Shoot Apical Meristem Development Research Articles

Arabidopsis ASL11/LBD15 is involved in shoot apical meristem development and regulates WUS expression

The ASYMMETRIC LEAVES2-LIKE/LATERAL ORGAN BOUNDARIES (LOB) DOMAIN (ASL/LBD) genes encode plant-specific nuclear proteins containing the conserved domain AS2/LOB. In this study, the function of a member of the Arabidopsis thaliana AS2/LOB gene family, ASL11/LBD15, was investigated. The results show that ASL11/LBD15 is expressed in the meristems of shoot apex, root apex, organ boundaries, and developing seeds. Overexpression of ASL11/LBD15 resulted in aberrant arrangements in the tunica cell layers of the shoot apical meristem (SAM). Two-week-old transgenic plants developed needle-like leaves in addition to regular leaves, while 6-week-old transformants displayed clustered cauline leaves suggesting altered SAM development. qRT-PCR analysis revealed that the WUSCHEL (WUS) transcript level was strongly up-regulated in plants overexpressing ASL11/LBD15 compared with the wild-type plants. Furthermore, inducible ASL11/LBD15 ectopic expression activated ectopic expression of WUS and affected the differentiation of leaf epidermal cells. Therefore, our results suggest that ASL11/LBD15 affects cellular differentiation in the SAM and regulates WUS expression.

Systems Analysis of Shoot Apical Meristem Growth and Development: Integrating Hormonal and Mechanical Signaling

The shoot apical meristem (SAM) is a small population of stem cells that continuously generates organs and tissues. This review covers our current understanding of organ initiation by the SAM in Arabidopsis thaliana. Meristem function and maintenance involves two major hormones, cytokinins and auxins. Cytokinins appear to play a major role in meristem maintenance and in controlling meristematic properties, such as cell proliferation. Self-organizing transport processes, which are still only partially understood, lead to the patterned accumulation of auxin at particular positions, where organs will grow out. A major downstream target of auxin-mediated growth regulation is the cell wall, which is a determinant for both growth rates and growth distribution, but feedbacks with metabolism and the synthetic capacity of the cytoplasm are crucial as well. Recent work has also pointed at a potential role of mechanical signals in growth coordination, but the precise mechanisms at work remain to be elucidated.

Ectopic expression of the Arabidopsis ASYMMETRIC LEAVES2-LIKE5 (ASL5) gene in cockscomb (Celosia cristata) generates vascular-pattern modifications in lateral organs

ASYMMETRIC LEAVES2-LIKE5/LATERAL ORGAN BOUNDARIES DOMAIN 12 (ASL5/LBD12), isolated from Arabidopsis, is a member of the LATERAL ORGAN BOUNDARIES (LOB) domain gene family. Previously, it has been reported that the dominant peacock1-D (pck1-D) mutant, wherein an ASL5 gene activated by a T-DNA lost apical dominance, had epinastic or abaxial leaves and was sterile; this suggested that ASL5 gene might be involved in ab-/adaxial determination of leaves or in the development of shoot apical meristems (SAMs), but no evidence for this has been provided hitherto. In this study, 35S:AtASL5-GFP transgenic cockscomb plants were obtained. In leaf epidermal cells, the AtASL5-GFP fusion protein displayed nuclear localization suggesting that AtASL5 might be a potential transcription factor. The 35S:AtASL5 transgenic cockscombs have dramatically altered phenotype. Histological analysis of petioles, leaf blades and lateral roots shows that these lateral organs have vascular-pattern modifications, suggesting the leaves might have ab-/adaxial defect, and the lateral roots have a central-peripheral defect. Moreover, ectopic protuberances were observed in transgenic plants, indicated that AtASL5 might have function in development of shoot apical meristem.

