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PubMed2001

Plant morphogenesis: long-distance coordination and local patterning.

The overall morphology of a plant is largely determined by developmental decisions taken within or near the terminally positioned apical meristems of shoots and roots. The spatial separation of these developmental centers emphasizes the need for long-distance signaling. The same signaling events may simultaneously coordinate differentiation within meristems and in the connecting vascular tissues. Recent genetic and molecular analyses not only confirm the proposed role of auxin as a coordinating signal across the plant, but also implicate auxin as a patterning signal in embryo and meristem organization.

Morphogenesis↗

PubMed2003

Persistence and prevention of aluminium- and paraquat-induced adaptive response to methyl mercuric chloride in plant cells in vivo.

Induction and persistence of adaptive response by aluminium (Al), 1 or 10 microM, and paraquat (PQ), 5 or 10 microM, against genotoxicity of methyl mercuric chloride (MMCl), 1.26 microM, a standard environmental genotoxin, was investigated in root meristem cells of Allium cepa. Subsequently, three metabolic inhibitors, namely, 3-aminobezamide (3-AB, 10 or 100 microM), an inhibitor of poly(ADP-ribose) polymerase (PARP) implicated in DNA repair and/or apoptosis, cycloheximide (CH, 0.1 or 1 microM), an inhibitor of protein synthesis, and buthionine sulfoximine (BSO, 100 microM or 1mM), an inhibitor of glutathione synthesis were tested for their ability to prevent the adaptive response induced by conditioning doses of Al, 10 or 100 microM; and PQ, 5 or 100 microM, against MMCl-challenge, 1.26 or 100 microM, in root meristems of A. cepa or embryonic shoots of Hordeum vulgare, respectively. The findings demonstrated that once triggered, the Al- or PQ-adaptive response to MMCl could persist for at least 48h in root meristems of A. cepa. Furthermore, the adaptive response could effectively be prevented by 3-AB, to a lesser degree by CH, and the least by BSO, suggesting primarily the involvement of PARP and implicating DNA repair in the underlying mechanisms of adaptive response in plant cells in vivo.

Adaptation, Physiological↗

PubMedDate not supplied

Characterization of an inducible promoter system in Catharanthus roseus hairy roots.

Transgenic hairy root cultures of Catharanthus roseus were established with a glucocorticoid-inducible promoter controlling the expression of green fluorescent protein (GFP), and GFP expression was characterized. The inducible system shows a tightly controlled, reversible, and dosage-dependent response to the glucocorticoid dexamethasone in C. roseus hairy roots. Full induction was noted after 12-18 h in the mature regions of the root tips and after 6 h in the meristem tissue. Upon removal of the inducing agent, GFP expression declined to undetectable levels in the mature tissues after 24 h and in the meristem after 48 h. Although no dosage-dependent response was noted in the meristem region, such a response was apparent in the mature region of the tip and verified by quantitative GFP analysis. The inducible promoter system allowed quantitative control of GFP expression between 0.01 and 10 microM dexamethasone with saturation occurring at higher levels. Using GFP as a model system allowed demonstration of the ability to control temporal and quantitative gene expression with the glucocorticoid-inducible promoter in transgenic C. roseus hairy roots.

Catharanthus↗

PubMed1999

ZmOCL1, an HDGL2 family homeobox gene, is expressed in the outer cell layer throughout maize development.

The formation of a morphologically distinct outer cell layer or protoderm is one of the first and probably one of the most important steps in patterning of the plant embryo. Here we report the isolation of ZmOCL1 (OCL for outer cell layer), a member of the HDGL2 (also known as HD-ZIP IV) subclass of plant-specific HD-ZIP homeodomain proteins from maize. ZmOCL1 transcripts are detected very early in embryo development, before a morphologically distinct protoderm is visible, and expression then becomes localised to the protoderm of the embryo as it develops. Subsequently, expression is observed in the L1 cell layer of both the developing primary root and shoot meristems, and is maintained in developing leaves and floral organs. We propose that ZMOCL1 may play a role in the specification of protoderm identity within the embryo, the organisation of the primary root primordium or in the maintenance of the L1 cell layer in the shoot apical meristem. We also show that the expression of ZmOCL1 is different from that of another epidermal marker gene, LTP2 (lipid transfer protein) and, in meristems, is complementary to that of Kn1 (Knotted) which is transcribed only in underlying cell layers.

Amino Acid Sequence↗

PubMed2000

Cell cycle regulation in the course of nodule organogenesis in Medicago.

