Molecular analysis of the Arabidopsis pattern formation of gene GNOM: gene structure and intragenic complementation

N-terminal domain of ARF-GEF GNOM prevents heterodimerization with functionally divergent GNL1 in Arabidopsis

Evolutionary change following gene duplication can lead to functionally divergent paralogous proteins. If comprising identical subunits their random assortment would also form potentially detrimental heteromeric proteins. In Arabidopsis, the ARF GTPase guanine-nucleotide exchange factor GNOM is essential for polar recycling of auxin-efflux transporter PIN1 from endosomes to the basal plasma membrane whereas its paralog GNL1 mediates retrograde Golgi-endoplasmic reticulum traffic. Here we show that both GNOM and GNL1 form homodimers but no heterodimers. To assess the biological significance of this, we generated transgenic plants expressing engineered heterodimer-compatible GNOM variants. Those plants showed developmental defects such as the failure to produce lateral roots. To identify mechanisms underlying heterodimer prevention, we analyzed interactions of the N-terminal dimerization and cyclophilin-binding (DCB) domain. Each DCB domain interacted with the complementary fragment (ΔDCB) both of their own and of the paralogous protein. However, only DCB^GNOM interacted with itself whereas DCB^GNL1 failed to interact with itself and with DCB^GNOM . GNOM variants in which the DCB domain was removed or replaced by DCB^GNL1 revealed a role for DCB-DCB interaction in the prevention of GNOM-GNL1 heterodimers whereas DCB-ΔDCB interaction was essential for dimer formation and GNOM function. Our data suggest a model of early DCB-DCB interaction that facilitates GNOM homodimer formation, indirectly precluding formation of detrimental heterodimers.

Connected function of PRAF/RLD and GNOM in membrane trafficking controls intrinsic cell polarity in plants

Cell polarity is a fundamental feature underlying cell morphogenesis and organismal development. In the Arabidopsis stomatal lineage, the polarity protein BASL controls stomatal asymmetric cell division. However, the cellular machinery by which this intrinsic polarity site is established remains unknown. Here, we identify the PRAF/RLD proteins as BASL physical partners and mutating four PRAF members leads to defects in BASL polarization. Members of PRAF proteins are polarized in stomatal lineage cells in a BASL-dependent manner. Developmental defects of the praf mutants phenocopy those of the gnom mutants. GNOM is an activator of the conserved Arf GTPases and plays important roles in membrane trafficking. We further find PRAF physically interacts with GNOM in vitro and in vivo. Thus, we propose that the positive feedback of BASL and PRAF at the plasma membrane and the connected function of PRAF and GNOM in endosomal trafficking establish intrinsic cell polarity in the Arabidopsis stomatal lineage.

Of Cells, Strands, and Networks: Auxin and the Patterned Formation of the Vascular System

Throughout plant development, vascular cells continually form from within a population of seemingly equivalent cells. Vascular cells connect end to end to form continuous strands, and vascular strands connect at both or either end to form networks of exquisite complexity and mesmerizing beauty. Here we argue that experimental evidence gained over the past few decades implicates the plant hormone auxin-its production, transport, perception, and response-in all the steps that lead to the patterned formation of the plant vascular system, from the formation of vascular cells to their connection into vascular networks. We emphasize the organizing principles of the cell- and tissue-patterning process, rather than its molecular subtleties. In the picture that emerges, cells compete for an auxin-dependent, cell-polarizing signal; positive feedback between cell polarization and cell-to-cell movement of the polarizing signal leads to gradual selection of cell files; and selected cell files differentiate into vascular strands that drain the polarizing signal from the neighboring cells. Although the logic of the patterning process has become increasingly clear, the molecular details remain blurry; the future challenge will be to bring them into razor-sharp focus.

Genetic activity during early plant embryogenesis

Seeds are essential for human civilization, so understanding the molecular events underpinning seed development and the zygotic embryo it contains is important. In addition, the approach of somatic embryogenesis is a critical propagation and regeneration strategy to increase desirable genotypes, to develop new genetically modified plants to meet agricultural challenges, and at a basic science level, to test gene function. We briefly review some of the transcription factors (TFs) involved in establishing primary and apical meristems during zygotic embryogenesis, as well as TFs necessary and/or sufficient to drive somatic embryo programs. We focus on the model plant Arabidopsis for which many tools are available, and review as well as speculate about comparisons and contrasts between zygotic and somatic embryo processes.

