Cell lineage, cell signaling and the control of plant morphogenesis

Flow cytometric analysis of somaclonal variation in lineages of Hosta sports detects polyploidy and aneuploidy chimeras

Somaclonal variation of some 124 specially selected cultivars of Hosta Tratt. (Hostaceae) was investigated. Nuclear DNA contents (2C-value) were measured by flow cytometry of leaves and roots of L1, L2 and L3 layers derived from apical meristems. These values were then converted to inferred ploidies by comparing the measured 2C-values and ploidy with those of the parent plant. During tissue-culture propagation, on occasion diploid (L1-L2-L3 = 2-2-2) hostas give rise to polyploids, such as fully tetraploids (4-4-4), and periclinal chimeras, such as partial tetraploids (4-2-2). Continual propagation can result in partial tetraploids becoming full tetraploids. Nuclear DNA of some diploids increased with incomplete chromosome sets resulting in fully aneuploids, such as hostas with a DNA ploidy of L1-L2-L3 = 2.5-2.5-2.5 and 3.7-3.7-3.7, and even in aneuploid periclinal chimeras, such as L1-L2-L3 = 2.5-2-2 and 3.8-2-2. The polyploidy of L1, irrespective of the ploidy of L2 and L3, is found to mainly determine the thickness of leaves. Also the higher the ploidy of L1, the wider and more intense in color is the leaf margin. The measurements of Hosta cultivars and their lineages of sports show that chromosome losses or gains are an important source of new cultivars. The complexity of chromosomal distribution in lineages of several Hosta cultivars is discussed.

Morphogengineering roots: comparing mechanisms of morphogen gradient formation

BACKGROUND

In developmental biology, there has been a recent focus on the robustness of morphogen gradients as possible providers of positional information. It was shown that functional morphogen gradients present strong biophysical constraints and lack of robustness to noise. Here we explore how the details of the mechanism which underlies the generation of a morphogen gradient can influence those properties.

RESULTS

We contrast three gradient-generating mechanisms, (i) a source-decay mechanism; and (ii) a unidirectional transport mechanism; and (iii) a so-called reflux-loop mechanism. Focusing on the dynamics of the phytohormone auxin in the root, we show that only the reflux-loop mechanism can generate a gradient that would be adequate to supply functional positional information for the Arabidopsis root, for biophysically reasonable kinetic parameters.

CONCLUSIONS

We argue that traits that differ in spatial and temporal time-scales can impose complex selective pressures on the mechanism of morphogen gradient formation used for the development of the particular organism.

To gate, or not to gate: regulatory mechanisms for intercellular protein transport and virus movement in plants

Cell-to-cell signal transduction is vital for orchestrating the whole-body physiology of multi-cellular organisms, and many endogenous macromolecules, proteins, and nucleic acids function as such transported signals. In plants, many of these molecules are transported through plasmodesmata (Pd), the cell wall-spanning channel structures that interconnect plant cells. Furthermore, Pd also act as conduits for cell-to-cell movement of most plant viruses that have evolved to pirate these channels to spread the infection. Pd transport is presumed to be highly selective, and only a limited repertoire of molecules is transported through these channels. Recent studies have begun to unravel mechanisms that actively regulate the opening of the Pd channel to allow traffic. This macromolecular transport between cells comprises two consecutive steps: intracellular targeting to Pd and translocation through the channel to the adjacent cell. Here, we review the current knowledge of molecular species that are transported though Pd and the mechanisms that control this traffic. Generally, Pd traffic can occur by passive diffusion through the trans-Pd cytoplasm or through the membrane/lumen of the trans-Pd ER, or by active transport that includes protein-protein interactions. It is this latter mode of Pd transport that is involved in intercellular traffic of most signal molecules and is regulated by distinct and sometimes interdependent mechanisms, which represent the focus of this article.

Transcription profile analysis reveals that zygotic division results in uneven distribution of specific transcripts in apical/basal cells of tobacco

BACKGROUND

Asymmetric zygotic division in higher plants results in the formation of an apical cell and a basal cell. These two embryonic cells possess distinct morphologies and cell developmental fates. It has been proposed that unevenly distributed cell fate determinants and/or distinct cell transcript profiles may be the underlying reason for their distinct fates. However, neither of these hypotheses has convincing support due to technical limitations.

