How plants split hairs.

The unicellular three-branched trichomes, or 'hairs', of Arabidopsis provide a model system for studying cell morphogenesis in plants. Recent results, including the characterization of a newly identified mutant with multicellular trichomes, have led to a new view of how trichome morphogenesis might be controlled.

Cell growth: how to grow and where to grow.

Root hairs provide a model system for studying tip growth in plants. The recent cloning of genes required for tip growth has shed new light on the link between ionic regulation, cell wall assembly and the cytoskeleton in cell growth.

CPR5 is involved in cell proliferation and cell death control and encodes a novel transmembrane protein.

Plants often respond to pathogens by sacrificing cells at the infection site. This type of programmed cell death is mimicked by the constitutive pathogene response5 (cpr5) mutant in Arabidopsis in the absence of pathogens, suggesting a role for CPR5 in programmed cell death control. The analysis of the cellular phenotypes of two T-DNA-tagged cpr5 alleles revealed an additional role for CPR5 in the regulation of endoreduplication and cell division. In cpr5 mutant trichomes, endoreduplication cycles stop after two rounds instead of four, and trichome cells have fewer branches than normal. Eventually, cpr5 trichomes die, the nucleus disintegrates, and the cell collapses. Similarly, leaf growth stops earlier than in wild-type, and, frequently, regions displaying spontaneous cell death are observed. The cloning of the CPR5 gene revealed a novel putative transmembrane protein with a cytosolic domain containing a nuclear-targeting sequence. The dual role of CPR5 in cell proliferation and cell death control suggests a regulatory link between these two processes.

Genetic Evidence for a Long-Range Activity That Directs Pollen Tube Guidance in Arabidopsis.

The fertilization process of plants is governed by different kinds of cell-cell interactions. In higher plants, these interactions are required both for recognition of the pollen grain by the female reproductive system and to direct the growth of the pollen tube inside the ovary. Despite many years of study, the signaling mechanisms that guide the pollen tube toward its target, the ovule, are largely unknown. Two distinct types of principles, mechanical and chemotropic, have been suggested to account for the directed growth of the pollen tube. The first of these two types of models implies that the guidance of the pollen tube depends on the architecture and chemical properties of the female reproductive tissues, whereas the latter suggests that the ovule provides a signal for the target-directed growth of the pollen tube. To examine such a role for the ovules, we analyzed the growth path of pollen tubes in mutants defective in ovule development in Arabidopsis. The results presented here provide unique in vivo evidence for an ovule-derived, long-range activity controlling pollen tube guidance. A morphological comparison of the ovule mutants used in this study indicates that within the ovule, the haploid embryo sac plays an important role in this long-range signaling process.

Plant cells - young at heart?

Dolly has become a synonym for one of the greatest breakthroughs in animal reproductive biology: the regeneration of a whole mammal from a somatic cell nucleus. The equivalent experiments in plants - the regeneration of whole plants from single differentiated cells - are comparatively easy. Does this apparent difference in the developmental potential of animal and plant somatic cells reflect mechanistic differences in the regulation and maintenance of their respective cell differentiation?

Endoreduplication and development: rule without dividing?

Endoreduplication, a strategy to amplify nuclear DNA without cell division, is very common but poorly understood in plants. Recent findings in Drosophila provide a first picture of the molecular mechanism, which appears to be conserved between plants and animals. In Arabidopsis, the study of trichomes, leaf epidermis and hypocotyl cells sheds new light on the developmental regulation of this process, and its relation to cell expansion.

Pattern formation and cell differentiation: trichomes in Arabidopsis as a genetic model system.

Arabidopsis trichomes are single-celled hairs that originate from epidermal cells and are distributed regularly on most aerial body parts. During the last decade, trichome formation in Arabidopsis has been established as a genetic and molecular model system to study various general developmental and cellular mechanisms. This review summarizes progress in the investigation of several aspects of trichome development: the spatial regulation of cell fate determination, the regulation of cell differentiation in response to exogenous signals and plant hormones, and the regulation of endoreplication, cell growth, and cell morphogenesis.