Asymmetric cell division in plants: mechanisms of symmetry breaking and cell fate determination

Asymmetric cell division is a fundamental mechanism that generates cell diversity while maintaining self-renewing stem cell populations in multicellular organisms. Both intrinsic and extrinsic mechanisms underpin symmetry breaking and differential daughter cell fate determination in animals and plants. The emerging picture suggests that plants deal with the problem of symmetry breaking using unique cell polarity proteins, mobile transcription factors, and cell wall components to influence asymmetric divisions and cell fate. There is a clear role for altered auxin distribution and signaling in distinguishing two daughter cells and an emerging role for epigenetic modifications through chromatin remodelers and DNA methylation in plant cell differentiation. The importance of asymmetric cell division in determining final plant form provides the impetus for its study in the areas of both basic and applied science.

Identification of new adventitious rooting mutants amongst suppressors of the Arabidopsis thaliana superroot2 mutation

The plant hormone auxin plays a central role in adventitious rooting and is routinely used with many economically important, vegetatively propagated plant species to promote adventitious root initiation and development on cuttings. Nevertheless the molecular mechanisms through which it acts are only starting to emerge. The Arabidopsis superroot2-1 (sur2-1) mutant overproduces auxin and, as a consequence, develops excessive adventitious roots in the hypocotyl. In order to increase the knowledge of adventitious rooting and of auxin signalling pathways and crosstalk, this study performed a screen for suppressors of superroot2-1 phenotype. These suppressors provide a new resource for discovery of genetic players involved in auxin signalling pathways or at the crosstalk of auxin and other hormones or environmental signals. This study reports the identification and characterization of 26 sur2-1 suppressor mutants, several of which were identified as mutations in candidate genes involved in either auxin biosynthesis or signalling. In addition to confirming the role of auxin as a central regulator of adventitious rooting, superroot2 suppressors indicated possible crosstalk with ethylene signalling in this process.

Strigolactone can promote or inhibit shoot branching by triggering rapid depletion of the auxin efflux protein PIN1 from the plasma membrane

Shoot branching is regulated by competition between branches to export the phytohormone auxin into the main stem. The phytohormone strigolactone balances shoot system growth by making auxin export harder to establish, thus modulating the auxin transport network.

Plants continuously extend their root and shoot systems through the action of meristems at their growing tips. By regulating which meristems are active, plants adjust their body plans to suit local environmental conditions. The transport network of the phytohormone auxin has been proposed to mediate this systemic growth coordination, due to its self-organising, environmentally sensitive properties. In particular, a positive feedback mechanism termed auxin transport canalization, which establishes auxin flow from active shoot meristems (auxin sources) to the roots (auxin sinks), has been proposed to mediate competition between shoot meristems and to balance shoot and root growth. Here we provide strong support for this hypothesis by demonstrating that a second hormone, strigolactone, regulates growth redistribution in the shoot by rapidly modulating auxin transport. A computational model in which strigolactone action is represented as an increase in the rate of removal of the auxin export protein, PIN1, from the plasma membrane can reproduce both the auxin transport and shoot branching phenotypes observed in various mutant combinations and strigolactone treatments, including the counterintuitive ability of strigolactones either to promote or inhibit shoot branching, depending on the auxin transport status of the plant. Consistent with this predicted mode of action, strigolactone signalling was found to trigger PIN1 depletion from the plasma membrane of xylem parenchyma cells in the stem. This effect could be detected within 10 minutes of strigolactone treatment and was independent of protein synthesis but dependent on clathrin-mediated membrane trafficking. Together these results support the hypothesis that growth across the plant shoot system is balanced by competition between shoot apices for a common auxin transport path to the root and that strigolactones regulate shoot branching by modulating this competition.

Plants can adapt their form to suit the environment in which they are growing. For example, genetically identical plants can develop as a single unbranched stem or as a highly ramified bush. This broad developmental potential is possible because the shoot system is produced continuously by growing tips, known as shoot meristems. Meristems produce the stem and leaves of a shoot, and at the base of each leaf, a new meristem is formed. This meristem can remain dormant as a small bud or activate to produce a branch. Thus, the shoot system is a community of shoot meristems, the combined activity and inactivity of which shape shoot form. Here we provide evidence that growth is balanced across the Arabidopsis shoot system by competition between the shoot meristems. This competition is likely mediated by the requirement of meristems to export the plant hormone auxin in order to activate bud outgrowth. In our model, auxin in the main stem, exported from active branches, can prevent auxin export by dormant buds, thus preventing their activation. Our findings show that a second hormone, strigolactone, increases the level of competition between branches by making auxin export harder to establish. Together, these hormones balance growth across the shoot system, adjusting it according to the environmental conditions in which a plant is growing.