Differentiation of Multiple Shoot Apical Meristems in Mutant Rice with One Embryo Causing Multiple Plumuples

The proper formation and development of shoot apical meristem during plant embryonic development is prerequisite to generate leaves,buds and stems.Rice(Oryza sativa L.) mutant line 4001 produces multiple plumules in one seed and is consi dered as an important mutant with abnormal differentiation of shoot apical meristem.While the embryonic basis of the mutant with multiple plumules is yet unknown.To elucidate the cytological basis,mature embryo sac structure and process of double fertilization were observed with whole-mount eosin B-staining confocal laser scanning microscopy(WE-CLSM),semi-thin plastic sections and scanning electron microscopy(SEM).The observation by WE-CLSM showed that both embryo sac structure and double fertilization process were normal in the mutant line,suggesting that the abnormal traits were related to the later embryonic development.By semi-thin plastic sections and SEM,the retardation of embryonic development and the formation of multiple shoot apical meristems(SAM) under abnormal orientation were observed in one embryo of the mutant line,compared with those of wild-type rice.At three days after pollination,the process of embryonic development was slower in the mutant line than in the wild type rice.The abnormality included the diverse number of SAMs under irregular orientation and the changes of dorsal-ventral axis.Two separate plumules,two plumules in one coleoptile and two connected plumules with one separate plumule were also observed in the mutant line 4001.It was concluded that differentiation of multiple SAMs and the changes of dorsal-ventral axis in one embryo contribute to the multiple plumuples in the mutant line.It was inferred that mutant trait-related genes might play important roles in the regulations of the timing of shoot apical meristem differentiation,the number of undifferentiated cells and the establishment of dorsal-ventral axis.

CCS52A2/FZR1, a cell cycle regulator, is an essential factor for shoot apical meristem maintenance in Arabidopsis thaliana

BackgroundCell division and cell fate decisions regulate organ formation and function in plant growth and development. It is still unclear how specific meristematic regulatory networks operate with the cell cycle machinery to translate stem cell identity and maintenance into cellular behavior. In this study, we address these questions by analysis of a shoot apex defective mutant, namely xcm9.ResultsPhenotypic analysis of the xcm9 mutant reveals concomitant premature termination of floral shoots with frequent bifurcation of the shoot apices, stems, and flowers. Microscopic observations show irregular cell organization in shoot apical meristems of xcm9. Positional cloning revealed that xcm9 is a loss of function allele of the CCS52A2/FZR1 gene, which has previously been implicated in root development. Expression analysis demonstrated that CCS52A2 maintains a higher transcriptional expression level in actively dividing tissue. Genetic studies indicated that the CCS52A2 gene functions together with WUSCHEL (WUS) and CLAVATA3 (CLV3) in regulating the development of the shoot meristem, and also contributes to this regulation together with the chromatin remodeling pathway. In addition, fewer xcm9 cells express CYCLIN B1:1, showing that cell cycle progression is disrupted in the mutant.ConclusionWe propose that the CCS52A2 gene is a mediator that functions together with meristematic genes to regulate meristem organization, and cross-functions with chromatin regulators in cell cycle progression during shoot apical meristem development.

Somatic embryogenesis in tamarillo (Cyphomandra betacea): approaches to increase efficiency of embryo formation and plant development

Somatic embryogenesis induction and somatic embryo development of the solanaceous tamarillo tree were previously established and successfully used for plant regeneration from different explants and varieties. Somatic embryogenesis was induced in Murashige and Skoog medium containing 2,4-dichlorophenoxyacetic acid (2,4-D) or picloram and high sucrose concentrations (0.25 M). The embryogenic tissues were transferred to an auxin-free medium, with reduced sucrose levels, to permit embryo development and conversion into plantlets. This two-step protocol is often impaired by an ineffective transition from the proembryogenic masses to embryo development. In this work, attempts to optimize the somatic embryogenesis system of tamarillo by improving the quality of somatic embryo and embryo conversion were carried out. The results showed that the presence of a high number of abnormal somatic embryos did not significantly inhibit plant conversion, hence indicating that shoot apical meristem development was not affected in abnormal somatic embryos. It was also shown that the manipulation of sucrose concentration in the development medium (0.11 M) and dark conditions before conversion increased the number of morphologically normal somatic embryos. The comparison between mature cotyledonary zygotic and somatic embryos showed an inefficient accumulation of storage compounds, mainly lipids, in somatic embryos. These reduced levels of lipid storage could be responsible for the abnormal patterns of embryo development found in tamarillo somatic embryos.