Differential regulation of target genes by different alleles of the segmentation gene hunchback in Drosophila.

hunchback (hb) is a key regulatory gene in the early segmentation gene hierarchy of Drosophila. It codes for a transcription factor of the Cys2-His2 zinc finger type and shows two separate zinc finger domains in its coding region. hb forms a morphogenetic gradient in the middle of the embryo that is required for setting the spatial boundaries of several target genes. We have analyzed the molecular lesions found in the different hb alleles and have studied the differential effects of these alleles on a number of such target genes. We find that in mutants in which the HB protein lacks a functional second finger domain, the regulation of the target genes Krüppel (Kr) and knirps (kni) is differentially affected. While this domain is required for the correct regulation of Kr, it is not necessary for the repression of kni. Furthermore, mutations affecting this domain lead to a decreased protein stability. The integration of the expression pattern of target genes was found to be distorted in a second class of mutants between the two finger domains which lead to gain of function or neomorphic phenotypes. The effects of these mutations were studied in detail and it was found that they fall into two classes, the first one interfering with the function of the maternal hb product, the second leading to a delayed segmentation. The function of the latter class appears to be linked to the secondary expression of hb in the parasegment 4 (PS4) stripe at blastoderm stage.

Trichome cell growth in Arabidopsis thaliana can be derepressed by mutations in at least five genes.

Leaf trichomes in Arabidopsis are unicellular epidermal hairs with a branched morphology. They undergo successive endoreduplication rounds early during cell morphogenesis. Mutations affecting trichome nuclear DNA content, such as triptychon or glabra3, alter trichome branching. We isolated new mutants with supernumerary trichome branches, which fall into three unlinked complementation groups: KAKTUS and the novel loci, POLYCHOME and RASTAFARI. They map to chromosomes IV, II, and V, respectively. The trichomes of these mutants presented an increased DNA content, although to a variable extent. The spindly-5 mutant, which displays a constitutive gibberellin response, also produces overbranched trichomes containing more nuclear DNA. We analyzed genetic interactions using double mutants and propose that two independent pathways, defined by SPINDLY and TRIPTYCHON, act to limit trichome growth. KAKTUS and POLYCHOME might have redundant actions mediating gibberellin control via SPINDLY. The overall leaf polysomaty was not notably affected by these mutations, suggesting that they affect the control of DNA synthesis in a tissue- or cell type-specific manner. Wild-type tetraploids also produce overbranched trichomes; they displayed a shifted polysomaty in trichomes and in the whole leaf, suggesting a developmental program controlling DNA increases via the counting of endoreduplication rounds.

Isolation of ethyl methanesulfonate-induced gametophytic mutants in Arabidopsis thaliana by a segregation distortion assay using the multimarker chromosome 1.

The life cycle of plants comprises two alternating generations, the diploid sporophyte (spore-bearing plant) and the haploid gametophyte (gamete-bearing plant). In contrast to animals, the postmeiotic cells give rise to haploid organisms whose function is to produce the gametes and to mediate fertilization. Analysis of gametophyte development and function has been hampered by the difficulty of identifying haplo-phase-specific mutants in conventional mutagenesis screens. Here we use a genetic strategy that is based on segregation distortion of nearby visible markers to screen for EMS-induced gametophytic mutants in Arabidopsis thaliana. Using the multiple marker chromosome mm1 we have isolated seven lines that displayed an altered segregation of markers. Reciprocal backcrosses of these lines showed a marked reduction of the transmission of the male and/or female gametes. Phenotypic analysis revealed that different aspects of either gametophytic development or function were affected. Three male gametophytic lines showed specific arrests during pollen development. One male gametophytic line was specifically defective in pollen tube elongation. Three gametophytic lines showed variable defects in both male and female gametophytic development.