Complex evolutionary events at a tandem cluster of Arabidopsis thaliana genes resulting in a single-locus genetic incompatibility

Non-additive interactions between genomes have important implications, not only for practical applications such as breeding, but also for understanding evolution. In extreme cases, genes from different genomic backgrounds may be incompatible and compromise normal development or physiology. Of particular interest are non-additive interactions of alleles at the same locus. For example, overdominant behavior of alleles, with respect to plant fitness, has been proposed as an important component of hybrid vigor, while underdominance may lead to reproductive isolation. Despite their importance, only a few cases of genetic over- or underdominance affecting plant growth or fitness are understood at the level of individual genes. Moreover, the relationship between biochemical and fitness effects may be complex: genetic overdominance, that is, increased or novel activity of a gene may lead to evolutionary underdominance expressed as hybrid weakness. Here, we describe a non-additive interaction between alleles at the Arabidopsis thaliana OAK (OUTGROWTH-ASSOCIATED PROTEIN KINASE) gene. OAK alleles from two different accessions interact in F₁ hybrids to cause a variety of aberrant growth phenotypes that depend on a recently acquired promoter with a novel expression pattern. The OAK gene, which is located in a highly variable tandem array encoding closely related receptor-like kinases, is found in one third of A. thaliana accessions, but not in the reference accession Col-0. Besides recruitment of exons from nearby genes as promoter sequences, key events in OAK evolution include gene duplication and divergence of a potential ligand-binding domain. OAK kinase activity is required for the aberrant phenotypes, indicating it is not recognition of an aberrant protein, but rather a true gain of function, or overdominance for gene activity, that leads to this underdominance for fitness. Our work provides insights into how tandem arrays, which are particularly prone to frequent, complex rearrangements, can produce genetic novelty.

While intraspecific hybrids are vitally important in modern agriculture because they often perform better than their inbred parents, certain hybrid combinations fail to develop normally and are inferior to their parents. We have identified an Arabidopsis thaliana hybrid with several aberrant growth phenotypes that are caused by divergence at a single locus encoding the receptor-like kinase OUTGROWTH-ASSOCIATED PROTEIN KINASE (OAK). OAK belongs to a group of similar genes arranged in a tandem cluster that varies substantially between A. thaliana strains. OAK originated through duplication within the cluster with concurrent recruitment of coding sequences from nearby genes to form a new promoter with a novel expression pattern. Kinase activity of OAK is required for its effects, indicating that it is not recognition of an aberrant protein but rather a true gain of function that leads to the incompatibility. Most of the incompatibility seems to come from divergence within the extracellular ligand-binding domain of the OAK protein, indicating that heterodimers of OAK may have higher affinity for a natural substrate compared to either homodimer. Finally, mis-expression of the incompatible OAK alleles from the promoter present in the reference strain of A. thaliana also leads to genetic incompatibility, but with different phenotypic outcomes.

A statistical model for estimating maternal-zygotic interactions and parent-of-origin effects of QTLs for seed development

Proper development of a seed requires coordinated exchanges of signals among the three components that develop side by side in the seed. One of these is the maternal integument that encloses the other two zygotic components, i.e., the diploid embryo and its nurturing annex, the triploid endosperm. Although the formation of the embryo and endosperm contains the contributions of both maternal and paternal parents, maternally and paternally derived alleles may be expressed differently, leading to a so-called parent-of-origin or imprinting effect. Currently, the nature of how genes from the maternal and zygotic genomes interact to affect seed development remains largely unknown. Here, we present a novel statistical model for estimating the main and interaction effects of quantitative trait loci (QTLs) that are derived from different genomes and further testing the imprinting effects of these QTLs on seed development. The experimental design used is based on reciprocal backcrosses toward both parents, so that the inheritance of parent-specific alleles could be traced. The computing model and algorithm were implemented with the maximum likelihood approach. The new strategy presented was applied to study the mode of inheritance for QTLs that control endoreduplication traits in maize endosperm. Monte Carlo simulation studies were performed to investigate the statistical properties of the new model with the data simulated under different imprinting degrees. The false positive rate of imprinting QTL discovery by the model was examined by analyzing the simulated data that contain no imprinting QTL. The reciprocal design and a series of analytical and testing strategies proposed provide a standard procedure for genomic mapping of QTLs involved in the genetic control of complex seed development traits in flowering plants.