The molecular mechanisms of de novo meristem formation, cell differentiation and the integration of the cell cycle machinery into appropriate stages of the developmental programmes are still largely unknown in plants. Legume root nodules, which house nitrogen-fixing rhizobia, are unique plant organs and their development may serve as a model for organogenetic processes in plants. Nodules form and are essential for the plant only under limitation of combined nitrogen in the soil. Moreover, their development is triggered by external mitogenic signals produced by their symbiotic partners, the rhizobia. These signals, the lipochitooligosaccharide Nod factors, act as host-specific morphogens and induce the re-entry of root cortical cells into mitotic cycles. Maintenance of cell division activity leads to the formation of a persistent nodule meristem from which cells exit continuously and enter the nodule differentiation programme, involving multiple cycles of endoreduplication and enlargement of nuclear and cell volumes. While the small diploid 2C cells remain uninfected, the large polyploid cells can be invaded and, after completing the differentiation programme, host the nitrogen-fixing bacteroids. This review summarizes the present knowledge on cell cycle reactivation and meristem formation in response to Nod factors and reports on a novel plant cell cycle regulator that can switch mitotic cycles to differentiation programmes.

Cell Cycle↗

PubMed2002

Spatial and temporal expression of the orchid floral homeotic gene DOMADS1 is mediated by its upstream regulatory regions.

The orchid floral homeotic gene, DOMADSI, is a marker gene specifically expressed in the transitional shoot apical meristem during floral transition in Dendrobium Madame Thong-In. DOMADSI is not detectable in vegetative tissues except a weak expression in the stem. Its transcript is uniformly localized in both of the inflorescence meristem and floral primordia, and later expressed in almost all of the floral organs. We isolated and sequenced a 3.5 kb DOMADSI promoter fragment upstream of the transcription start site, demonstrating the location of several putative DNA-binding sites, through which MADS-box and class I knox genes may modulate the DOMADSI expression. To gain insight into the molecular basis of the regulation of DOMADS1, deletion analysis of the DOMADSI::beta-glucuronidase (GUS) gene fusions was performed by means of the stable orchid transformation systems. The study shows that the full-length upstream promoter sequence confers the same spatial and temporal GUS staining pattern as that of the distribution of DOMADSI RNA during orchid development. We also identified the distinct cis-acting regulatory regions required for the control of DOMADS1 expression in vegetative and reproductive tissues, as well as the shoot apical meristem during floral transition.

5' Flanking Region↗

PubMed2002

Pyrimidine nucleotide and nucleic acid synthesis in embryos and megagametophytes of white spruce (Picea glauca) during germination.

Pyrimidine nucleotide synthesis was investigated in isolated germinating zygotic embryos and separated megagametophytes of white spruce by following the metabolic fate of 14C-labelled orotic acid, uridine, and uracil, as well as by measuring the activities of the major enzymes participating in nucleotide synthesis. The rate of nucleic acid synthesis in these tissues was also examined by tracer experiments and autoradiographic studies conducted with labelled thymidine, and by conventional light microscopy. From our results, it emerges that changes in the contribution of the de novo and salvage pathways of pyrimidines play an important role during the initial stages of zygotic embryo germination. Preferential utilization of uridine for nucleic acid synthesis, via the salvage pathway, was observed at the onset of germination, before the restoration of a fully functional de novo pathway. Similar metabolic changes, not observed in the gametophytic tissue, were also documented in somatic embryos previously. These alterations of the overall pyrimidine metabolism may represent a strategy for ensuring the germinating embryos with a large nucleotide pool. Utilization of 14C-thymidine for nucleic acid synthesis increased in both dissected embryos and megagametophytes during germination. Autoradiographic and light microscopic studies indicated that soon after imbibition, DNA synthesis was preferentially initiated along the embryonic axis, especially in the cortical cells. Apical meristem reactivation was a later event, and the root meristem became activated before the shoot meristem. Taken together, these results indicate that precise changes in nucleotide and nucleic acid metabolism occur during the early phases of embryo germination.

Journal Article↗

PubMed1996

Control of root growth and development by cyclin expression.