METHODOLOGY/PRINCIPAL FINDINGS

Using laser-controlled microdissection, we isolated apical and basal cells and constructed cell type-specific cDNA libraries. Transcript profile analysis revealed difference in transcript composition. PCR and qPCR analysis confirmed that transcripts of selected embryogenesis-related genes were cell-type preferentially distributed. Some of the transcripts that existed in zygotes were found distinctly existed in apical or basal cells. The cell type specific de novo transcription was also found after zygotic cell division.

CONCLUSIONS/SIGNIFICANCE

Thus, we found that the transcript diversity occurs between apical and basal cells. Asymmetric zygotic division results in the uneven distribution of some embryogenesis related transcripts in the two-celled proembryos, suggesting that a differential distribution of some specific transcripts in the apical or basal cells may involve in guiding the two cell types to different developmental destinies.

High frequency targeted mutagenesis in Arabidopsis thaliana using zinc finger nucleases

We report here an efficient method for targeted mutagenesis of Arabidopsis genes through regulated expression of zinc finger nucleases (ZFNs)-enzymes engineered to create DNA double-strand breaks at specific target loci. ZFNs recognizing the Arabidopsis ADH1 and TT4 genes were made by Oligomerized Pool ENgineering (OPEN)-a publicly available, selection-based platform that yields high quality zinc finger arrays. The ADH1 and TT4 ZFNs were placed under control of an estrogen-inducible promoter and introduced into Arabidopsis plants by floral-dip transformation. Primary transgenic Arabidopsis seedlings induced to express the ADH1 or TT4 ZFNs exhibited somatic mutation frequencies of 7% or 16%, respectively. The induced mutations were typically insertions or deletions (1-142 bp) that were localized at the ZFN cleavage site and likely derived from imprecise repair of chromosome breaks by nonhomologous end-joining. Mutations were transmitted to the next generation for 69% of primary transgenics expressing the ADH1 ZFNs and 33% of transgenics expressing the TT4 ZFNs. Furthermore, approximately 20% of the mutant-producing plants were homozygous for mutations at ADH1 or TT4, indicating that both alleles were disrupted. ADH1 and TT4 were chosen as targets for this study because of their selectable or screenable phenotypes (adh1, allyl alcohol resistance; tt4, lack of anthocyanins in the seed coat). However, the high frequency of observed ZFN-induced mutagenesis suggests that targeted mutations can readily be recovered by simply screening progeny of primary transgenic plants by PCR and DNA sequencing. Taken together, our results suggest that it should now be possible to obtain mutations in any Arabidopsis target gene regardless of its mutant phenotype.

Transcriptome profiling of maize anthers using genetic ablation to analyze pre-meiotic and tapetal cell types

Oligonucleotide arrays were used to profile gene expression in dissected maize anthers at four stages: after-anther initiation, at the rapid mitotic proliferation stage, pre-meiosis, and meiotic prophase I. Nearly 9200 sense and antisense transcripts were detected, with the most diverse transcriptome present at the pre-meiotic stage. Three male-sterile mutants lacking a range of normal cell types resulting from a temporal progression of anther failure were compared with fertile siblings at equivalent stages by transcription profiles. The msca1 mutant has the earliest visible phenotype, develops none of the normal anther cell types and exhibits the largest deviation from fertile siblings. The mac1 mutant has an excess of archesporial derivative cells and lacks a tapetum and middle layer, resulting in moderate transcriptional deviations. The ms23 mutant lacks a differentiated tapetum and shows the fewest differences from fertile anthers. By combining the data sets from the comparisons between individual sterile and fertile anthers, candidate genes predicted to play important roles during maize anther development were assigned to stages and to likely cell types. Comparative analyses with a data set of anther-specific genes from rice highlight remarkable quantitative similarities in gene expression between these two grasses.

Abscisic acid rescues the root meristem defects of the Medicago truncatula latd mutant

The LATD gene of the model legume, Medicago truncatula, is required for the normal function of three meristems, i.e. the primary root, lateral roots and nitrogen-fixing nodules. In latd mutants, primary root growth eventually arrests, resulting in a disorganized root tip lacking a presumptive meristem and root cap columella cells. Lateral root organs are more severely affected; latd lateral roots and nodules arrest immediately after emerging from the primary root, and reveal a lack of organization. Here we show that the plant hormone, abscisic acid (ABA), can rescue the latd root, but not nodule, meristem defects. Growth on ABA is sufficient to restore formation of small, cytoplasm-rich cells in the presumptive meristem region, rescue meristem organization and root growth and formation of root cap columella cells. In contrast, inhibition of ethylene synthesis or signaling fails to restore latd primary root growth. We find that latd mutants have normal levels of ABA, but exhibit reduced sensitivity to the hormone in two other ABA-dependent processes: seed germination and stomatal closure. Together, these observations demonstrate that the latd mutant is defective in the ABA response and indicate a role for LATD-dependent ABA signaling in M. truncatula root meristem function.