Regulation of shoot apical meristem development by SEUSS and SEUSS-LIKE 2 in Arabidopsis

Regulation of shoot apical meristem development by SEUSS and SEUSS-LIKE 2 in Arabidopsis

Overexpression of AtBMI1C, a Polycomb Group Protein Gene, Accelerates Flowering in Arabidopsis

Polycomb group protein (PcG)-mediated gene silencing is emerging as an essential developmental regulatory mechanism in eukaryotic organisms. PcGs inactivate or maintain the silenced state of their target chromatin by forming complexes, including Polycomb Repressive Complex 1 (PRC1) and 2 (PRC2). Three PRC2 complexes have been identified and characterized in Arabidopsis; of these, the EMF and VRN complexes suppress flowering by catalyzing the trimethylation of lysine 27 on histone H3 of FLOWER LOCUS T (FT) and FLOWER LOCUS C (FLC). However, little is known about the role of PRC1 in regulating the floral transition, although AtRING1A, AtRING1B, AtBMI1A, and AtBMI1B are believed to regulate shoot apical meristem and embryonic development as components of PRC1. Moreover, among the five RING finger PcGs in the Arabidopsis genome, four have been characterized. Here, we report that the fifth, AtBMI1C, is a novel, ubiquitously expressed nuclear PcG protein and part of PRC1, which is evolutionarily conserved with Psc and BMI1. Overexpression of AtBMI1C caused increased H2A monoubiquitination and flowering defects in Arabidopsis. Both the suppression of FLC and activation of FT were observed in AtBMI1C-overexpressing lines, resulting in early flowering. No change in the H3K27me3 level in FLC chromatin was detected in an AtBMI1C-overexpressing line. Our results suggest that AtBMI1C participates in flowering time control by regulating the expression of FLC; moreover, the repression of FLC by AtBMI1C is not due to the activity of PRC2. Instead, it is likely the result of PRC1 activity, into which AtBMI1C is integrated.

Plant Hormones: Metabolism, Signaling and Crosstalk

Plant Hormones: Metabolism, Signaling and Crosstalk

Arabidopsis Argonaute10 Specifically Sequesters miR166/165 to Regulate Shoot Apical Meristem Development

The shoot apical meristem (SAM) comprises a group of undifferentiated cells that divide to maintain the plant meristem and also give rise to all shoot organs. SAM fate is specified by class III HOMEODOMAIN-LEUCINE ZIPPER (HD-ZIP III) transcription factors, which are targets of miR166/165. In Arabidopsis, AGO10 is a critical regulator of SAM maintenance, and here we demonstrate that AGO10 specifically interacts with miR166/165. The association is determined by a distinct structure of the miR166/165 duplex. Deficient loading of miR166 into AGO10 results in a defective SAM. Notably, the miRNA-binding ability of AGO10, but not its catalytic activity, is required for SAM development, and AGO10 has a higher binding affinity for miR166 than does AGO1, a principal contributor to miRNA-mediated silencing. We propose that AGO10 functions as a decoy for miR166/165 to maintain the SAM, preventing their incorporation into AGO1 complexes and the subsequent repression of HD-ZIP III gene expression.

Reaction-diffusion pattern in shoot apical meristem of plants.