Root development is plastic, with post-embryonic organogenesis being mediated by meristems. Although cell division is intrinsic to meristem initiation, maintenance and proliferative growth, the role of the cell cycle in regulating growth and development is unclear. To address this question, we examined the expression of cdc2 and cye genes, which encode the catalytic and regulatory subunits, respectively, of cyclin-dependent protein kinases that control progression through the cell cycle. Unlike cdc2, which is expressed not only in apical meristems but also before lateral root initiation in quiescent, pericycle cells arrested in the G2 phase of the cell cycle, cyc1At transcripts accumulate specifically in dividing cells immediately before cytokinesis. Ectopic expression of cyc1At under the control of the cdc2aAt promoter in Arabidopsis plants markedly accelerates growth without altering the pattern of lateral root development or inducing neoplasia. Thus cyclin expression is a limiting factor for growth, which in turn drives indeterminate development of the root system.

Arabidopsis↗

PubMed1995

Specification of chimeric flowering shoots in wild-type Arabidopsis.

Within wild-type Arabidopsis populations, a subset of the plants were found to have a single chimeric shoot on their primary shoot axes. The chimeric shoots were located below the lowest primary-axis flower; and they exhibited features of both flowers and paraclades (lateral flowering shoots). Morphological analyses of chimeric shoots indicated that they developed from single primordia. In each chimeric shoot, the side furthest from the apical meristem was specified as 'flower'--while the side closest to the meristem was specified as 'paraclade'--suggesting that a stimulus from outside the apical meristem can directly induce primordia to develop as flowers. It is concluded that the development of the teratological chimeric shoots resulted from the overlap of the vegetative and floral specification processes within single primordia.

Arabidopsis↗

PubMed1997

Functional analysis of the Arabidopsis thaliana SBP-box gene SPL3: a novel gene involved in the floral transition.

The isolation and initial characterization of the Arabidopsis thaliana SPL3 gene are described. SPL3 belongs to a gene family encoding putative transcription factors characterized by a conserved DNA-binding domain, the SBP domain. SPL3 transcription is developmentally regulated and is localized mainly in vegetative and inflorescence apical meristems, floral meristems and in leaf and floral organ primordia. SPL3 recognizes a conserved sequence motif in the promoter region of the A. thaliana floral meristem identity gene AP1. Similarly to AP1, constitutive expression of SPL3 results in early flowering. However, constitutive expression of SPL3 in an ap1 mutant background showed that AP1 is not required for the early flowering phenotype of the SPL3 transgenic plants. The function of SPL3 during flowering as well as its possible functional redundancy are discussed.

Amino Acid Sequence↗

PubMed2005

Arabidopsis TCP20 links regulation of growth and cell division control pathways.

During postembryonic plant development, cell division is coupled to cell growth. There is a stringent requirement to couple these processes in shoot and root meristems. As cells pass through meristems, they transit through zones with high rates of cell growth and proliferation during organogenesis. This transition implies a need for coordinate regulation of genes underpinning these two fundamental cell functions. Here, we report a mechanism for coregulation of cell division control genes and cell growth effectors. We identified a GCCCR motif necessary and sufficient for high-level cyclin CYCB1;1 expression at G2/M. This motif is overrepresented in many ribosomal protein gene promoters and is required for high-level expression of the S27 and L24 ribosomal subunit genes we examined. p33(TCP20), encoded by the Arabidopsis TCP20 gene, binds to the GCCCR element in the promoters of cyclin CYCB1;1 and ribosomal protein genes in vitro and in vivo. We propose a model in which organ growth rates, and possibly shape in aerial organs, are regulated by the balance of positively and negatively acting teosinte-branched, cycloidea, PCNA factor (TCP) genes in the distal meristem boundary zone where cells become mitotically quiescent before expansion and differentiation.

Arabidopsis↗

PubMed2002

AGAMOUS-LIKE 24, a dosage-dependent mediator of the flowering signals.

The most dramatic phase change in plants is the transition from vegetative to reproductive growth. This flowering process is regulated by several interacting pathways that monitor both the developmental state of the plants and environmental cues such as light and temperature. The flowering-time genes FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CO1 (SOC1), together with the floral meristem identity gene LEAFY (LFY), are three essential regulators integrating floral signals from multiple pathways in Arabidopsis thaliana. Part of the crosstalk among these genes is mediated by a putative transcription factor, AGAMOUS-LIKE 24 (AGL24). This gene is gradually activated in shoot apical meristems during the floral transition and later located in the whole zone of both inflorescence and floral meristems. Loss and reduction of AGL24 activity by double-stranded RNA-mediated interference result in late flowering, whereas constitutive overexpression of AGL24 causes precocious flowering. The correlation between the level of AGL24 accumulation and the alteration of flowering time suggests that AGL24 is a dosage-dependent flowering promoter. Analysis of AGL24 expression in various flowering-time mutants shows that it is regulated in several floral inductive pathways. Further genetic analyses of epistasis indicate that AGL24 may act downstream of SOC1 and upstream of LFY.

