Are there symplastic connections between the endosperm and embryo in some angiosperms?--a lesson from the Crassulaceae family

Epigenetic silencing of callose synthase by VIL1 promotes bud-growth transition in lily bulbs

In plants, restoring intercellular communication is required for cell activity in buds during the growth transition from slow to fast growth after dormancy release. However, the epigenetic regulation of this phenomenon is far from understood. Here we demonstrate that lily VERNALIZATION INSENSITIVE 3-LIKE 1 (LoVIL1) confers growth transition by mediating plasmodesmata opening via epigenetic repression of CALLOSE SYNTHASE 3 (LoCALS3). Moreover, we found that a novel transcription factor, NUCLEAR FACTOR Y, SUBUNIT A7 (LoNFYA7), is capable of recruiting the LoVIL1-Polycomb Repressive Complex 2 (PRC2) and enhancing H3K27me3 at the LoCALS3 locus by recognizing the CCAAT cis-element (Cce) of its promoter. The LoNFYA7-LoVIL1 module serves as a key player in orchestrating the phase transition from slow to fast growth in lily bulbs. These studies also indicate that LoVIL1 is a suitable marker for the bud-growth-transition trait following dormancy release in lily cultivars.

Seed transmission of raspberry bushy dwarf virus is blocked in Nicotiana benthamiana plants by preventing virus entry into the embryo from the infected embryo sac and endosperm

Raspberry bushy dwarf virus (RBDV) is transmitted through seed in infected red raspberry plants after pollination with pollen grains from healthy red raspberry plants. Here, we show that RBDV is not transmitted through seeds in infected Nicotiana benthamiana (Nb) plants after pollination with virus-free Nb pollen grains. Chromogenic in situ hybridization revealed that the virus invades the shoot apical meristem and the ovule, including the embryo sac, of RBDV-infected Nb plants; however, in seeds that developed from infected embryo sacs after fertilization by virus-free sperm cells, RBDV was absent in the embryos and present in the endosperms. When we analyzed seed transmission of RBDV in Nb mutants with mutations in dicer-like enzyme 2 and 4 (NbDCL2&4) or RNA-dependent RNA polymerase 6 (NbRDR6), RBDV was not present in the offspring from seeds with embryos and endosperms that did not express NbDCL2&4 or NbRDR6. These results suggest that seed transmission of RBDV is prevented by evasion of infection by the embryo and that RNA silencing is not essential for preventing seed transmission of RBDV in Nb plants.

Non-cell-autonomous small RNA silencing in Arabidopsis female gametes

In recent years, small RNA movement has been both hypothesized and shown to be an integral part of the epigenetic DNA methylation reprogramming occurring during plant reproduction.¹It was suggested that the release of epigenetic silencing in accessory cell types or tissues is necessary to reinforce epigenetic silencing in the gametes (egg cell and sperm cells), which would in turn ensure the genomic stability of the next generation plant.^2,3 In Arabidopsis thaliana, small RNA (sRNA) movement was indeed shown to occur during male gametogenesis.^4,5,6 However, the situation within the female gametophyte and in early seed development is mostly unknown. Here, we show that small RNAs can induce non-cell-autonomous silencing from the central cell toward the egg cell but also from the synergids to the egg cell and central cell. Our data show that in addition to the movement of sRNAs during pollen development, hairpin RNAs can have non-cell-autonomous effects in the female gametes.

Cytosolic phosphoglucose isomerase is essential for microsporogenesis and embryogenesis in Arabidopsis

Phosphoglucose isomerase (PGI) catalyzes the interconversion of fructose-6-phosphate and glucose-6-phosphate, which impacts cell carbon metabolic flow. Arabidopsis (Arabidopsis thaliana) contains two nuclear PGI genes respectively encoding plastidial PGI1 and cytosolic PGI (cPGI). The loss of PGI1 impairs the conversion of F6P of the Calvin-Benson cycle to G6P for the synthesis of transitory starch in leaf chloroplasts. Since cpgi knockout mutants have not yet been obtained, they are thought to be lethal. The cpgi lethality can be rescued by expressing CaMV 35S promoter (p35S)-driven cPGI; however, the complemented line is completely sterile due to pollen degeneration. Here, we generated a cpgi mutant expressing p35S::cPGI-YFP in which YFP fluorescence in developing anthers was undetectable specifically in the tapetum and in pollen, which could be associated with male sterility. We also generated RNAi-cPGI knockdown lines with strong cPGI repression in floral buds that exhibited reduced male fertility due to the degeneration of most pollen. Histological analyses indicated that the synthesis of intersporal callose walls was impaired, causing microsporocytes to fail to separate haploid daughter nuclei to form tetrads, which might be responsible for subsequent pollen degeneration. We successfully isolated cpgi knockout mutants in the progeny of a heterozygous cpgi mutant floral-dipped with sugar solutions. The rescued cpgi mutants exhibited diminished young vegetative growth, reduced female fertility, and impaired intersporal callose wall formation in a meiocyte, and, thus, male sterility. Collectively, our data suggest that cPGI plays a vital role in carbohydrate partitioning, which is indispensable for microsporogenesis and early embryogenesis.