A fundamental question in developmental biology is how spatial patterns are self-organized from homogeneous structures. In 1952, Turing proposed the reaction-diffusion model in order to explain this issue. Experimental evidence of reaction-diffusion patterns in living organisms was first provided by the pigmentation pattern on the skin of fishes in 1995. However, whether or not this mechanism plays an essential role in developmental events of living organisms remains elusive. Here we show that a reaction-diffusion model can successfully explain the shoot apical meristem (SAM) development of plants. SAM of plants resides in the top of each shoot and consists of a central zone (CZ) and a surrounding peripheral zone (PZ). SAM contains stem cells and continuously produces new organs throughout the lifespan. Molecular genetic studies using Arabidopsis thaliana revealed that the formation and maintenance of the SAM are essentially regulated by the feedback interaction between WUSHCEL (WUS) and CLAVATA (CLV). We developed a mathematical model of the SAM based on a reaction-diffusion dynamics of the WUS-CLV interaction, incorporating cell division and the spatial restriction of the dynamics. Our model explains the various SAM patterns observed in plants, for example, homeostatic control of SAM size in the wild type, enlarged or fasciated SAM in clv mutants, and initiation of ectopic secondary meristems from an initial flattened SAM in wus mutant. In addition, the model is supported by comparing its prediction with the expression pattern of WUS in the wus mutant. Furthermore, the model can account for many experimental results including reorganization processes caused by the CZ ablation and by incision through the meristem center. We thus conclude that the reaction-diffusion dynamics is probably indispensable for the SAM development of plants.

Analysis of expressed sequence tags in developing secondary xylem and shoot of Acacia mangium

Acacia mangium is a fast-growing tree widely planted in tropical countries because of its rapid growth, high wood density, high fiber quality, and good adaptability. Despite its importance as a fiber source in the pulp and paper industry, a large-scale analysis of expressed sequence tags (ESTs) has not been performed in A. mangium. In this study, we sequenced 10 752 clones of a normalized complementary DNA (cDNA) library prepared from A. mangium developing secondary xylem and shoot, and obtained a total of 8963 ESTs. The ESTs were assembled into 6220 unigenes comprising 1614 contigs and 4606 singletons. The unigene set was then subjected to various bioinformatic analyses. BlastN searches of the unigene set against the Gene Index Databases of soybean, Medicago truncatula, Lotus japonicus, grape, poplar, spruce, and pine demonstrated that the largest number of unigenes shared homologies with the soybean Gene Indices. BlastX searches against the TAIR9 peptide database enabled us to annotate the unigenes. Based on the annotation, we discussed whether the unigenes involved in the cell cycle, cell growth, shoot apical meristem development, and cell wall biosynthesis were present. This new genomic resource will accelerate the functional genomics of wood formation and molecular breeding to improve the wood properties of A. mangium.

Analysis of interactions among the CLAVATA3 receptors reveals a direct interaction between CLAVATA2 and CORYNE in Arabidopsis

In Arabidopsis, CORYNE (CRN), a new member of the receptor kinase family, was recently isolated as a key player involved in the CLAVATA3 (CLV3) signaling pathway, thereby playing an important role in regulating the development of shoot and root apical meristems. However, the precise relationships among CLAVATA1 (CLV1), CLAVATA2 (CLV2), and CRN receptors remain unclear. Here, we demonstrate the subcellular localization of CRN and analyze the interactions among CLV1, CLV2, and CRN using firefly luciferase complementation imaging (LCI) assays in both Arabidopsis mesophyll protoplasts and Nicotiana benthamiana leaves. Fluorescence targeting showed that CRN was localized to the plasma membrane. The LCI assays coupled with co-immunoprecipitation assays demonstrated that CLV2 can directly interact with CRN in the absence of CLV3. Additional LCI assays showed that CLV1 did not interact with CLV2, but can interact weakly with CRN. We also found that CLV1 can interact with CLV2-CRN heterodimers, implying that these three proteins may form a complex. Moreover, CRN, rather than CLV1 and CLV2, was able to form homodimers without CLV3 stimulation. Taken together, our results add direct evidence to the newly proposed two-parallel receptor pathways model and therefore provide new insights into the CLV3 signaling pathway.