Regulatory and coding regions of the segmentation gene hunchback are functionally conserved between Drosophila virilis and Drosophila melanogaster.

The segmentation gene hunchback (hb) is involved in setting up the anterior-posterior axis of the Drosophila embryo. It is expressed maternally and zygotically and it plays a key role in integrating the effects of the anterior and posterior maternal systems. The hb gene from D. virilis has previously been cloned and was shown to be well conserved in its coding region, but less so in its upstream region which shows a more patchy pattern of conserved and diverged sequences. This work deals with the functional conservation of hb between the two species. We have mapped two additional regulatory elements for the expression of hb in the early embryo, namely the enhancer for the maternal expression and the enhancer region for the late blastoderm expression. Fragments containing these two elements, the previously identified bicoid dependent element for the early blastoderm expression of hb and the coding region were taken from D. virilis and tested in the D. melanogaster background. We find that all enhancer elements as well as the coding region are functionally conserved between the two species. Comparison of the upstream sequences that include the enhancer region for the late blastoderm expression reveal seven highly conserved blocks. Some of these contain consensus binding sites for transregulatory factors that are likely to control the respective expression domains.

Epidermal pattern formation in the root and shoot of Arabidopsis.

Root hair formation, stomata development on hypocotyls and trichome formation on leaves in Arabidopsis represent three model systems for epidermal patterning in plants that involve a common set of genes or corresponding homologues. The resulting pattern and the developmental readout are, however, strikingly different. Trichomes become regularly spaced on the leaf surface. Root hairs and stomata-bearing cells are formed in rows at specific locations with reference to the underlying cortex cells. In this review, we summarize the mechanistic similarities and discuss differences that might account for the different outcome of patterning in each system.

Spatial regulation of trichome formation in Arabidopsis thaliana.

Trichomes in Arabidopsis are single-celled hairs that develop from epidermal cells regularly distributed on leaves, sepals and stems. Trichome formation in Arabidopsis is emerging as a genetic and molecular model system for the analysis of the spatial regulation of cell type specification in plants. In this review we focus on two aspects of trichome initiation: the regulation of organ and tissue layer specificity and the establishment of an epidermal spacing pattern.

Gap genes and gradients--the logic behind the gaps.

Gap genes stand at the top of the zygotic segmentation hierarchy in Drosophila. Their expression domains are set up by a combination of maternal regulatory signals and interactions among themselves. In addition, these regulatory pathways are partially redundantly specified by the maternal and zygotic genomes, or by multiple zygotic gene products. The gap genes code for transcription factors which exert their function by forming short-range morphogenetic gradients; differential concentrations of these transcription factors can either act as activators or as repressors for the expression domains of neighboring genes. It is possible to view the gap genes as a system of genes that subdivides the embryo by forming an activation-repression cascade proceeding from anterior towards posterior.

Trichome morphogenesis: a cell-cycle perspective.

Arabidopsis leaf hairs (trichomes) are polyploid epidermal cells with a predictable branching pattern. More than 15 genes have been identified that are involved in the regulation of branching. The cloning of the ZWICHEL, ANGUSTIFOLIA and STICHEL genes points to two mechanistic aspects of branch formation: (i) a role of the microtubule cytoskeleton; and (ii) a link to the regulation of cell divisions. The latter aspect is supported by the recent identification of an Arabidopsis mutant with multicellular trichomes, the siamese mutant, suggesting that Arabidopsis trichomes are evolutionarily derived from multicellular forms. We speculate that the spatial information for branch formation is derived from mechanisms employed in cell divisions.

Cell morphogenesis of trichomes in Arabidopsis: differential control of primary and secondary branching by branch initiation regulators and cell growth.