Arabidopsis↗

PubMed1997

Expression of an ortholog of replication protein A1 (RPA1) is induced by gibberellin in deepwater rice.

Internodes of deepwater rice are induced to grow rapidly when plants become submerged. This adaptation enables deepwater rice to keep part of its foliage above the rising flood waters during the monsoon season and to avoid drowning. This growth response is, ultimately, elicited by the plant hormone gibberellin (GA). The primary target tissue for GA action is the intercalary meristem of the internode. Using differential display of mRNA, we have isolated a number of genes whose expression in the intercalary meristem is regulated by GA. The product of one of these genes was identified as an ortholog of replication protein A1 (RPA1). RPA is a heterotrimeric protein involved in DNA replication, recombination, and repair and also in regulation of transcription. A chimeric construct, in which the single-stranded DNA-binding domain of rice RPA1 was spliced into the corresponding region of yeast RPA1, was able to complement a yeast rpa1 mutant. The transcript level of rice RPA1 is high in tissues containing dividing cells. RPA1 mRNA levels increase rapidly in the intercalary meristem during submergence and treatment with GA before the increase in the level of histone H3 mRNA, a marker for DNA replication.

Amino Acid Sequence↗

PubMed2002

Comparative developmental anatomy of seedlings in nine species of podostemaceae (subfamily Podostemoideae).

The developmental anatomy is described for seedlings of nine Asian and Australian species of Podostemaceae, subfamily Podostemoideae. The hypocotyl is rudimentary (except in Zeylanidium olivaceum) and does not form a primary root in any of the species examined. An adventitious root forms endogenously in the hypocotyl of six species with ribbon-like or flattened subcylindrical roots, and in Z olivaceum with foliose roots. In contrast, it forms exogenously in Hydrobryum griffithii and Synstylis micranthera with foliose roots. The juvenile root becomes flattened and dorsiventral, branches exogenously (in Polypleurum stylosum, P. wallichii and Z. lichenoides) and produces shoots endogenously (in P. stylosum, P. wallichii, S. micranthera and Z. lichenoides). The root meristem is simple, composed of surface and uniform inner cells, and is devoid of root cap initials in all species. The reduced meristem morphology of seedling roots may be primitive in the Asian-Australian Podostemoideae. A root cap or protective tissue did not form during the culture period, even in the seven species with capped adult roots, probably due to its delayed development. It was absent throughout ontogeny in the other two species. No obvious shoot apical meristem forms between the cotyledons. One to several leaves occupy the shoot apical area in species with endogenous adventitious roots, while no leaves are formed in species with exogenous roots. These differences suggest recurrent origins of foliose roots in the Asian clade. Similarities between the unique seedling morphology and mutant Arabidopsis phenotypes are discussed.

Biological Evolution↗

PubMed2006

Unique and redundant functional domains of APETALA1 and CAULIFLOWER, two recently duplicated Arabidopsis thaliana floral MADS-box genes.

APETALA1 (AP1) and CAULIFLOWER (CAL) are closely related MADS box genes that are partially redundant during Arabidopsis thaliana floral meristem determination. AP1 is able to fully substitute for CAL functions, but not vice versa, and AP1 has unique sepal and petal identity specification functions. In this study, the unique and redundant functions of these two genes has been mapped to the four protein domains that characterize type-II MADS-domain proteins by expressing all 15 chimeric combinations of AP1 and CAL cDNA regions under control of the AP1 promoter in ap1-1 loss-of-function plants. The "in vivo" function of these chimeric genes was analysed in Arabidopsis plants by expressing the chimeras. Rescue of flower meristem and sepal/petal identities was scored in single and multiple insert homozygous transgenic lines. Using these chimeric lines, it was found that distinct residues of the AP1 K domain not shared by the same CAL domain are necessary and sufficient for complete recovery of floral meristem identity, in the context of the CAL protein sequence, while both AP1 COOH and K domains are indispensable for complete rescue of sepal identity. By contrast, either one of these two AP1 domains is necessary and sufficient for complete petal identity recovery. It was also found that there were positive and negative synergies among protein domains and their combinations, and that multiple-insert lines showed relatively better rescue than equivalent single-insert lines. Finally, several lines had flowers with extra sepals and petals suggesting that chimeric proteins yield abnormal transcriptional complexes that may alter the expression or regulation of genes that control floral organ number under normal conditions.