Embryo-Endosperm Interactions

In angiosperms, double fertilization triggers the concomitant development of two closely juxtaposed tissues, the embryo and the endosperm. Successful seed development and germination require constant interactions between these tissues, which occur across their common interface. The embryo-endosperm interface is a complex and poorly understood compound apoplast comprising components derived from both tissues, across which nutrients transit to fuel embryo development. Interface properties, which affect molecular diffusion and thus communication, are themselves dynamically regulated by molecular and physical dialogues between the embryo and endosperm. We review the current understanding of embryo-endosperm interactions, with a focus on the structure, properties, and function of their shared interface. Concentrating on Arabidopsis, but with reference to other species, we aim to situate recent findings within the broader context of seed physiology, developmental biology, and genetic factors such as parental conflicts over resource allocation.

Leaf Plasmodesmata Respond Differently to TMV, ToBRFV and TYLCV Infection

Macromolecule and cytosolic signal distribution throughout the plant employs a unique cellular and intracellular mechanism called plasmodesmata (PD). Plant viruses spread throughout plants via PD using their movement proteins (MPs). Viral MPs induce changes in plasmodesmata’s structure and alter their ability to move macromolecule and cytosolic signals. The developmental distribution of a family member of proteins termed plasmodesmata located proteins number 5 (PDLP5) conjugated to GFP (PDLP5-GFP) is described here. The GFP enables the visual localization of PDLP5 in the cell via confocal microscopy. We observed that PDLP5-GFP protein is present in seed protein bodies and immediately after seed imbibition in the plasma membrane. The effect of three different plant viruses, the tobacco mosaic virus (TMV), tomato brown rugose fruit virus (ToBRFV, tobamoviruses), and tomato yellow leaf curl virus (TYLCV, begomoviruses), on PDLP5-GFP accumulation at the plasmodesmata was tested. In tobacco leaf, TMV and ToBRFV increased PDLP5-GFP amount at the plasmodesmata of cell types compared to control. However, there was no statistically significant difference in tomato leaf. On the other hand, TYLCV decreased PDLP5-GFP quantity in plasmodesmata in all tomato leaf cells compared to control, without any significant effect on plasmodesmata in tobacco leaf cells.

Developmental, ultrastructural and cytochemical investigations of the female gametophyte in Sedum rupestre L. (Crassulaceae)

This article describes the development of female gametophyte in Sedum rupestre L. New embryological information about the processes of megasporogenesis and megagametogenesis provided in this paper expand the current knowledge about the embryology of the studied species. S. rupestre is characterized by monosporic megasporogenesis and the formation of Polygonum-type embryo sac. The process of megasporogenesis is initiated by one megaspore mother cell, resulting in the formation of a triad of cells after meiosis and cytokinesis. The functional megaspore, which is located chalazally, is a mononuclear cell present next to the megaspore in the centre of the triad. Only one of the two non-functional cells of the triad is binucleate, which occur at the micropylar pole. In this paper, we explain the functional ultrastructure of the female gametophytic cells in S. rupestre. Initially, the cytoplasm of the gametophytic cells does not differ from each other; however, during differentiation, the cells reveal different morphologies. The antipodals and the synergids gradually become organelle-rich and metabolically active. The antipodal cells participate in the absorption and transport of nutrients from the nucellar cells towards the megagametophyte. Their ultrastructure shows the presence of plasmodesmata with electron-dense material, which is characteristic of Crassulaceae, and wall ingrowths in the outer walls. The ultrastructure of synergid cells is characterized by the presence of filiform apparatus and cytoplasm with active dictyosomes, abundant profiles of endoplasmic reticulum and numerous vesicles, which agrees with their main function-the secretion of pollen tube attractants. Reported data can be used to resolve the current taxonomic problems within the genus Sedum ser. Rupestria.

