The shoot apical meristem: the dynamics of a stable structure

Cotton BLH1 and KNOX6 antagonistically modulate fiber elongation via regulation of linolenic acid biosynthesis

BEL1-LIKE HOMEODOMAIN (BLH) proteins are known to function in various plant developmental processes. However, the role of BLHs in regulating plant cell elongation is still unknown. Here, we identify a BLH gene, GhBLH1, that positively regulates fiber cell elongation. Combined transcriptomic and biochemical analyses reveal that GhBLH1 enhances linolenic acid accumulation to promote cotton fiber cell elongation by activating the transcription of GhFAD7A-1 via binding of the POX domain of GhBLH1 to the TGGA cis-element in the GhFAD7A-1 promoter. Knockout of GhFAD7A-1 in cotton significantly reduces fiber length, whereas overexpression of GhFAD7A-1 results in longer fibers. The K2 domain of GhKNOX6 directly interacts with the POX domain of GhBLH1 to form a functional heterodimer, which interferes with the transcriptional activation of GhFAD7A-1 via the POX domain of GhBLH1. Overexpression of GhKNOX6 leads to a significant reduction in cotton fiber length, whereas knockout of GhKNOX6 results in longer cotton fibers. An examination of the hybrid progeny of GhBLH1 and GhKNOX6 transgenic cotton lines provides evidence that GhKNOX6 negatively regulates GhBLH1-mediated cotton fiber elongation. Our results show that the interplay between GhBLH1 and GhKNOX6 modulates regulation of linolenic acid synthesis and thus contributes to plant cell elongation.

Chimeric mutations in grapevine ENHANCED DISEASE RESISTANCE1 improve resistance to powdery mildew without growth penalty

Grapevine (Vitis vinifera L.) incurs severe quality degradation and yield loss from powdery mildew, a major fungal disease caused by Erysiphe necator. ENHANCED DISEASE RESISTANCE1 (EDR1), a Raf-like mitogen-activated protein kinase kinase kinase, negatively regulates defense responses against powdery mildew in Arabidopsis (Arabidopsis thaliana). However, little is known about the role of the putatively orthologous EDR1 gene in grapevine. In this study, we obtained grapevine VviEDR1-edited lines using CRISPR/Cas9. Plantlets containing homozygous and bi-allelic indels in VviEDR1 developed leaf lesions shortly after transplanting into the soil and died at the seedling stage. Transgenic plants expressing wild-type VviEDR1 and mutant Vviedr1 alleles as chimera (designated as VviEDR1-chi) developed normally and displayed enhanced resistance to powdery mildew. Interestingly, VviEDR1-chi plants maintained a spatiotemporally distinctive pattern of VviEDR1 mutagenesis: while almost no mutations were detected from terminal buds, ensuring normal function of the apical meristem, mutations occurred in young leaves and increased as leaves matured, resulting in resistance to powdery mildew. Further analysis showed that the resistance observed in VviEDR1-chi plants was associated with callose deposition, increased production of salicylic acid and ethylene, H2O2 production and accumulation, and host cell death. Surprisingly, no growth penalty was observed with VviEDR1-chi plants. Hence, this study demonstrated a role of VviEDR1 in the negative regulation of resistance to powdery mildew in grapevine and provided an avenue for engineering powdery mildew resistance in grapevine.

Combined computational modeling and experimental analysis integrating chemical and mechanical signals suggests possible mechanism of shoot meristem maintenance

Stem cell maintenance in multilayered shoot apical meristems (SAMs) of plants requires strict regulation of cell growth and division. Exactly how the complex milieu of chemical and mechanical signals interact in the central region of the SAM to regulate cell division plane orientation is not well understood. In this paper, simulations using a newly developed multiscale computational model are combined with experimental studies to suggest and test three hypothesized mechanisms for the regulation of cell division plane orientation and the direction of anisotropic cell expansion in the corpus. Simulations predict that in the Apical corpus, WUSCHEL and cytokinin regulate the direction of anisotropic cell expansion, and cells divide according to tensile stress on the cell wall. In the Basal corpus, model simulations suggest dual roles for WUSCHEL and cytokinin in regulating both the direction of anisotropic cell expansion and cell division plane orientation. Simulation results are followed by a detailed analysis of changes in cell characteristics upon manipulation of WUSCHEL and cytokinin in experiments that support model predictions. Moreover, simulations predict that this layer-specific mechanism maintains both the experimentally observed shape and structure of the SAM as well as the distribution of WUSCHEL in the tissue. This provides an additional link between the roles of WUSCHEL, cytokinin, and mechanical stress in regulating SAM growth and proper stem cell maintenance in the SAM.

