily controlled by growth. GA-induced growth may however change the water status of cells which in turn results in altered PIP abundance.
The AP2/EREBP transcription factors play important roles in plant development and in the responses of plants to biotic and abiotic stresses. All members of the EREBP subfamily described to date are from dicotyledonous plants. In this paper, we describe the cloning and characterization of a rice gene, OsEBP-89, encoding a protein 326 amino acids long with a typical EREBP domain; this is the first report of an EREBP transcription factor in a monocotyledonous plant. Except for the EREBP domain, the OsEBP-89 protein does not have substantial sequence similarities to other members of the subfamily. The DNA-binding activity of the EREBP domain was confirmed by electrophoretic mobility-shift assays. An activation domain rich in acidic amino acids was identified by using a yeast one-hybrid system. Two putative nuclear-localization signals were also identified. The results of northern blot hybridization experiments showed that the transcript of the OsEBP-89 gene accumulates primarily in immature seeds, roots, and leaves (low levels). More detailed information about the pattern of OsEBP-89 gene expression was obtained by histochemical studies of transgenic rice plants carrying an OsEBP-89 5'/GUS reporter gene. The reporter gene was expressed in the endosperm starting at 7 days after pollination and in the intercalary meristem of plants. Expression of OsEBP-89 was induced in roots of rice seedlings by treatment with ACC, NaCl, or 2,4-D. Two cis-acting elements, an endosperm motif and a primary PERE, are present upstream of the OsEBP-89 coding region and may be involved in regulating its expression. Collectively, these results suggest that the OsEBP-89 gene is a new member of the EREBP subfamily and may be involved in ethylene-dependent seed maturation and shoot development of rice.
The binding protein BiP is an endoplasmic reticulum (ER)-resident member of the HSP70 stress-related protein family, which is essential for the constitutive function of the ER. In addition to responding to a variety of environmental stimuli, plant BiP exhibits a tissue-specific regulation. We have isolated two soybean BiP genomic clones, designated gsBiP6 and gsBiP9, and different extensions of their 5' flanking sequences were fused to beta-glucuronidase (GUS) reporter gene and introduced into Nicotiana tabacum by Agrobacterium tumefaciens-mediated transformation. Transgenic plants displayed prominent GUS activity in the vascular bundles of roots and shoots as well as in regions of intense cell division, such as procambial region and apical meristems. Promoter deletion analyses identified two cis-regulatory functional domains that are important for the spatially-regulated activation of BiP expression under normal plant development. While an AT-rich enhancer-like sequence, designated cis-acting regulatory domain 1, CRD1 (-358 to -211, on gsBiP6), activated expression of the BiP minimal promoter in all organs analyzed, BiP promoter activity in meristematic tissues and phloem cells required the presence of a second activating domain, CRD2 (-211 to -80). Apparently, the CRD2 sequence also harbors negative cis-acting elements, because removal of this region caused activation of gsBiP6 promoter in parenchymatic xylem rays. These results suggest that the tissue-specific control of BiP gene expression requires a complex integration of multiple cis-acting regulatory elements on the promoter.
The regulation of cell differentiation at meristems is crucial to developmental patterning in plants. Rapid progress has been made in identifying the genes that regulate differentiation and the receptor-mediated signalling events that have a key role in this process. In particular, we are now learning how the CLAVATA receptor kinase signalling pathway promotes stem cell differentiation in balance with the initiation of stem cells by the transcription factor WUSCHEL.
To examine the role of gibberellin hormones (GAs) in tolerance to apical meristem damage (AMD), we characterized the reaction norms of several GA-deficient and insensitive mutants of Arabidopsis thaliana in response to AMD and compared them to those of the wild type, Landsberg, from which they were derived. We included 'natural' genotypes of A. thaliana--accessions with shorter lab histories--in order to evaluate how representative Landsberg is of other genotypes. The GA mutations did not alter the level of tolerance to AMD, which was consistent with equal compensation for all genotypes. Generally, the reaction norms to AMD did not differ among the GA mutants themselves, or between the GA mutants and Landsberg. The GA mutations did affect the overall phenotypes of the plants, but these effects were not simply related to whether the mutation was early or late in the biochemical pathways. The GA-insensitive mutant was phenotypically different from the GA-deficient mutants and from Landsberg. The natural populations differed significantly from Landsberg, particularly in attributes related to size and inflorescence production, one more example of the need for researchers to be careful when generalizing the results of studies based upon laboratory strains. Our results indicate that early-flowering genotypes of A. thaliana can be remarkably tolerant to AMD, and that GA deficiency/insensitivity does not hinder tolerance to AMD, at least in this genetic background. Moreover, we confirm that mutations at regulatory loci can have noncatastrophic effects on fitness, as recently found by other investigators.