NtGNL1 is involved in embryonic cell division patterning, root elongation, and pollen tube growth in tobacco

The function of the ARF-GEF family has drawn great attention recently, especially GNOM and GNL1, owing to their important role in plant development. A homolog of GBF was identified in Nicotiana tabacum, named NtGNL1, which is ubiquitously expressed throughout the tobacco life cycle. In NtGNL1 RNAi plants, irregular orientation of cell division and asynchronous cell development during early embryogenesis disrupted the symmetry of the developing embryo. In addition, root growth in transgenic lines was significantly slower than that in wild-type plants, although the structure of the root tip was largely intact. Pollen germination and pollen tube growth were also inhibited in the transgenic lines, and the tip of the pollen tube presented various aberrant morphologies in one of the transgenic lines. The phenotypes of different NtGNL1 RNAi transgenic lines suggest that the NtGNL1 is likely to be involved not only in embryogenesis and postembryonic development, but also in sexual reproduction; thus, NtGNL1 may play multiple and critical roles in plant development.

New insight into the biochemical mechanisms regulating auxin transport in plants

The transport of the plant hormone auxin has been under intense investigation since its identification 80 years ago. Studies have gradually refined our understanding of the importance of auxin transport in many aspects of plant signalling and development, and the focus has intensified in recent years towards the identification of the proteins involved in auxin transport and their functional mechanism. Within the past 18 months, the field has progressed rapidly, with confirmation that several distinct classes of proteins, previously dubbed as 'putative auxin permeases' or 'auxin transport facilitators', are bona fide transporters of IAA (indol-3-ylacetic acid). In this review we will appraise the recent transport data and highlight likely future research directions, including the characterization of auxiliary proteins necessary for the regulation of auxin transporters.

MINISEED3 (MINI3), a WRKY family gene, and HAIKU2 (IKU2), a leucine-rich repeat (LRR) KINASE gene, are regulators of seed size in Arabidopsis

We have identified mutant alleles of two sporophytically acting genes, HAIKU2 (IKU2) and MINISEED3 (MINI3). Homozygotes of these alleles produce a small seed phenotype associated with reduced growth and early cellularization of the endosperm. This phenotype is similar to that described for another seed size gene, IKU1. MINI3 encodes WRKY10, a WRKY class transcription factor. MINI3 promoter::GUS fusions show the gene is expressed in pollen and in the developing endosperm from the two nuclei stage at approximately 12 hr postfertilization to endosperm cellularization at approximately 96 hr. MINI3 is also expressed in the globular embryo but not in the late heart stage of embryo development. The early endosperm expression of MINI3 is independent of its parent of origin. IKU2 encodes a leucine-rich repeat (LRR) KINASE (At3g19700). IKU2::GUS has a similar expression pattern to that of MINI3. The patterns of expression of the two genes and their similar phenotypes indicate they may operate in the same genetic pathway. Additionally, we found that both MINI3 and IKU2 showed decreased expression in the iku1-1 mutant. IKU2 expression was reduced in a mini3-1 background, whereas MINI3 expression was unaltered in the iku2-3 mutant. These data suggest the successive action of the three genes IKU1, IKU2, and MINI3 in the same pathway of seed development.

DRP1A is responsible for vascular continuity synergistically working with VAN3 in Arabidopsis

In most dicotyledonous plants, vascular tissues in the leaf have a reticulate venation pattern. We have isolated and characterized an Arabidopsis (Arabidopsis thaliana) mutant defective in the vascular network defective mutant, van3. van3 mutants show a discontinuous vascular pattern, and VAN3 is known to encode an ADP-ribosylation-factor-GTPase-activating protein that regulates membrane trafficking in the trans-Golgi network. To elucidate the molecular nature controlling the vein patterning process through membrane trafficking, we searched VAN3-interacting proteins using a yeast (Saccharomyces cerevisiae) two hybrid system. As a result, we isolated the plant Dynamin-Related Protein 1A (DRP1A) as a VAN3 interacting protein. The spatial and temporal expression patterns of DRP1AGUS and VAN3GUS were very similar. The subcellular localization of VAN3 completely overlapped to that of DRP1A. drp1a showed a disconnected vascular network, and the drp1a mutation enhanced the phenotype of vascular discontinuity of the van3 mutant in the drp1a van3 double mutant. Furthermore, the drp1 mutation enhanced the discontinuous vascular pattern of the gnom mutant, which had the same effect as that of the van3 mutation. These results indicate that DRP1 modulates the VAN3 function in vesicle budding from the trans-Golgi network, which regulates vascular formation in Arabidopsis.