Fluctuations shape plants through proprioception

Plants constantly experience fluctuating internal and external mechanical cues, ranging from nanoscale deformation of wall components, cell growth variability, nutating stems, and fluttering leaves to stem flexion under tree weight and wind drag. Developing plants use such fluctuations to monitor and channel their own shape and growth through a form of proprioception. Fluctuations in mechanical cues may also be actively enhanced, producing oscillating behaviors in tissues. For example, proprioception through leaf nastic movements may promote organ flattening. We propose that fluctuation-enhanced proprioception allows plant organs to sense their own shapes and behave like active materials with adaptable outputs to face variable environments, whether internal or external. Because certain shapes are more amenable to fluctuations, proprioception may also help plant shapes to reach self-organized criticality to support such adaptability.

Spatio-Temporal Dynamics of the Patterning of Arabidopsis Flower Meristem

The qualitative model presented in this work recovers the onset of the four fields that correspond to those of each floral organ whorl of Arabidopsis flower, suggesting a mechanism for the generation of the positional information required for the differential expression of the A, B, and C identity genes according to the ABC model for organ determination during early stages of flower development. Our model integrates a previous model for the emergence of WUS pattern in the floral meristem, and shows that this pre-pattern is a necessary but not sufficient condition for the posterior information of the four fields predicted by the ABC model. Furthermore, our model predicts that LFY diffusion along the L1 layer of cells is not a necessary condition for the patterning of the floral meristem.

Chimerism of chrysanthemum stems changes at the nodes during vegetative growth

The shoot apical meristem (SAM) is typically divided into three cell layers: the outermost epidermal layer (L1), the subepidermal layer (L2) and the inner corpus region (L3). Structures within the cell layers are normally maintained throughout development; however, through vegetative propagation of a periclinal chimeric chrysanthemum expressing a fluorescent protein gene only in the L1 layer, we collected twelve independent shoots that had partially mosaic fluorescent inner cells (L2, L3) in addition to fluorescent epidermal cells (L1). Furthermore, the elongated tissues of nine shoots out of the twelve had no internal fluorescent cells, i.e., they had the original L1 chimerism. Observations of the fluorescence distribution suggested that the change in chimerism occurred at the nodes, indicating previously unnoticed cell layer dynamics occurring at the nodes.

Identification of candidate genes for drought tolerance in coffee by high-throughput sequencing in the shoot apex of different Coffea arabica cultivars

BACKGROUND

Drought is a widespread limiting factor in coffee plants. It affects plant development, fruit production, bean development and consequently beverage quality. Genetic diversity for drought tolerance exists within the coffee genus. However, the molecular mechanisms underlying the adaptation of coffee plants to drought are largely unknown. In this study, we compared the molecular responses to drought in two commercial cultivars (IAPAR59, drought-tolerant and Rubi, drought-susceptible) of Coffea arabica grown in the field under control (irrigation) and drought conditions using the pyrosequencing of RNA extracted from shoot apices and analysing the expression of 38 candidate genes.

RESULTS

Pyrosequencing from shoot apices generated a total of 34.7 Mbp and 535,544 reads enabling the identification of 43,087 clusters (41,512 contigs and 1,575 singletons). These data included 17,719 clusters (16,238 contigs and 1,575 singletons) exclusively from 454 sequencing reads, along with 25,368 hybrid clusters assembled with 454 sequences. The comparison of DNA libraries identified new candidate genes (n = 20) presenting differential expression between IAPAR59 and Rubi and/or drought conditions. Their expression was monitored in plagiotropic buds, together with those of other (n = 18) candidates genes. Under drought conditions, up-regulated expression was observed in IAPAR59 but not in Rubi for CaSTK1 (protein kinase), CaSAMT1 (SAM-dependent methyltransferase), CaSLP1 (plant development) and CaMAS1 (ABA biosynthesis). Interestingly, the expression of lipid-transfer protein (nsLTP) genes was also highly up-regulated under drought conditions in IAPAR59. This may have been related to the thicker cuticle observed on the abaxial leaf surface in IAPAR59 compared to Rubi.

