Shoot Meristem Initiation Research Articles

The CUC1 and CUC2 genes promote carpel margin meristem formation during Arabidopsis gynoecium development.

Carpel margin meristems (CMMs), a pair of meristematic tissues present along the margins of two fused carpel primordia of Arabidopsis thaliana, are essential for the formation of ovules and the septum, two major internal structures of the gynoecium. Although a number of regulatory factors involved in shoot meristem activity are known to be required for the formation of these gynoecial structures, their direct roles in CMM development have yet to be addressed. Here we show that the CUP-SHAPED COTYLEDON genes CUC1 and CUC2, which are essential for shoot meristem initiation, are also required for formation and stable positioning of the CMMs. Early in CMM formation, CUC1 and CUC2 are also required for expression of the SHOOT MERISTEMLESS gene, a central regulator for stem cell maintenance in the shoot meristem. Moreover, plants carrying miR164-resistant forms of CUC1 and CUC2 resulted in extra CMM activity with altered positioning. Our results thus demonstrate that the two regulatory proteins controlling shoot meristem activity also play critical roles in elaboration of the female reproductive organ through the control of meristematic activity.

小分子RNA合成経路を介したイネのシュート構築機構

小分子RNA合成経路を介したイネのシュート構築機構

Differential transcriptional expression following thidiazuron-induced callus differentiation developmental shifts in rice

Very little is known about molecular events associated with callus differentiation in indica rice. The genes expressed differentially during shoot meristem initiation were identified on genomic arrays applied to efficiently regenerating rice calli. A thidiazuron (TDZ; N-phenyl-N-thiadiazol-1,2,3-5,ylurea)-dependent regeneration protocol was developed for efficient embryogenesis in indica rice. The regenerating embryogenic calli induced by TDZ for 10 days showed transcriptional modulation of a number of genes associated with photosynthesis, hormone metabolism, plant development, signal transduction, light response, and plant defense. Eighteen candidate miRNAs were predicted to target the genes expressed differentially in the embryogenic calli grown in TDZ-containing medium. The majority of the photosynthesis-related genes up-regulated in differentiating calli were not expressed or were down-regulated in developing seeds and inflorescences. Most of the genes down-regulated in differentiating calli were up-regulated in developing seeds. The transcriptome of differentiating callus most closely resembled that of the germinating whole seed.

Arabidopsis Cytokinin Signaling Pathway

Cytokinins are essential plant hormones that control cell division, shoot meristem initiation, leaf and root differentiation, vasculature patterning, chloroplast biogenesis, photomorphogenesis, fertility, seed development, senescence, and stress tolerance. The Arabidopsis cytokinin signal transduction pathway involves hybrid histidine protein kinases [AHK2, AHK3, and AHK4 (also known as CRE1or WOL)] as cytokinin receptors, histidine phosphotransfer proteins (AHPs), and nuclear response regulators (ARRs) that serve as transcriptional regulators. There are four major steps in the cytokinin phosphorelay: (i) AHK sensing and signaling, (ii) AHP nuclear translocation, (iii) ARR-dependent transcriptional activation, and (iv) a negative-feedback loop through cytokinin-inducible ARR gene products. Each step is executed by components encoded by multigene families. The effects of cytokinin depend on cell type, environment, and developmental stage. The response is frequently the outcome of interactions with other plant signaling pathways.

Pattern formation during de novo assembly of theArabidopsisshoot meristem

Most multicellular organisms have a capacity to regenerate tissue after wounding. Few, however, have the ability to regenerate an entire new body from adult tissue. Induction of new shoot meristems from cultured root explants is a widely used, but poorly understood, process in which apical plant tissues are regenerated from adult somatic tissue through the de novo formation of shoot meristems. We characterize early patterning during de novo development of the Arabidopsis shoot meristem using fluorescent reporters of known gene and protein activities required for shoot meristem development and maintenance. We find that a small number of progenitor cells initiate development of new shoot meristems through stereotypical stages of reporter expression and activity of CUP-SHAPED COTYLEDON 2 (CUC2), WUSCHEL (WUS), PIN-FORMED 1 (PIN1), SHOOT-MERISTEMLESS (STM), FILAMENTOUS FLOWER (FIL, also known as AFO), REVOLUTA (REV), ARABIDOPSIS THALIANA MERISTEM L1 LAYER (ATML1) and CLAVATA 3 (CLV3). Furthermore, we demonstrate a functional requirement for WUS activity during de novo shoot meristem initiation. We propose that de novo shoot meristem induction is an easily accessible system for the study of patterning and self-organization in the well-studied model organism Arabidopsis.