Development of Embryo Suspensors for Five Genera of Crassulaceae with Special Emphasis on Plasmodesmata Distribution and Ultrastructure

The suspensor in the majority of angiosperms is an evolutionally conserved embryonic structure functioning as a conduit that connects ovule tissues with the embryo proper for nutrients and growth factors flux. This is the first study serving the purpose of investigating the correlation between suspensor types and plasmodesmata (PD), by the ultrastructure of this organ in respect of its full development. The special attention is paid to PD in representatives of Crassulaceae genera: Sedum, Aeonium, Monanthes, Aichryson and Echeveria. The contribution of the suspensor in transporting nutrients to the embryo was confirmed by the basal cell structure of the suspensor which produced, on the micropylar side of all genera investigated, a branched haustorium protruding into the surrounding ovular tissue and with wall ingrowths typically associated with cell transfer. The cytoplasm of the basal cell was rich in endoplasmic reticulum, mitochondria, dictyosomes, specialized plastids, microtubules, microbodies and lipid droplets. The basal cell sustained a symplasmic connection with endosperm and neighboring suspensor cells. Our results indicated the dependence of PD ultrastructure on the type of suspensor development: (i) simple PD are assigned to an uniseriate filamentous suspensor and (ii) PD with an electron-dense material are formed in a multiseriate suspensor. The occurrence of only one or both types of PD seems to be specific for the species but not for the genus. Indeed, in the two tested species of Sedum (with the distinct uniseriate/multiseriate suspensors), a diversity in the structure of PD depends on the developmental pattern of the suspensor. In all other genera (with the multiseriate type of development of the suspensor), the one type of electron-dense PD was observed.

Female gametophyte development in Sedum sediforme (Jacq.) Pau (Crassulaceae): an anatomical, cytochemical and ultrastructural analysis

Available documentation about the development of the female gametophyte of Crassulaceae is very limited. The aim of this study was to extend the embryological knowledge of Crassulaceae by analysing the development of the embryo sac in Sedum sediforme. Transmission electron microscopy and light microscopy including Nomarski optics (DIC) were used to observe individual stages of female gametophyte development. Cytochemical staining enabled detection of lipids, insoluble polysaccharides and proteins in gametophyte cells during their formation. Their increased accumulation was observed during nucellar cell and unfunctional cell degeneration in the embryo sac at the coenocytic and cellular stages (megagametogenesis). The female gametophyte develops in anatropous, bitegmic and crassinucellate ovules. The mature embryo sac is built of seven cells but after antipodes degeneration it is formed by the egg apparatus and a central cell. The monosporic Polygonum type was observed. One megaspore mother cell (MMC) formed three cells after meiosis. A triad was formed from a functional megaspore (placed chalazally), one uninucleate megaspore and a binucleate cell located at the micropylar end. Plasmodesmata with adhering electron-dense dome were noticed in walls of the coenocytic embryo sac and in the outer walls of ephemeral antipodes. Moreover, similar to synergids, antipodes form wall ingrowths. Here, we report new structural features of the antipodal cells (the presence of plasmodesmata with an electron-dense dome) which have not been described before. This new structural observation indicates that these cells participate in substance transport and that this process can probably be additionally regulated.

Ultrastructural and cytochemical aspects of female gametophyte development in Sedum hispanicum L. (Crassulaceae)