In plants, cell proliferation occurs mostly within meristems but a significant amount also occurs at other well-defined sites during specific stages of development. We have developed molecular markers to follow the location and progress of cell division within multicellular plant organs and thereby gain some insight into how cell division might be regulated during morphogenesis. As in other eukaryotes, cell division in plants is regulated by a highly conserved set of protein kinases and phosphatases. The molecular information available on these molecules from other eukaryotes has allowed the design of strategies by which plant homologues can be isolated. In this report we describe the identification of a nimA-like gene from Antirrhinum majus and describe the pattern in which its transcript is expressed. Comparison of the pattern of AmnimA gene expression with that of genes which are expressed in a cell cycle-dependent manner suggests that this gene is expressed in actively dividing tissues but expression is not specific to any particular phase of the cell cycle nor specific to any particular tissue type.
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.
The Arabidopsis PINHEAD/ZWILLE (PNH/ZLL) gene is thought to play an important role in the formation of the shoot apical meristem (SAM) and in leaf adaxial cell specification. To investigate the molecular mechanisms of rice development, we have isolated a rice homologue of PNH/ZLL, called OsPNH1. Around the SAM, OsPNH1 was strongly expressed in developing leaf primordia, specifically in the presumptive vascular domains, developing vascular tissues, a few cell-layers of the adaxial region, and future bundle sheath extension cells. In the SAM, only weak expression was observed in the central region, whereas strong expression was detected in the mid-vein region of leaf founder cells in the peripheral SAM domain. We produced transgenic rice plants containing the antisense OsPNH1 strand. The antisense OsPNH1 plants developed malformed leaves with an altered vascular arrangement and abnormal internal structure. These plants also formed an aberrant SAM with reduced KNOX gene expression. We examined the subcellular localization of the OsPNH1-GFP fusion protein and found that it was localized in the cytoplasm. On the basis of these observations, we propose that OsPNH1 functions not only in SAM maintenance as previously thought, but also in leaf formation through vascular development.
Critical developmental and gene expression profiles were charted during the formation of shoots from root explants in Arabidopsis tissue culture. Shoot organogenesis is a two-step process involving pre-incubation on an auxin-rich callus induction medium (CIM) during which time root explants acquire competence to form shoots during subsequent incubation on a cytokinin-rich shoot induction medium (SIM). At a histological level, the organization of shoot apical meristems (SAMs) appears to occur during incubation on SIM about the time of shoot commitment, i.e. the transition from hormone-dependent to hormone-independent shoot development. Genes involved in SAM formation, such as SHOOTMERISTEMLESS (STM) and CLAVATA1 (CLV1), were upregulated at about the time of shoot commitment, while WUSCHEL (WUS) was upregulated somewhat earlier. Genes required for STM expression, such as CUP-SHAPED COTYLEDON 1 and 2 (CUC1 and 2) were upregulated prior to shoot commitment. Gene expression patterns were determined for two GFP enhancer trap lines with tissue-specific expression in the SAM, including one line reporting on CUC1 expression. CUC1 was generally expressed in callus tissue during early incubation on SIM, but later CUC1 was expressed more locally in presumptive sites of shoot formation. In contrast, the expression pattern of the enhancer trap lines during zygotic embryogenesis was more localized to the presumptive SAM even in early stages of embryogenesis.
Reproductive transition, inflorescence architecture, meristem patterning, and floral organ identity have been studied as distinct research areas in plant science. By using the ornamental plant Gerbera, we demonstrate that all of these keystone aspects of reproductive meristematic fate are integrated genetically by a single SEPALLATA-like MADS-box gene from a functional class designated previously as "floral homeotic" or "organ identity." This extended regulatory network has not been elaborated in the model plant systems, which have a floral design and inflorescence-determinacy state that obscures these relationships.
The RB/E2F pathway is involved in the control of the G(1)/S transition of the eukaryotic cell cycle where various S phase genes are activated by specific E2F factors. Ribonucleotide reductase (RNR) plays an essential role in the DNA synthesis pathway. Earlier studies showed that there are at least two RNR1 genes (RNR1a and RNR1b) and one RNR2 gene in tobacco. In synchronized tobacco BY2 cells, RNR1b gene expression is at its highest level in S phase. To investigate transcriptional regulation of the RNR1b gene, its promoter region was cloned and sequenced. Unlike its animal counterparts, the tobacco RNR1b promoter contains a consensus E2F-binding site. Surprisingly, this site is found in the leader sequence of the gene. We show here by gel shift analysis and antibody competition that one nuclear complex specifically binds this motif, and an E2F factor is part of this complex. Using reporter gene analysis, tobacco RNR1b promoter activity was detected during S phase in synchronized cells and in plant meristematic tissues. Mutation of the E2F element substantially reduced both activities. For the first time in plants, a single E2F motif found in the leader sequence plays an important role in the meristem and S phase-specific expression of the tobacco RNR1b gene.