CONCLUSIONS

The full transcriptome assembly of C. arabica, followed by functional annotation, enabled us to identify differentially expressed genes related to drought conditions. Using these data, candidate genes were selected and their differential expression profiles were confirmed by qPCR experiments in plagiotropic buds of IAPAR59 and Rubi under drought conditions. As regards the genes up-regulated under drought conditions, specifically in the drought-tolerant IAPAR59, several corresponded to orphan genes but also to genes coding proteins involved in signal transduction pathways, as well as ABA and lipid metabolism, for example. The identification of these genes should help advance our understanding of the genetic determinism of drought tolerance in coffee.

Real-Time Lineage Analysis to Study Cell Division Orientation in the Arabidopsis Shoot Meristem

Cells in the Arabidopsis shoot apical meristem are small and divide frequently throughout the life-time of the organism making them good candidates for studying the mechanisms of cell division in plants. But tracking these cell divisions requires multiple images to be taken of the same specimen over time which means the specimen must stay alive throughout the process. This chapter provides details on how to prepare plants for live imaging, keep them alive and growing through multiple time points, and how to process the data to extract cell boundary coordinates from three-dimensional images.

Differential induction of meristematic stem cells of Catharanthus roseus and their characterization

Plant cell culture technology has been introduced for the mass production of the many useful components. A variety of plant-derived compounds is being used in various fields, such as pharmaceuticals, foods, and cosmetics. Plant cell cultures are believed to be derived from the dedifferentiation process. In the present study, an undifferentiated cambial meristematic cell (CMCs) of Catharanthus is isolated using histological and genetic methods, and compared with dedifferentiation-derived callus (DDCs) cultures. Furthermore, differential culture conditions for both DDCs- and CMCs-derived cell lines were established. A suitable media for the increased accumulation of terpenoid indole alkaloids (TIAs) was also standardized. Compared with DDCs, CMCs showed marked accumulation of TIAs in cell lines grown on media with 1.5 mg·mL(-1) of NAA and 0.5 mg·mL(-1) of kinetin. CMCs-derived cultures of Catharanthus, as a source of key anticancer drugs (viblastine and vincristine), would overcome the obstacles usually associated with the production of natural metabolites through the use of DDCs. Cell culture systems that are derived from CMCs may also provide a cost-effective and eco-friendly basis for the sustainable production of a number of important plant natural products.

Analysis of cell division patterns in the Arabidopsis shoot apical meristem

The stereotypic pattern of cell shapes in the Arabidopsis shoot apical meristem (SAM) suggests that strict rules govern the placement of new walls during cell division. When a cell in the SAM divides, a new wall is built that connects existing walls and divides the cytoplasm of the daughter cells. Because features that are determined by the placement of new walls such as cell size, shape, and number of neighbors are highly regular, rules must exist for maintaining such order. Here we present a quantitative model of these rules that incorporates different observed features of cell division. Each feature is incorporated into a "potential function" that contributes a single term to a total analog of potential energy. New cell walls are predicted to occur at locations where the potential function is minimized. Quantitative terms that represent the well-known historical rules of plant cell division, such as those given by Hofmeister, Errera, and Sachs are developed and evaluated against observed cell divisions in the epidermal layer (L1) of Arabidopsis thaliana SAM. The method is general enough to allow additional terms for nongeometric properties such as internal concentration gradients and mechanical tensile forces.

Ectopic expression of Kxhkn5 in the viviparous species Kalanchoe × Houghtonii induces a novel pattern of epiphyll development

KxhKN5 (class 1 KNOX gene) was cloned from Kalanchoe × houghtonii with strong tendency to form epiphylls on leaves. KxhKN5 appear to be homologue of BP of A. thaliana on the basis of phylogeny, expression and phenotype analysis. Beside the modification of several plant and leaf traits, the appearance of epiphylls was drastically reduced by both the silencing and the over-expression of KxhKN5 in most of the generated clones. In silenced clones, epiphyll production followed the morphogenetic pathway of the WT plants: somatic embryos outbreak in the centre of each leaf-pedestal, grown in the notch between leaf indentations and were supported by a suspensor. The connection between the epiphyll and the mother plant did not include any vasculature and as a result, the epiphylls dropped easily from the mother plant. The most represented category of over expressor clones, disclosed a novel pattern of epiphyll development: the leaf-pedestals were absent and single bud outbreaks in each leaf notch. Buds developed into shoots which remained attached to the maternal plant by a strong vascular connection. The leaves supporting shoots, produced a thickened midrib and veins, and their lamina ceased expanding. Finally, the leaf/shoot structure resembles a lateral branch. The ectopic expression of KxhKN5 in K. × houghtonii induces a process comparable to the alternation of leaf and shoot formation in other species and support the idea, that it is the variation in shared molecular and developmental processes which produces the growth of specific structures.