Auxin and embryo axis formation: the ends in sight?

The major axis of polarity of the plant embryo serves as a reference for the formation of meristems and, thus, for all subsequent development. Mechanisms underlying the establishment of the embryo axis itself have remained elusive. This is now changing with recent reports documenting a role for auxin in embryo axis formation. Auxin accumulates dynamically at specific positions that correlate with developmental decisions in early embryogenesis, and this ties developmental decisions to both transport regulators and components of the response machinery. A major challenge for the future is to determine how auxin-dependent processes interact with other as yet unknown factors to mediate differential gene expression patterns in early embryogenesis.

Pattern formation in the vascular system of monocot and dicot plant species

Plant vascular tissues are organised in continuous strands, the longitudinal and radial patterns of which are intimately linked to the signals that direct plant architecture as a whole. Therefore, understanding the mechanisms underlying vascular tissue patterning is expected to shed light on patterning events beyond those that organise the vascular system, and thus represents a central issue in plant developmental biology. A number of recent advances, reviewed here, are leading to a more precise definition of the signals that control the formation of vascular tissues and their integration into a larger organismal context. Contents Summary 209 I. Introduction 209 II. The plant vascular system 210 III. Ontogeny of the vascular tissues 210 IV. Procambium development 210 V. The organisation of the vascular tissues 212 VI. The regulation of longitudinal vascular pattern formation 214 VII. The regulation of radial vascular pattern formation 220 VIII. Genetic screens for vascular development mutants 231 IX. Genes involved in vascular development identified through reverse genetics approaches 235 X. Conclusions and perspectives 235 Note added at the revision stage 236 Acknowledgements 236 References 236.

A novel Golgi membrane protein is a partner of the ARF exchange factors Gea1p and Gea2p

The Sec7 domain guanine nucleotide exchange factors (GEFs) for the GTPase ARF are highly conserved regulators of membrane dynamics and protein trafficking. The interactions of large ARF GEFs with cellular membranes for localization and/or activation are likely to participate in regulated recruitment of ARF and effectors. However, these interactions remain largely unknown. Here we characterize Gmh1p, the first Golgi transmembrane-domain partner of any of the high-molecular-weight ARF-GEFs. Gmh1p is an evolutionarily conserved protein. We demonstrate molecular interaction between the yeast Gmh1p and the large ARF-GEFs Gea1p and Gea2p. This interaction involves a domain of Gea1p and Gea2p that is conserved in the eukaryotic orthologues of the Gea proteins. A single mutation in a conserved amino acid residue of this domain is sufficient to abrogate the interaction, whereas the overexpression of Gmh1p can compensate in vivo defects caused by mutations in this domain. We show that Gmh1p is an integral membrane protein that localizes to the early Golgi in yeast and in human HeLa cells and cycles through the ER. Hence, we propose that Gmh1p acts as a positive Golgi-membrane partner for Gea function. These results are of general interest given the evolutionary conservation of both ARF-GEFs and the Gmh proteins.

Division decisions and the spatial regulation of cytokinesis

Cytokinesis in plant cells in accomplished when a membranous cell plate is guided to a pre-established division site. The orientation of the new wall establishes the starting position of a cell in a growing tissue, but the impact of this position on future development varies. Recently, proteins have been identified that participate in forming, stabilizing and guiding the cell plate to the correct division site. Mutations that affect cytokinesis with varying impacts on plant development are providing information about the mechanics of cytokinesis and also about how the division site is selected.

Divide and conquer: cytokinesis in plant cells

Plant cells divide in two by constructing a new cell wall (cell plate) between daughter nuclei after mitosis. Golgi-derived vesicles are transported to the equator of a cytoskeletal structure called a phragmoplast, where they fuse together to form the cell plate. Orientation of new cell walls involves actindependent guidance of phragmoplasts and associated cell plates to cortical sites established prior to mitosis. Recent work has provided new insights into how actin filaments and other proteins in the phragmoplast and cell plate contribute to cytokinesis. Newly discovered mutations have identified a variety of genes required for cytokinesis or its spatial regulation.