The small interfering RNA production pathway is required for shoot meristem initiation in rice

The shoot apical meristem (SAM) is a group of stem cells that are responsible for plant development. Mutations in rice SHOOTLESS2 (SHL2), SHL4/SHOOT ORGANIZATION2 (SHO2), and SHO1 cause complete deletion or abnormal formation of the SAM. In this study we showed that defects in SAM formation in shl mutants are associated with the loss of expression of the homeodomain-leucine zipper (HD-ZIPIII) family genes. Rice SHL2, SHL4/SHO2, and SHO1 encoded orthologues of Arabidopsis RNA-dependent RNA polymerase 6, ARGONAUTE (AGO) 7, and DICER-like 4, respectively, whose mutations affect leaf development through the trans-acting siRNA (ta-siRNA) pathway. This suggested that the ta-siRNA pathway regulates the critical step of SAM formation during rice embryogenesis. The gain-of-function experiment by the ectopic expression of SHL4 resulted in reduced accumulation of an microRNA, miR166, and partial adaxialization of leaves, supporting a role for the ta-siRNA pathway in the maintenance of leaf polarity as previously reported in maize. Analysis of the spatiotemporal expression patterns of HD-ZIPIII and miR166 in wild-type and shl mutant embryos suggested that the loss of HD-ZIPIII expression in the SAM region of the developing embryo is the result of ectopic expression of miR166. Our analysis of shl mutants demonstrated that HD-ZIPIII expression regulated by miR166 is sensitive to the ta-siRNA pathway during SAM formation in rice embryogenesis.

Monitoring genome-wide changes in gene expression in response to endogenous cytokinin reveals targets in Arabidopsis thaliana

Cytokinins have been implicated in developmental and growth processes in plants including cell division, chloroplast biogenesis, shoot meristem initiation and senescence. The regulation of these processes requires changes in cytokinin-responsive gene expression. Here, we induced the expression of a bacterial isopentenyl transferase gene, IPT, in transgenic Arabidopsis thaliana seedlings to study the regulation of genome-wide gene expression in response to endogenous cytokinin. Using MPSS (massively parallel signature sequencing) we identified 823 and 917 genes that were up- and downregulated, respectively, following 24 h of IPT induction. When comparing the response to cytokinin after 6 and 24 h, we identified different clusters of genes showing a similar course of regulation. Our study provides researchers with the opportunity to rapidly assess whether genes of interest are regulated by cytokinins.

The SHOOTLESS2 and SHOOTLESS1 genes are involved in both initiation and maintenance of the shoot apical meristem through regulating the number of indeterminate cells.

To characterize the SHL2 and SHL1 genes in detail, we analyzed three strains carrying weak alleles of SHL2, shl2-6, shl2-7, and shl2-8, and one weak allele of SHL1, shl1-3. In contrast to strong alleles, which result in lack of shoot meristem, strains bearing these weak alleles formed shoot meristem frequently during embryogenesis. In shl2-6 and shl2-7 mutants, the meristem was lost during seed development. Only the shl2-8 mutant could survive after germination, but it showed abnormal initiation pattern and morphology of leaves. In strains bearing the weak alleles, the shoot meristem was composed of a small number of indeterminate cells and ultimately converted into leaf primordium. The shl1-3 mutant showed phenotypes similar to those of shl2-8. Thus SHL2 and SHL1 are required for both initiation and maintenance of shoot meristem. In shl2 mutants, there was a positive correlation between the size of the expression domain of OSH1 representing the number of indeterminate cells, the frequency of shoot meristem initiation, and the duration of meristem survival. Thus the shoot meristem will not initiate in an "all-or-nothing" fashion, but is formed in various degrees depending on the strength of the alleles. Double-mutant analyses indicate that SHL2 functions upstream of SHO to establish proper organization of the shoot meristem.

Production and characterization of diverse developmental mutants of Medicago truncatula.