RNA-Seq reveals genotype-specific molecular responses to water deficit in eucalyptus

BACKGROUND

In a context of climate change, phenotypic plasticity provides long-lived species, such as trees, with the means to adapt to environmental variations occurring within a single generation. In eucalyptus plantations, water availability is a key factor limiting productivity. However, the molecular mechanisms underlying the adaptation of eucalyptus to water shortage remain unclear. In this study, we compared the molecular responses of two commercial eucalyptus hybrids during the dry season. Both hybrids differ in productivity when grown under water deficit.

RESULTS

Pyrosequencing of RNA extracted from shoot apices provided extensive transcriptome coverage - a catalog of 129,993 unigenes (49,748 contigs and 80,245 singletons) was generated from 398 million base pairs, or 1.14 million reads. The pyrosequencing data enriched considerably existing Eucalyptus EST collections, adding 36,985 unigenes not previously represented. Digital analysis of read abundance in 14,460 contigs identified 1,280 that were differentially expressed between the two genotypes, 155 contigs showing differential expression between treatments (irrigated vs. non irrigated conditions during the dry season), and 274 contigs with significant genotype-by-treatment interaction. The more productive genotype displayed a larger set of genes responding to water stress. Moreover, stress signal transduction seemed to involve different pathways in the two genotypes, suggesting that water shortage induces distinct cellular stress cascades. Similarly, the response of functional proteins also varied widely between genotypes: the most productive genotype decreased expression of genes related to photosystem, transport and secondary metabolism, whereas genes related to primary metabolism and cell organisation were over-expressed.

CONCLUSIONS

For the most productive genotype, the ability to express a broader set of genes in response to water availability appears to be a key characteristic in the maintenance of biomass growth during the dry season. Its strategy may involve a decrease of photosynthetic activity during the dry season associated with resources reallocation through major changes in the expression of primary metabolism associated genes. Further efforts will be needed to assess the adaptive nature of the genes highlighted in this study.

Dormancy cycling at the shoot apical meristem: transitioning between self-organization and self-arrest

To survive winter deciduous perennials of the temperate zones cease growth and acquire a cold-acclimated state. Timing of these events is guided by sensory systems in the leaves that register critical alterations in photoperiod. Growth cessation on its own is not sufficient to develop adequate freezing tolerance, which requires entry of the shoot apical meristem (SAM) into dormancy. To fully appreciate perennial dormancy as a precondition for cold acclimation it is necessary to assess how it is brought about in a timely fashion, what the nature of it is, and how it is released. Short day (SD) exposure results in growth cessation, bud set, dormancy establishment at the SAM, and a moderate to high level of freezing tolerance. Subsequent chilling releases the SAM from dormancy and enhances freezing tolerance further. Recent investigations indicate that dormancy is a state of self-arrest that is brought about by an enzyme-based system which disrupts the intrinsic signal network of the SAM. Release from this state requires a complimentary enzyme-based system that is preformed during SD and mobilized by chilling. These findings are in agreement with the paradigm of dormancy cycling, which defines the seasonal alternations at the SAM as transitions between states of self-organization and self-arrest. The success of this survival strategy is based on the adequate scheduling of a complex array of events. The appreciation is growing that this involves signal cascades that are, mutatis mutandis, also recruited in floral evocation in many annuals, including Arabidopsis. A heuristic model is presented of dormancy cycling at the SAM, which depicts crucial molecular and cellular events that drive the cycle.

Plant development: a TALE story

Plant development depends on the activity of a group of dividing cells called the meristem. Extensive genetic analyses have identified the major regulators of the shoot apical meristem (SAM), which control the development of all aerial organs. Among them, the three-amino-acid-loop-extension (TALE) class of homeoproteins has been shown to control meristem formation and/or maintenance, organ morphogenesis, organ position, and several aspects of the reproductive phase. This family contains the KNOTTED-like homeodomain (KNOX) and BEL1-like Homeodomain (BELL) members, which function as heterodimers. In this review, we have reported the functions of the TALE members throughout the Arabidopsis life cycle. Genetic analyses revealed a complex network, as TALE members exhibit both overlapping and antagonistic activities. The characterization of a new KNOX member (KNATM), which lacks a homeodomain and interacts with other members to modulate their activities, adds another layer of complexity to this network. While the mode of action of these transcription factors is still largely unknown, they have been implicated in the regulation of several hormonal pathways, providing a link between gene regulatory networks and signaling in the SAM.