A map of KNAT gene expression in the Arabidopsis root

Homeodomain proteins are key regulators of patterning during the development of animal and plant body plans. Knotted1-like TALE homeodomain proteins have been found to play important roles in the development of the Arabidopsis shoot apical meristem and are part of a complex regulatory network of protein interactions. We have investigated the possible role of the knotted1-like genes KNAT1, KNAT3, KNAT4, and KNAT5 in Arabidopsis root development. Root growth is indeterminate, and the organ shows distinct zones of cell proliferation, elongation and differentiation along its longitudinal axis. Here we show that KNAT1, KNAT3, KNAT4 and KNAT5 show cell type specific expression patterns in the Arabidopsis root. Moreover, they are expressed in different spatially restricted patterns along the longitudinal root axis and in lateral root primordia. Hormones play an important role in maintenance of root growth, and we have studied their effect on KNAT gene expression. We show that KNAT3 expression is repressed by moderate levels of cytokinin. In addition, we show that the subcellular localization of KNAT3 and KNAT4 is regulated, indicating post-translational control of the activities of these transcription factors. The regulated expression of KNAT1, KNAT3, KNAT4 and KNAT5 within the Arabidopsis root suggests a role for these genes in root development. Our data provide the first systematic survey of KNAT gene expression in the Arabidopsis root.

Drosophila neuroblast 7-3 cell lineage: a model system for studying programmed cell death, Notch/Numb signaling, and sequential specification of ganglion mother cell identity

Cell lineage studies provide an important foundation for experimental analysis in many systems. Drosophila neural precursors (neuroblasts) sequentially generate ganglion mother cells (GMCs), which generate neurons and/or glia, but the birth order, or cell lineage, of each neuroblast is poorly understood. The best-characterized neuroblast is NB7-3, in which GMC-1 makes the EW1 serotonergic interneuron and GW motoneuron; GMC-2 makes the EW2 serotonergic interneuron and a programmed cell death; and GMC-3 gives rise to the EW3 interneuron. However, the end of this lineage has not been determined. Here, we use positively marked genetic clones, bromodeoxyuridine (BrdU) labeling, mutations that affect Notch signaling, and antibody markers to further define the end of the cell lineage of NB7-3. We provide evidence that GMC-3 directly differentiates into EW3 and that the sibling neuroblast undergoes programmed cell death. Our results confirm and extend previous work on the early portion of the NB7-3 lineage (Novotny et al. [2002] Development 129:1027-1036; Lundell et al. [ 2003] Development 130:4109-4121).

Targeted mutagenesis using zinc-finger nucleases in Arabidopsis

Targeted mutagenesis is an essential tool of reverse genetics that could be used experimentally to investigate basic plant biology or modify crop plants for improvement of important agricultural traits. Although targeted mutagenesis is routine in several model organisms including yeast and mouse, efficient and widely usable methods to generate targeted modifications in plant genes are not currently available. In this study we investigated the efficacy of a targeted-mutagenesis approach based on zinc-finger nucleases (ZFNs). In this procedure, ZFNs are used to generate double-strand breaks at specific genomic sites, and subsequent repair produces mutations at the break site. To determine whether ZFNs can cleave and induce mutations at specific sites within higher plant genomes, we introduced a construct carrying both a ZFN gene, driven by a heat-shock promoter, and its target into the Arabidopsis genome. Induction of ZFN expression by heat shock during seedling development resulted in mutations at the ZFN recognition sequence at frequencies as high as 0.2 mutations per target. Of 106 ZFN-induced mutations characterized, 83 (78%) were simple deletions of 1-52 bp (median of 4 bp), 14 (13%) were simple insertions of 1-4 bp, and 9 (8%) were deletions accompanied by insertions. In 10% of induced individuals, mutants were present in the subsequent generation, thus demonstrating efficient transmission of the ZFN-induced mutations. These data indicate that ZFNs can form the basis of a highly efficient method for targeted mutagenesis of plant genes.