Cell morphogenesis, i.e. the acquisition of a particular cell shape, can be examined genetically in the three-branched trichomes that differentiate from single epidermal cells on the leaves of Arabidopsis thaliana. In normal development, the growing trichome cell undergoes two successive branching events, resulting in a proximal side stem and a distal main stem which subsequently splits in two branches. Using new and previously described trichome mutants, we have analyzed the branching pattern in single and double mutants affecting branch number or cell size in order to determine underlying mechanisms. Our results suggest that primary branching is genetically distinct from subsequent branching events and that the latter, secondary events are initiated in response to positive and negative regulators of branching as well as subject to control by cell growth. We propose a model of how trichome cell morphogenesis is regulated during normal development.

Tissue layer and organ specificity of trichome formation are regulated by GLABRA1 and TRIPTYCHON in Arabidopsis.

In animal development, cellular diversity is generated within tissues which in turn are derived from germ layers. Similar to the germ layers in animals, plants establish three distinct tissue layers early in development which each give rise to a distinct set of cell types. To investigate the role of tissue-layer-specific cues in generating plant cellular diversity we studied the spatial regulation of an epidermal cell type, trichomes (hairs), by the two genes, GLABRA1 (GL1) and TRIPTYCHON (TRY). Ubiquitous expression of the positive regulator GL1 in the absence of the negative regulator TRY leads to ectopic trichome formation not only on additional organs but also in subepidermal tissue layers. Trichomes in inner tissue layers can differentiate the same morphology and show a spacing pattern comparable to trichomes in the epidermis. This clearly shows that cell type specification takes place downstream of tissue-specific cues. We propose a model of how the tissue and organ specificity of trichome induction is regulated in normal development.

A morphogenetic gradient of hunchback protein organizes the expression of the gap genes Krüppel and knirps in the early Drosophila embryo.

Segmentation of the Drosophila embryo depends on a hierarchy of interactions among the maternal and zygotic genes in the early embryo. The anterior region is organized maternally by the bicoid (bcd) gene product, which forms a concentration gradient in the anterior half of the embryo. The gap genes are also involved in establishing the body plan, with hunchback (hb) being expressed both maternally and zygotically. Zygotic expression of hb is directly activated by the bcd gene product, leading to a subdivision of the embryo into an anterior half expressing zygotically provided hb protein and a posterior half that does not. A similar effect on maternally provided hb protein is caused by the gene nanos, which represses the translation of maternally provided transcripts in the posterior half. This regulation of hb protein is a prerequisite for abdomen development, because the presence of hb protein in the posterior half represses posterior segmentation. This repression mechanism suggests that posterior segmentation might not directly depend on maternal positional cues, but be solely organized at the zygotic level. Here we report further evidence to support this hypothesis and show that the hb protein itself is crucially involved in organizing abdominal segmentation. Differential concentrations of hb protein determine the anterior and posterior borders of expression of the gap gene Krüppel (Kr) and the anterior border of the gap gene knirps (kni), thus defining three positional values. These regulatory pathways are controlled in a redundant way, in part by bcd and in part by the maternal hb gene product.

Cell polarity in Arabidopsis trichomes.

Arabidopsis leaf trichomes are unicellular hairs that display a highly characteristic cell form that has a fixed orientation with respect to the basal-distal leaf axis. The genetic, molecular and cell biological analysis of trichome morphogenesis reveal that various cellular processes need to be coordinated including regulation of the cell cycle, the cell size and the actin and tubulin cytoskeleton. Here we will focus on what is known about the establishment and maintenance of positional information during trichome formation.

Genetic dissection of trichome cell development in Arabidopsis.

Trichomes are specialized epidermal cells that are regularly distributed on the leaves of Arabidopsis plants. During leaf development, each trichome undergoes cell morphogenesis, resulting in a very precise architecture. We have isolated and characterized more than 70 trichome mutants that represent 21 different genes. By analyzing their cellular phenotypes, it has been possible to define specific steps in trichome development. Although these steps normally occur in a temporal sequence, genetic dissection indicates that the morphology of the mature cell is largely assembled in independent events. The results suggest a model of how a specialized cell type originates from a uniform cell layer.