The diploid annual legume Medicago truncatula has been developed as a tractable genetic system for studying biological questions that are unique to, or well suited for study in legume species. An efficient mutagenesis protocol using ethyl-methyl sulfonate and a polymorphic ecotype with properties appropriate for use as a mapping parent are described. Isolation and characterization of three developmental mutants are described. The mtapetala mutation results in homeotic conversions of floral organ whorls 2 and 3 into sepals and carpelloid structures, respectively, similar to mutations in the apetala3/pistillata genes of Arabidopsis. The palmyra mutation primarily affects seedling shoot meristem initiation, and thus phenocopies meristem function mutations identified in Arabidopsis such as the zwille locus. The phenotype of the palmyra and mtapetala double mutant is additive, with seedling shoot meristems and floral organs indistinguishable from those of the single palmyra and mtapetala mutants, respectively. These results are consistent with a lack of genetic interaction between these loci. A third mutant, speckle, is characterized by spontaneous necrotic lesion formation on leaves, root, and stems, similar to necrosis mutants identified in other plant species. In addition to documenting the efficient mutagenesis of M. truncatula, the availability of developmental mutants that phenocopy characterized Arabidopsis mutants will provide a basis for establishing orthologous gene function between M. truncatula and Arabidopsis, once the genes responsible are cloned. Moreover, the male-sterile, female-fertile nature of the mtapetala mutant provides a convenient tool for genetic analyses in M. truncatula.

Effect of mutations in the PINHEAD gene of Arabidopsis on the formation of shoot apical meristems

AbstractThe primary shoot apical meristem of angiosperm plants is formed during embryogenesis. Lateral shoot apical meristems arise postembryonically in the axils of leaves. Recessive mutations at the PINHEAD locus of Arabidopsis interfere with the ability of both the primary shoot apical meristem as well as lateral shoot apical meristems to form. However, adventitious shoot apical meristems can form in pinhead mutant seedlings from the axils of the cotyledons and also from cultred root explants. In this report, the phenotype of pinhead mutants is described, and a hypothesis for the role of the wild‐type PINHEAD gene product in shoot meristem initiation is presented. © 1995 Wiley‐Liss, Inc.

Activity and Accumulation of Cell Division-Promoting Phenolics in Tobacco Tissue Cultures

Dehydrodiconiferyl alcohol glucosides (DCGs) are derivatives of the phenylpropanoid pathway that have been isolated from Catharansus roseus L. (Vinca rosea) crown gall tumors. Fractions containing purified DCGs have been shown previously to promote the growth of cytokinin-requiring tissues of tobacco in the absence of exogenous cytokinins. In this study, we utilized synthetic DCG isomers to confirm the cell division-promoting activity of DCG isomers A and B and show that they neither promote shoot meristem initiation on Nicotiana tabacum L., cv Havana 425, leaf explants nor induce betacyanin synthesis in amaranth seedlings. Analysis of cultured tobacco pith tissue demonstrated that DCG accumulation was stimulated by cytokinin treatment and correlated with cytokinin-induced cell division. Thus, the accumulation of metabolites that could replace cytokinin in cell division bioassays is stimulated by cytokinins. These data support the model that DCGs are a component of a cytokinin-mediated regulatory circuit controlling cell division.

Gibberellic acid regulation of adventitious shoot formation from tuber discs of potato

The formation of adventitious shoots from potato tuber discs explanted onto a modified Murashige and Skoog (MS) medium containingN 6-benzylaminopurine (BAP) (3.0 mg/l), and α-naphthaleneacetic, acid (NAA) (0.01 mg/l), was affected by gibberellic acid (GA). The presence of GA in the explant medium was required for shoot formation and 3×10−10 M GA appeared optimum. However, microscopic examination of the tissue protuberances on the surface of the tuber discs from which shoots arose revealed that GA inhibited the formation of shoot meristems. Tuber discs cultured for 6 wk on MS medium containing BAP and NAA without GA did not initiate adventitious shoots that could be determined visually, but microscopic examination of the tissue protuberances revealed the presence of numerous shoot meristems. Subsequent transfer of these tuber discs to medium with GA but without BAP or NAA resulted in the formation of shoots from 100% of the recultrued dises. Thus it appears that although GA inhibits shoot meristem initiation from potato tuber discs, it is required for shoot development once meristems are initiated.