We have devised a protocol for the isolation and identification of a proliferative antigen of the initial cells of wheat stem meristems (termed PAI). We have carried out a variety of immunochemical and immunocytochemical methods, using colloidal gold (CG) complexed with monospecific antibodies to PAI as the marker for the detection of PAI. We have been able to determine the effectiveness of immunoaffinity chromatography in isolating PAI from plant tissues and have shown the advantages of CG over enzyme labels for identification of the antigen. Finally, we have obtained a purified preparation of PAI and have determined its molecular mass (approximately 83 kDa).
The use of bromodeoxyuridine (BrdU) and subsequent immunocytochemical visualization for studying cell proliferation in plant meristems was investigated in Allium cepa L. root-tips. We describe the optimization of an indirect immunoperoxidase method for detecting incorporation of this DNA precursor in pulse-labeled cells. The basic object of this study is to quantify the extent to which the fraction of S-phase cells can reliably be estimated in asynchronous populations. A matrix of parallel labeling schedules with tritiated-thymidine or BrdU was developed, and the labeling indices provided by autoradiography or immunocytochemistry were compared. Thus, 0.5 mM BrdU assured saturation S-phase labeling after an exposure time of 30 min, and the mean length of the S-phase determined under such conditions was similar to that previously reported for this plant system. Interestingly, Feulgen staining did not interfere with subsequent detection of the BrdU probe. This allowed comparative evaluations of the nuclear DNA content by Feulgen-microdensitometry and the position of a given cell in G1, S or G2 compartments. We also explored the possibility of quantifying BrdU-incorporation in single nuclei by densitometry measurement of the peroxidase label.
Micrographs of dictyosomes in face view and in profile, together with serial sections representing both these planes, are reproduced from three sample cells at different developmental stages in the meristem of Anthoceros. The stages are: a vegetative cell at anaphase of a mitotic division, a vegetative cell in an early stage of postmitotic extension growth, and a young spore mother cell in the act of rounding up before the onset of meiosis. The observations suggest that proliferation of tubules from the edges of the dictyosomal cisternae into the cytoplasm is occurring with varying intensity and with slightly different morphological expression in all three cells. In all, the tubules are joined into a reticulum which exhibits local swellings at varying distances from the unfenestrated part of the subtending cisterna. A comparison is suggested between the observed reticulum and "smooth" endoplasmic reticulum of animals but it is not claimed that all the cytoplasmic tubules detectable in Anthoceros need have arisen in this way. Morphological differences discernible between tubules near their point of attachment to dictyosomes and others apparently involved in the formation of the new nuclear membrane at the end of a cell division could mean that more than one category of tube may exist in these cells. A plea is registered for restraint in the formulation of far reaching theories until more facts are available on unequivocal evidence.
BACKGROUND AND AIMS: Shoot apical meristems (SAMs) in most seed plants are quite uniform in size and zonation, and molecular genetic studies of Arabidopsis and other model plants are revealing details of SAM morphogenesis. Some cacti have SAMs much larger than those of A. thaliana and other seed plants. This study examined how SAM size affects leaf primordium (LP) size, phyllotaxy and shoot diameter. METHODS: Apices from 183 species of cacti were fixed, microtomed and studied by light microscopy. KEY RESULTS: Cactus SAM diameter varies from 93 to 2565 microm, the latter being 36 times wider than SAMs of A. thaliana and having a volume 45 thousand times larger. Phyllotaxy ranges from distichous to having 56 rows of leaves and is not restricted to Fibonacci numbers. Leaf primordium diameter ranges from 44 to 402 microm, each encompassing many more cells than do LP of other plants. Species with high phyllotaxy have smaller LP, although the correlation is weak. There is almost no correlation between SAM diameter and LP size, but SAM diameter is strongly correlated with shoot diameter, with shoots being about 189.5 times wider than SAMs. CONCLUSIONS: Presumably, genes such as SHOOT-MERISTEMLESS, WUSCHEL and CLAVATA must control much larger volumes of SAM tissue in cacti than they do in A. thaliana, and genes such as PERIANTHIA might establish much more extensive fields of inhibition around LP. These giant SAMs should make it possible to more accurately map gene expression patterns relative to SAM zonation and LP sites.