Dark-induced hormone changes coincide with the resumption of light-inhibited shoot growth in Catasetum fimbriatum (Orchidaceae)

Catasetum fimbriatum plants cultivated in the absence of light exhibit continuous shoot growth leading to the formation of nodes and internodes. On the other hand, when these plants are incubated in the presence of light, shoot longitudinal growth is inhibited and pseudobulbs develop just below the shoot apical meristem. These facts provide evidence of a possible influence of light on mitotic cell division in the shoot apex as well as on pseudobulb initiation. The effects of light and dark on the interruption and/or maintenance of shoot apex mitotic activity and the subsequent formation of pseudobulbs in the sub-meristematic regions were investigated by means of histological and hormonal studies. The interruption of shoot apex development occurred around the 150th d of light incubation and seems to have resulted from the establishment of a strong storage sink in the region of the future pseudobulb, in detriment to the continuous activity of the shoot apical meristem. The reduced total cytokinins/IAA ratio in the apex, mainly due to high levels of IAA, could be a key factor in the interruption of cell divisions. Transfer to the dark brings about the resumption of shoot apex development of plants through the re-entrance of cells in the cell cycle which coincides with a significant increase in the total cytokinins/IAA ratio.

Auxin: a major regulator of organogenesis

Plant development is characterized by the continuous initiation of tissues and organs. The meristems, which are small stem cell populations, are involved in this process. The shoot apical meristem produces lateral organs at its flanks and generates the growing stem. These lateral organs are arranged in a stereotyped pattern called phyllotaxis. Organ initiation in the peripheral zone of the meristem involves accumulation of the plant hormone auxin. Auxin is transported in a polar way by influx and efflux carriers located at cell membranes. Polar localization of the PIN1 efflux carrier in meristematic cells generates auxin concentration gradients and PIN1 localization depends, in turn, on auxin gradients: this feedback loop generates a dynamic auxin distribution which controls phyllotaxis. Furthermore, PIN-dependent local auxin gradients represent a common module for organ initiation, in the shoot and in the root.

From genes to shape: understanding the control of morphogenesis at the shoot meristem in higher plants using systems biology

The shoot apical meristem is a population of stem cells which controls the initiation of leaves, flowers and branches during the entire life of the plant. Although we have gained significant new insight in the nature of the genetic networks and cellular processes that control meristem function, major questions have remained unsolved. It has been difficult, for instance, to define the precise role of genetic determinants in controlling morphogenesis and the control of shape is currently a major and largely unresolved issue in plant biology. This is a difficult task, notably because it is close to impossible to predict the activity of a single gene, in a context where thousands of genes interact. Systems biology has emerged as a powerful tool to address this type of issue. Systems biology analyses processes such as plant development at different scales, describing not only the properties of individual cells but also their interactions. The complexity of the information involved is such, that it cannot be understood and integrated on a purely intuitive basis. For this reason, building on the acquisition of quantitative data, computer models have become more and more important. The first models have begun to reproduce gene network behaviours and dynamical shape changes, providing new insight in the control of morphogenesis.

Turning a plant tissue into a living cell froth through isotropic growth

The forms resulting from growth processes are highly sensitive to the nature of the driving impetus, and to the local properties of the medium, in particular, its isotropy or anisotropy. In turn, these local properties can be organized by growth. Here, we consider a growing plant tissue, the shoot apical meristem of Arabidopsis thaliana. In plants, the resistance of the cell wall to the growing internal turgor pressure is the main factor shaping the cells and the tissues. It is well established that the physical properties of the walls depend on the oriented deposition of the cellulose microfibrils in the extracellular matrix or cell wall; this order is correlated to the highly oriented cortical array of microtubules attached to the inner side of the plasma membrane. We used oryzalin to depolymerize microtubules and analyzed its influence on the growing meristem. This had no short-term effect, but it had a profound impact on the cell anisotropy and the resulting tissue growth. The geometry of the cells became similar to that of bubbles in a soap froth. At a multicellular scale, this switch to a local isotropy induced growth into spherical structures. A theoretical model is presented in which a cellular structure grows through the plastic yielding of its walls under turgor pressure. The simulations reproduce the geometrical properties of a normal tissue if cell division is included. If not, a "cell froth" very similar to that observed experimentally is obtained. Our results suggest strong physical constraints on the mechanisms of growth regulation.