Bystander/Abscopal Effects Induced in Intact Arabidopsis Seeds by Low-Energy Heavy-Ion Radiation

To date, radiation-induced bystander effects have been observed largely in in vitro single-cell systems; verification of both the effects and the mechanisms in multicellular systems in vivo is important. Previously we showed that bystander/ abscopal effects can be induced by irradiating the shoot apical meristem cells in Arabidopsis embryos. In this study, we investigated the in vivo effects induced by 30 keV 40Ar ions in intact Arabidopsis seeds and traced the postembryonic development of both irradiated and nonirradiated shoot apical meristem and root apical meristem cells. Since the range of 30 keV 40Ar ions in water is about 0.07 microm, which is less than the distance from the testa to shoot apical meristem and root apical meristem in Arabidopsis seeds (about 100 microm), the incident low-energy heavy ions generally stop in the proximal surface. Our results showed that, after the 30 keV 40Ar-ion irradiation of shielded and nonshielded Arabidopsis seeds at a fluence of 1.5 x 10(17) ions/cm2, short- and long-term postembryonic development, including germination, root hair differentiation, primary root elongation, lateral root initiation and survival, was significantly inhibited. Since shoot apical meristem and root apical meristem cells were not damaged directly by radiation, the results suggested that a damage signal(s) is transferred from the irradiated cells to shoot apical meristem and root apical meristem cells and causes the ultimate developmental alterations, indicating that long-distance bystander/ abscopal effects exist in the intact seed. A further study of mechanisms showed that the effects are associated with either enhanced generation of reactive oxygen species (ROS) or decreased auxin-dependent transcription in postembryonic development. Treatment with the ROS scavenger dimethyl sulfoxide (DMSO) or synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) can significantly reverse both the alterations in postembryonic development and auxin-dependent transcription, suggesting that ROS and auxin-dependent transcription processes play essential roles in the low-energy heavy-ion radiation-induced long-distance bystander/abscopal effects in the intact organism.

A model for Arabidopsis class-1 KNOX gene function

The Arabidopsis class-1 KNOX genes STM, BP/KNAT1, KNAT2 and KNAT6 encode homeodomain transcriptional regulators important for shoot apical meristem (SAM) and carpel development. During vegetative growth, STM is required to establish and maintain the stem cell pool of the SAM, a function replaceable by ectopic expression of BP/KNAT1 but not of other class-1 KNOX genes. We recently demonstrated additional STM roles in the development of the central floral whorl and subsequent formation of carpels, a function replaceable by ectopic KNAT2 expression. However, STM is normally required for BP/KNAT1 and KNAT2 expression, explaining why it is essential for both SAM and carpel development. We propose therefore that STM provides the critical KNOX function in these processes and that the SAM- and carpel-promoting activities of STM are redundantly duplicated in BP/KNAT1 and KNAT2 respectively. Here we show that ectopic KNAT2 expression can restore carpel development to stm mutants, but fails to restore proper development of the central floral whorl, which requires a function analogous to the SAM-promoting activities of STM and BP/KNAT1. Similarly, we show that ectopic KNAT2 expression does not restore normal meristem organization to the SAM. We propose a model for discrete and overlapping class-1 KNOX gene function in Arabidopsis.

A Role for KNAT Class II Genes in Root Development

Homeodomain proteins set up domains of gene expression during the development of animal and plant body plans. In plants, homeodomain proteins of the KNOX class I family have been shown to play a role in shoot apical meristem development. Recently, we have investigated the role of the Arabidopsis thaliana KNOX class II genes KNAT3, KNAT4 and KNAT5 in root development. These genes showed root domain and cell type specific expression patterns, and their expression was regulated by hormones that influence root growth. Moreover, sub-cellular localization of the KNAT proteins exhibited regulation, suggesting that post-transcriptional control contributes to KNOX class II protein activity. Our data provide a survey of KNAT gene expression in the root and indicate that the investigated KNAT genes might play distinct roles during root development.