High-resolution boundary analysis during Arabidopsis thaliana flower development

We report a comparative analysis of cell proliferation patterns during Arabidopsis flower development. Cell division was evaluated by a direct method, i.e. the 5-bromo-2'-deoxyuridine (BrdU) incorporation/immunodetection procedure. BrdU patterns in wild-type plants were correlated with the expression profiles of both several cell cycle genes involved in the control of the G(1)/S transition and cell cycle-related repressor genes, MSI4 and MSI5, encoding WD-repeat proteins. To evaluate how proliferation patterns arise with respect to boundaries and vice versa, the expression of a boundary gene, CUP SHAPED COTYLEDON (CUC)2, was determined. Combining these approaches, we demonstrate that boundaries between inflorescence and floral meristems and between floral whorls are narrow bands of non-dividing cells. In addition, we show that negative and positive regulators of cell proliferation are simultaneously and continuously expressed in dividing meristematic domains, being excluded from boundary cells. Finally, BrdU incorporation and CUC2 in situ hybridisation patterns were analysed in two mutant backgrounds, agamous (ag)-1 and superman (sup)-1, in order to assess changes in boundary establishment and different levels of indeterminacy under conditions of altered proliferation at the floral meristem centre.

The globby1-1 (glo1-1) mutation disrupts nuclear and cell division in the developing maize seed causing alterations in endosperm cell fate and tissue differentiation

Cereal endosperm tissues account for most of the world's calorific intake, yet the regulation of monocot seed development remains poorly understood. The maize endosperm originates with a series of free-nuclear divisions, followed by cellularisation and subsequent formation of a range of functional cellular domains. We describe the isolation and characterisation of a mutation that induces aberrant globular embryo and endosperm morphology, globby1-1 (glo1-1). Our data indicate that glo1-1 plays a role in nuclear division and cytokinesis in the developing seed. Pattern formation in the embryo is severely impaired with development arresting at premature stages, while in the endosperm, the effects of the glo1-1 mutation are manifest at the free-nuclear or syncytial stage. During cellularisation, and at later stages of development, aberrant cell division and localised domains of cell proliferation are apparent in glo1-1 endosperms. As a consequence, cell fate acquisition and subsequent differentiation of endosperm tissues are affected to varying degrees of severity. To date, it has been hypothesised that BETL cell fate is specified in the syncytium and that cell files subsequently develop in response to a gradient of signal(s) derived from the maternal pedicel region. Based on our findings, however, we propose that specification of BETL cells is an irreversible event that occurs within a narrow window of syncytial development, and that BETL cell identity is subsequently inherited in a lineage-dependent manner. Additionally, our data suggest that acquisition of aleurone cell fate does not solely rely upon signalling from the maternal surrounding tissue to the periphery of the endosperm, as previously thought, but that other factor(s) present within the endosperm are involved.

Brassinosteroids. Plant counterparts to animal steroid hormones?

Brassinosteroids are polyhydroxylated derivatives of common plant membrane sterols such as campesterol. They occur throughout the plant kingdom and have been shown by genetic and biochemical analyses to be essential for normal plant growth and development. Numerous reviews have detailed the recent progress in our understanding of the biosynthesis, physiological responses, and molecular modes of action of brassinosteroids. It is clear that like their animal steroid counterparts, brassinosteroids have a defined receptor, can regulate the expression of specific genes, and can orchestrate complex physiological responses involved in growth. This review summarizes the current status of BR research, pointing out where appropriate the similarities and differences between the mechanism of action of brassinosteroids and the more thoroughly studied animal steroid hormones.

Origins of differentiation via phenotypic plasticity

How cell types of multicellular organisms came to be differentiated is still an open issue. Here I offer a model that posits that the origins of some cell differentiation patterns were originally passive outcomes of environmental effects. As cells' contact with the external environment was diminished, their patterns of gene expression were altered, due to changes in concentrations of externally supplied substances. Later, as multicellular growth continued, the relationships of cell layers to each other shifted, producing concentration gradients of signaling molecules. These gradients emanated both from the external cell layer toward the inside and from internal cell layers to adjacent layers. In this scenario then, differentiation arose initially as a by-product of the changing patterns of gene expression and of the complex mixtures and changing concentrations of substances passing among layers. Subsequent selection would operate to stabilize the expression patterns in those cell layers whose phenotypes provide a fitness advantage to the organism.