Stem fasciated, a recessive mutation in sunflower (Helianthus annuus), alters plant morphology and auxin level

BACKGROUND AND AIMS

Plant lateral organs such as leaves arise from a group of initial cells within the flanks of the shoot apical meristem (SAM). Alterations in the initiation of lateral organs are often associated with changes in the dimension and arrangement of the SAM as well as with abnormal hormonal homeostasis. A mutation named stem fasciated (stf) that affects various aspects of plant development, including SAM shape and auxin level, was characterized in sunflower (Helianthus annuus).

METHODS

F1, F2 and F3 generations were obtained through reciprocal crosses between stf and normal plants. For the genetic analysis, a chi2 test was used. Phenotypic observations were made in field-grown and potted plants. A histological analysis of SAM, hypocotyl, epicotyl, stem and root apical meristem was also conducted. To evaluate the level of endogenous indole-3-acetic acid (IAA), a capillary gas chromatography-mass spectrometry-selected ion monitoring analysis was performed.

KEY RESULTS

stf is controlled by a single nuclear recessive gene. stf plants are characterized by a dramatically increased number of leaves and vascular bundles in the stem, as well as by a shortened plastochron and an altered phyllotaxis pattern. By histological analysis, it was demonstrated that the stf phenotype is related to an enlarged vegetative SAM. Microscopy analysis of the mutant's apex also revealed an abnormal enlargement of nuclei in both central and peripheral zones and a disorganized distribution of cells in the L2 layer of the central zone. The stf mutant showed a high endogenous free IAA level, whereas auxin perception appeared normal.

CONCLUSIONS

The observed phenotype and the high level of auxin detected in stf plants suggest that the STF gene is necessary for the proper initiation of primordia and for the establishment of a phyllotactic pattern through control of both SAM arrangement and hormonal homeostasis.

Stem fasciated, a recessive mutation in sunflower (Helianthus annuus), alters plant morphology and auxin level

BACKGROUND AND AIMS

Plant lateral organs such as leaves arise from a group of initial cells within the flanks of the shoot apical meristem (SAM). Alterations in the initiation of lateral organs are often associated with changes in the dimension and arrangement of the SAM as well as with abnormal hormonal homeostasis. A mutation named stem fasciated (stf) that affects various aspects of plant development, including SAM shape and auxin level, was characterized in sunflower (Helianthus annuus).

METHODS

F1, F2 and F3 generations were obtained through reciprocal crosses between stf and normal plants. For the genetic analysis, a chi2 test was used. Phenotypic observations were made in field-grown and potted plants. A histological analysis of SAM, hypocotyl, epicotyl, stem and root apical meristem was also conducted. To evaluate the level of endogenous indole-3-acetic acid (IAA), a capillary gas chromatography-mass spectrometry-selected ion monitoring analysis was performed.

KEY RESULTS

stf is controlled by a single nuclear recessive gene. stf plants are characterized by a dramatically increased number of leaves and vascular bundles in the stem, as well as by a shortened plastochron and an altered phyllotaxis pattern. By histological analysis, it was demonstrated that the stf phenotype is related to an enlarged vegetative SAM. Microscopy analysis of the mutant's apex also revealed an abnormal enlargement of nuclei in both central and peripheral zones and a disorganized distribution of cells in the L2 layer of the central zone. The stf mutant showed a high endogenous free IAA level, whereas auxin perception appeared normal.

CONCLUSIONS

The observed phenotype and the high level of auxin detected in stf plants suggest that the STF gene is necessary for the proper initiation of primordia and for the establishment of a phyllotactic pattern through control of both SAM arrangement and hormonal homeostasis.