The effect of cadmium on shoot apical meristems of barley

We studied the effects of cadmium acetate at various concentrations (200 to 800 mg/kg substrate) on growth and development of shoot apical meristem of the barley plants (Hordeum vulgare L.) under the conditions of vegetative experiment. It was shown that in the presence of increasing cadmium concentrations in the soil substrate, the apex length and number of inflorescence elements decreased and the rate of organogenesis slowed down, thus affecting the spike potential productivity and morphological parameters of barley plants at the flowering phase. It is possible that the negative effect of cadmium on the shoot apical meristem is associated with its influence on division of the apex cells.

MIR166/165 genes exhibit dynamic expression patterns in regulating shoot apical meristem and floral development in Arabidopsis

The miR166/165 group and its target genes regulate diverse aspects of plant development, including apical and lateral meristem formation, leaf polarity, and vascular development. We demonstrate here that MIR166/165 genes are dynamically controlled in regulating shoot apical meristem (SAM) and floral development in parallel to the WUSCHEL (WUS)-CLAVATA (CLV) pathway. Although miR166 and miR165 cleave same target mRNAs, individual MIR166/165 genes exhibit distinct expression domains in different plant tissues. The MIR166/165 expression is also temporarily regulated. Consistent with the dynamic expression patterns, an array of alterations in SAM activities and floral architectures was observed in the miR166/165-overproducing plants. In addition, when a MIR166a-overexpressing mutant was genetically crossed with mutants defective in the WUS-CLV pathway, the resultant crosses exhibited additive phenotypic effects, suggesting that the miR166/165-mediated signal exerts its role via a distinct signaling pathway.

Physiological influences in the development and function of the shoot apical meristem of microspore-derived embryos ofBrassica napus‘Topas’

The effect of auxins and abscisic acid (ABA) on shoot apical meristem development and function was analyzed in microspore-derived embryos of Brassica napus L. ‘Topas’. Embryos were treated with an auxin transport inhibitor, tri-iodobenzoic acid (TIBA) or exogenous indole-3-acetic acid (IAA) at various stages during their development. If embryos were treated at the preglobular or globular stages of embryogenesis, they developed one fused cotyledon indicating a continuation of radial symmetry. However, if treated later in development, embryos were comparable to the control and formed two separate cotyledons. To demonstrate that this was not a pharmacological effect, an auxin antagonist, 2-(p-chlorophenoxy)-2-methylpropionic acid, included with the TIBA treatment produced normal embryos with two separate cotyledons. Histological observations revealed that the shoot apical meristem of the embryos treated with TIBA or IAA at the preglobular and globular stages was altered. Preglobular and globular stage embryos that were treated with TIBA exhibited a dramatic decline in conversion frequency compared with their controls. Embryos that were treated later in development had conversion frequencies comparable to their controls, and their shoot apical meristems also were similar to controls. Application of ABA at the preglobular and globular stages maintained meristem integrity and improved embryo conversion. However, ABA could not reverse the TIBA effect.

Ectopic expression of TaYAB1, a member of YABBY gene family in wheat, causes the partial abaxialization of the adaxial epidermises of leaves and arrests the development of shoot apical meristem in Arabidopsis

Adaxial–abaxial polarity is an important feature of lateral organs. To study the molecular mechanism of polarity establishment, a YABBY gene TaYAB1 was isolated from the young leaves of wheat. TaYAB1 has a zinc finger-like domain in the N terminus and a YABBY domain in the C terminus. Sequence comparison showed that TaYAB1 is a putative member of the YABBY gene family in wheat. Further expression analysis indicated that TaYAB1 was expressed in the lateral organs of wheat. Ectopic expression of TaYAB1 in Arabidopsis caused the partial abaxialization of the adaxial epidermises of leaves and arrested shoot apical meristem development with the reduction of the transcript levels of SHOOT MERISTEMLESS ( STM). Our results indicate that TaYAB1 affects the establishment of adaxial–abaxial polarity and the development of shoot apical meristem when ectopically expressed in Arabidopsis.