Amino Acid Sequence↗

PubMed2002

A novel DNA polymerase homologous to Escherichia coli DNA polymerase I from a higher plant, rice (Oryza sativa L.).

A novel DNA polymerase, designated as OsPolI-like, has been identified from the higher plant, rice (Oryza sativa L. cv. Nipponbare). The OsPolI-like cDNA was 3765 bp in length, and the open reading frame encoded a predicted product of 977 amino acid residues with a molecular weight of 100 kDa. The OsPolI-like gene has been mapped to chromosome 8 and contains 12 exons and 11 introns. The encoded protein showed a high degree of sequence and structural homology to Escherichia coli pol I protein, but differed from DNA polymerase gamma and theta. The DNA polymerase domain of OsPolI-like showed DNA polymerase activity. Subcellular fractionation analysis suggested that the protein is localized in the plastid. Northern and western blotting, and in situ hybridization analyses demonstrated preferential expression of OsPolI-like in meristematic tissues such as shoot apical meristem, root apical meristem, leaf primordia and the marginal meristem. Interestingly, no expression was detected in mature leaves, although they have a high chloroplast content. These properties indicated that OsPolI-like is a novel plant DNA polymerase. The function of OsPolI-like is discussed in relation to plastid maturation.

Chromosome Mapping↗

PubMed2001

EMF genes interact with late-flowering genes in regulating floral initiation genes during shoot development in Arabidopsis thaliana.

To investigate the mechanisms regulating the initiation of floral development in Arabidopsis, a construct containing beta-glucuronidase (GUS) gene driven by APETALA1 promoter (AP1::GUS) was introduced into emf fwa and emf ft double mutants. GUS activity was strongly detected on shoot meristem of emf1-1 single mutants harboring AP1::GUS construct just 5 d after germination. By contrast, GUS activity was undetectable on emf1-1 fwa-1, emf1-1 ft-1, emf2-1 fwa-1, emf2-3 fwa-1 and emf2-3 ft-1 double mutants harboring AP1::GUS construct 10 d after germination. GUS activity was only weakly detected on the apical meristem of 20-day-old emf1-1 fwa-1 and emf2-1 fwa-1 seedlings. During this time, only sessile leaves were produced. Further analysis indicated that AP1 was strongly expressed in 10-day-old emf1-1 and emf2-1 single mutants. Its expression was significantly reduced in all emf1-1 or emf2-1 late-flowering double mutants tested. Similar to AP1, the expression of LEAFY (LFY) was also high in emf1-1 and emf2-1 single mutants and reduced in emf1-1 or emf2-1 late-flowering double mutants. Our results indicate that the precocious expression of AP1 and LFY is dependent not only on the low EMF and FWA activities but also on the expression of most of the late-flowering genes such as FT, FCA, FE, CO and GI. These data also reveal that most late-flowering genes may function downstream of EMF or in pathways distinct from EMF to activate genes specified floral meristem identity during shoot maturation in Arabidopsis.

Alleles↗

PubMed2004

Cytokinins induce photomorphogenic development in dark-grown gametophytes of Ceratopteris richardii.

The cytokinins benzylaminopurine, kinetin and isopentenyladenine induce photomorphogenesis in dark-grown gametophytes of the fern Ceratopteris richardii. At sub-nanomolar concentrations each altered the rate and pattern of cell division, elongation and differentiation, mimicking aspects of the light-mediated transition from filamentous to prothallial growth. Untreated dark-grown gametophytes grow as narrow, elongate, asexual filaments with an apical meristem. Cytokinin treatments as low as 10(-12) M reduced the length-to-width ratio through decreased cell elongation, increased periclinal cell division and induced the formation of rhizoid initials in the cells immediately below the apical meristem. Higher concentrations (10(-9)-10(-8) M) induced conversion of the meristem from apical to notch morphology. Cytokinins induced both red- and blue-light-mediated photomorphogenic events, suggesting stimulation of both phytochrome and cryptochrome signaling; however, cytokinin treatment only partially substituted for light in that it did not induce hermaphroditic sexual development or spore germination in the dark. Additionally, cytokinins did not increase chlorophyll synthesis in dark-grown gametophytes, which unlike angiosperms are able to produce mature chloroplasts in the dark. Cytokinin treatment had only slight effects on light-grown gametophytes. These results suggest evolutionary conservation between angiosperms and pteridophytes in the role of cytokinins in regulating photomorphogenesis.

Cytokinins↗