ZmPIN1a and ZmPIN1b encode two novel putative candidates for polar auxin transport and plant architecture determination of maize

Shoot apical meristems produce organs in a highly stereotypic pattern that involves auxin. Auxin is supposed to be actively transported from cell to cell by influx (AUXIN/LIKE AUXIN proteins) and efflux (PIN-FORMED proteins) membrane carriers. Current hypotheses propose that, at the meristem surface, PIN proteins create patterns of auxin gradients that, in turn, create patterns of gene expression and morphogenesis. These hypotheses are entirely based on work in Arabidopsis (Arabidopsis thaliana). To verify whether these models also apply to other species, we studied the behavior of PIN proteins during maize (Zea mays) development. We identified two novel putative orthologs of AtPIN1 in maize and analyzed their expression pattern during development. The expression studies were complemented by immunolocalization studies using an anti-AtPIN1 antibody. Interestingly, the maize proteins visualized by this antibody are almost exclusively localized in subepidermal meristematic layers. Both tassel and ear were characterized by a compact group of cells, just below the surface, carrying PIN. In contrast to or to complement what was shown in Arabidopsis, these results point to the importance of internally localized cells in the patterning process. We chose the barren inflorescence2 (bif2) maize mutant to study the role of auxin polar fluxes in inflorescence development. In severe alleles of bif2, the tassel and the ear present altered ZmPIN1a and ZmPIN1b protein expression and localization patterns. In particular, the compact groups of cells in the tassel and ear of the mutant were missing. We conclude that BIF2 is important for PIN organization and could play a role in the establishment of polar auxin fluxes in maize inflorescence, indirectly modulating the process of axillary meristem formation and development.

Molecular control of stem cell maintenance in shoot apical meristem

Sustained post-embryonic organ initiation and development in plants depends on coordinating the formation and differentiation of pluripotent stem cells in apical meristems. Transcriptional regulation and intercellular signalling appear to play key roles in this coordination process. Here we discuss the current knowledge about the molecular regulation of stem cell maintenance in the shoot apical meristem and recent attempts to delineate the molecular signatures of "stemness" in flowering plants. We also outline contemporary molecular approaches for deciphering the process of stem cell renewal in the shoot apical meristem.

From leaf to flower: revisiting Goethe's concepts on the ¨metamorphosis¨ of plants

Goethe’s seminal scientific work, Versuch die Metamorphose der Pflanzen zu erklaren (An Attempt to Interpret the Metamorphosis of Plants) dated from 1790, has created the foundations for many domains of modern plant biology. The archetypal leaf concept, which considers floral organs as modified leaves, besides being the best known has been proven true, following the description of the ABC molecular model of floral organ identity determination during the last decade. Here we analyze the whole theoretical frame of Goethe’s 1790 publication and present two previously misconsidered aspects of this work: The "refinement of the sap" concept as a directional principle and the "cycles of contractions and expansions" as cycles of differential determination of the shoot apical meristem. The reinterpretation of these concepts are in line with the modern view that molecular networks integrate both environmental and endogenous cues and regulate plant development. This reassessment also helps to elaborate a theoretical frame that considers the evolutionary conservation of the molecular mechanisms that regulate plant development.

AtDEK1 is essential for specification of embryonic epidermal cell fate

The specification of epidermal (L1) identity occurs early during plant embryogenesis. Here we show that, in Arabidopsis, AtDEK1 encodes a key component of the embryonic L1 cell-layer specification pathway. Loss of AtDEK1 function leads to early embryo lethality characterized by a severe loss of cell organization in the embryo proper and abnormal cell divisions within the suspensor. Markers for L1 identity, ACR4 and ATML1, are not expressed in homozygous mutant embryos. In order to clarify the function of AtDEK1 further, an RNAi knockdown approach was used. This allowed embryos to partially complete embryogenesis before losing AtDEK1 activity. Resulting seedlings showed a specific loss of epidermal cell identity within large portions of the cotyledons. In addition, meristem structure and function was systematically either reduced or entirely lost. AtDEK1 expression is not restricted to the L1 epidermal cell layer at any stage in development. This is consistent with AtDEK1 playing an upstream role in the continuous generation or interpretation of positional information required for epidermal specification. Our results not only identify a specific role for AtDEK1 during embryogenesis, but underline the potential key importance of L1 specification at the globular stage for subsequent progression through embryogenesis.

Regulation of meristem activity by chromatin remodelling

The continuity and plasticity of plant development rely on the regulation of meristem activity in response to endogenous and environmental signals. Many plant development regulators involved in meristem function are transcription factors or signalling molecules. In the past few years, the role of chromatin remodelling in programming, maintaining or resetting specific gene expression profiles in subsequent cell generations has been shown to be crucial in plant development. Here, we summarize plant chromatin-remodelling factors required to regulate shoot apical meristem activity, particularly its maintenance during organogenesis and transitions between distinct developmental phases.