Digging out roots: pattern formation, cell division, and morphogenesis in plants

The analysis of plant development by genetic, molecular, and surgical approaches has accumulated a large body of data, and yet it remains a challenge to uncover the basic mechanisms that are operating. Early steps of development, when the zygote and its daughter cells organize the embryonic plant, are poorly understood despite considerable efforts toward the identification of relevant genes. Reported cases of genetic redundancy suggest that the difficulty in uncovering patterning genes may reflect overlapping gene activities. Our current knowledge on plant embryo development still leaves open whether mechanisms for axis formation and subsequent pattern formation are fundamentally different in animals and plants. Axis formation may follow the general principle of establishing a peripheral asymmetric cue and mobilizing the cytoskeleton toward this cue--in the case of plants possibly located in the cell wall--but the molecules involved may be entirely different. Embryonic pattern formation involves the establishment of different domains, but although there are candidates, it is not clear whether genes that define these domains are identified yet. Pattern formation continues postembryonically in the meristem, and the flexibility of this process may be explained by a feed-forward system of patterning cues originating from more mature cells. Control of cell division and differentiation, which is important in the meristems--regions of continuous development--has been studied intensively and appears to involve short-range signaling and transmembrane receptor kinase activation. Finally, although high importance of control of cell division rates and planes for plant morphogenesis have been often inferred, recent genetic studies as well as comparative morphological data point to a less decisive role of cell division and to global controls of as yet unknown nature.

Auxin-induced developmental patterns in Brassica juncea embryos

To investigate the mechanism of auxin action during pattern formation in dicot embryos, we tested the effects of the natural auxin indole-3-acetic acid (IAA), the auxin transport inhibitor N-(1-naphthyl)thalamic acid (NPA) and the antiauxin p-chlorophenoxyisobutyric acid (PCIB). In vitro treatments of isolated zygotic Brassica juncea embryos with these substances led to a wide range of morphogenetic alterations. Treatment of globular embryos with exogenous auxin (10-40 microM) either completely inhibited morphogenesis, resulting in ball-shaped embryos, or caused the development of egg- and cucumber-shaped embryos, which only consisted of a shortened hypocotyl without any apical structures. Axis duplication was observed sometimes after inhibition of auxin transport in globular embryos, and led to the development of twin embryos. During the transition from globular to heart stage, changes in auxin distribution or activity frequently caused the development of either split-collar or collar-cotyledons. Antiauxin inhibited cotyledon growth, leading to embryos with single or no cotyledons, or inhibited the development of the hypocotyl and the radicle. Inhibition of auxin transport in transition embryos sometimes led to axis broadening, which resulted in the development of two radicles. The described changes in embryo shapes represent arrests in different auxin-regulated developmental steps and phenocopy some Arabidopsis morphogenetic mutants.

The HOBBIT gene is required for formation of the root meristem in the Arabidopsis embryo

In Arabidopsis, the root meristem originates from the hypophyseal cell and from an adjoining cell tier that is distinct at the heart stage of embryogenesis. We have analysed mutations in the HOBBIT (HBT) gene that is essential for root meristem formation. hbt embryos display incorrect hypophyseal cell development from the quadrant stage onward. At the heart stage, the adjoining cell tier of hbt embryos develops abnormally, in that the activation of cell division and the formation of a lateral root cap layer are disturbed. Strong hbt mutants give rise to seedlings that lack an anatomically recognisable quiescent centre and differentiated columella root cap cells, the cell types derived from the wild-type hypophysis. Furthermore, they have no mitotically active root meristem and lack a differentiated lateral root cap. Secondary roots of hbt mutants and roots obtained from cultured cells of hbt mutants have similar defects. Therefore the HBT gene is required for root meristem formation in different developmental contexts.

Ovule and embryo development, apomixis and fertilization

Genetic analyses, particularly in Arabidopsis, have led to the identification of mutants that define different steps of ovule ontogeny, pollen stigma interaction, pollen tube growth, and fertilization. Isolation of the genes defined by these mutations promises to lead to a molecular understanding of these processes. Mutants have also been obtained in which processes that are normally triggered by fertilization, such as endosperm formation and initiation of seed development, occur without fertilization. These mutants may illuminate apomixis, a process of seed development without fertilization extant in many plants.