Comparative analysis of axillary and floral meristem development

Axillary and floral meristems are shoot meristems that initiate postembryonically. In Arabidopsis, axillary meristems give rise to branches during vegetative development while floral meristems give rise to flowers during reproductive development. This review compares the development of these meristems from their initiation at the shoot apical meristem up to the establishment of their specific developmental fates. Axillary and floral meristems originate from lateral primordia that form at flanks of the shoot apical meristem. Initial development of vegetative and reproductive primordia are similar, resulting in the formation of a morphologically defined primordium partitioned into adaxial and abaxial domains. The adaxial primordial domain is competent to form a meristem, while the abaxial domain correlates with the formation of a leaf. This review proposes that all primordia partition into domains competent to form the meristem and the leaf. According to this model, a vegetative primordium develops as leaf-bias while a reproductive primordium develops as meristem-bias.Key words: SHOOTMERISTEMLESS, LATERAL SUPPRESSOR, AINTEGUMENTA, adaxial primordial domain, abaxial primordial domain, shoot morphogenesis.

INCOMPOSITA: a MADS-box gene controlling prophyll development and floral meristem identity in Antirrhinum

INCOMPOSITA (INCO) is a MADS-box transcription factor and member of the functionally diverse StMADS11 clade of the MADS-box family. The most conspicuous feature of inco mutant flowers are prophylls initiated prior to first whorl sepals at lateral positions of the flower primordium. The developing prophylls physically interfere with subsequent floral organ development that results in aberrant floral architecture. INCO, which is controlled by SQUAMOSA, prevents prophyll formation in the wild type, a role that is novel among MADS-box proteins, and we discuss evolutionary implications of this function. Overexpression of INCO or SVP, a structurally related Arabidopsis MADS-box gene involved in the negative control of Arabidopsis flowering time,conditions delayed flowering in transgenic plants, suggesting that SVP and INCO have functions in common. Enhanced flowering of squamosa mutants in the inco mutant background corroborates this potential role of INCO as a floral repressor in Antirrhinum. One further,hitherto hidden, role of INCO is the positive control of Antirrhinumfloral meristem identity. This is revealed by genetic interactions between inco and mutants of FLORICAULA, a gene that controls the inflorescence to floral transition, together with SQUAMOSA. The complex regulatory and combinatorial relations between INCO, FLORICAULA and SQUAMOSA are summarised in a model that integrates observations from molecular studies as well as analyses of expression patterns and genetic interactions.

A high-resolution transcript profile across the wood-forming meristem of poplar identifies potential regulators of cambial stem cell identity

Plant growth is the result of cell proliferation in meristems, which requires a careful balance between the formation of new tissue and the maintenance of a set of undifferentiated stem cells. Recent studies have provided important information on several genetic networks responsible for stem cell maintenance and regulation of cell differentiation in the apical meristems of shoots and roots. Nothing, however, is known about the regulatory networks in secondary meristems like the vascular cambium of trees. We have made use of the large size and highly regular layered organization of the cambial meristem to create a high-resolution transcriptional map covering 220 microm of the cambial region of aspen (Populus tremula). Clusters of differentially expressed genes revealed substantial differences in the transcriptomes of the six anatomically homogenous cell layers in the meristem zone. Based on transcriptional and anatomical data, we present a model for the position of the stem cells and the proliferating mother cells in the cambial zone. We also provide sets of marker genes for different stages of xylem and phloem differentiation and identify potential regulators of cambial meristem activity. Interestingly, analysis of known regulators of apical meristem development indicates substantial similarity in regulatory networks between primary and secondary meristems.

Introduction

This is an auspicious time for plant cell biology. We have the recent genome sequence of the model plant Arabidopsis, together with powerful new sets of tools that include functional genomics and the dynamic imaging of green fluorescent protein (GFP)–conjugated proteins by confocal microscopy. We are well placed indeed for uncovering the ways in which all genes act out their roles within individual cells. It is the combinatorial properties of these individual cells that underpin both the higherorder processes of plant growth and development and of plant evolution. At a crucial level in the hierarchy of explanations and structures, between the linear DNA sequence at one extreme and whole plant phenomena at the other, lies the cell. It is to this cellular level, to the irreducible unit of life, that this collection of papers is addressed, literally ‘between genome and plant’. Most of the major conceptual advances in our understanding of cell biology have come from work with animal and fungal systems. The ways in which membranes create dynamic compartments within cells and the role of the cytoskeleton in providing an ordering framework are familiar from the textbooks. But plant cells have special features.