Until now, development of the female gametophyte has been investigated only in some species of Crassulaceae using a light microscope. To the best of our knowledge, this is the first report that describes the process of megasporogenesis and megagametogenesis in Crassulaceae in detail. To achieve this, we performed embryological studies on Sedum hispanicum L. (Crassulaceae). Cytochemical analysis detected the presence of proteins, lipids, and insoluble polysaccharides in individual cells of the gametophyte. The development of the embryo sac conforms to the monosporic or Polygonum-type in anatropous, crassinucellate, and bitegmic ovules. One megaspore mother cell initiates the process of megasporogenesis. Prior to the first meiotic division, the nucleus is centrally located within the meiocyte. Other organelles seem to be distributed evenly over the micropylar and chalazal parts during the development. Most storage reserves detected during megasporogenesis were observed in the megaspore mother cell. Three mitotic divisions within the chalazal functional megaspore resulted in the enlargement of the eight-nucleated embryo sac. In the seven-celled gametophyte, three chalazally located antipodes degenerated. A mature embryo sac was formed by the egg apparatus and central cell. When the antipodes degenerated, both synergids became organelle-rich and more active. The concentration of lipid droplets, starch grains, and proteins increased during megagametogenesis in the growing gametophyte. In the cellular embryo sac, the central cell can be distinguished by its largest accumulation. Our data confirm the hypothesis that plasmodesmata with electron-dense dome are formed during development of the female gametophyte in S. hispanicum and not just during the stages of embryogenesis. We observed these structures in megaspores and coenocytic embryo sac walls. Functions of observed plasmodesmata are discussed.

Integument cell gelatinisation-the fate of the integumentary cells in Hieracium and Pilosella (Asteraceae)

Members of the genera Hieracium and Pilosella are model plants that are used to study the mechanisms of apomixis. In order to have a proper understanding of apomixis, knowledge about the relationship between the maternal tissue and the gametophyte is needed. In the genus Pilosella, previous authors have described the specific process of the "liquefaction" of the integument cells that surround the embryo sac. However, these observations were based on data only at the light microscopy level. The main aim of our paper was to investigate the changes in the integument cells at the ultrastructural level in Pilosella officinarum and Hieracium alpinum. We found that the integument peri-endothelial zone in both species consisted of mucilage cells. The mucilage was deposited as a thick layer between the plasma membrane and the cell wall. The mucilage pushed the protoplast to the centre of the cell, and cytoplasmic bridges connected the protoplast to the plasmodesmata through the mucilage layers. Moreover, an elongation of the plasmodesmata was observed in the mucilage cells. The protoplasts had an irregular shape and were finally degenerated. After the cell wall breakdown of the mucilage cells, lysigenous cavities that were filled with mucilage were formed.

Molecular movement in the Arabidopsis thaliana female gametophyte

Size limits on molecular movement among female gametes. Cellular decisions can be influenced by information communicated from neighboring cells. Communication can occur via signaling or through the direct transfer of molecules. Movement of RNAs and proteins has frequently been observed among symplastically connected plant cells. In flowering plants, the female gametes, the egg cell and central cell, are closely apposed within the female gametophyte. Here we investigated the ability of fluorescently labeled dyes and small RNAs to move from the Arabidopsis thaliana central cell to the egg apparatus following microinjection. These results define a size limit of at least 20 kDa for symplastic movement between the two gametes, somewhat larger than that previously observed in Torenia fournieri. Our results indicate that symplastic connectivity in Arabidopsis thaliana changes after fertilization and suggest that prior to fertilization mechanisms are in place to facilitate small RNA movement from the central cell to the egg cell and synergids.

Identification of symplasmic domains in the embryo and seed of Sedum acre L. (Crassulaceae)

MAIN CONCLUSION

Our study demonstrated that symplasmic communication between Sedum acre seed compartments and the embryo proper is not uniform. The presence of plasmodesmata (PD) constitutes the structural basis for information exchange between cells, and symplasmic communication is involved in the regulation of cell differentiation and plant development. Most recent studies concerning an analysis of symplasmic communication between seed compartments and the embryo have been predominantly performed on Arabidopsis thaliana. The results presented in this paper describe the analysis of symplasmic communication on the example of Sedum acre seeds, because the ultrastructure of the seed compartments and the embryo proper, including the PD, have already been described, and this species represents an embryonic type of development different to Arabidopsis. Moreover, in this species, an unusual electron-dense dome associated with plasmodesmata on the border between the basal cell/chalazal suspensor cells and the basal cell/the endosperm has been described. This prompted the question as to whether these plasmodesmata are functional. Thus, the aim of this study was to describe the movement of symplasmic transport fluorochromes between different Sedum seed compartments, with particular emphasis on the movement between the basal cell and the embryo proper and endosperm, to answer the following questions: (1) are seeds divided into symplasmic domains; (2) if so, are they stable or do they change with the development? The results have shown that symplasmic tracers movement: (a) from the external integument to internal integument is restricted; (b) from the basal cell to the other part of the embryo proper and from the basal cell to the endosperm is also restricted;

Integument cell differentiation in dandelions (Taraxacum, Asteraceae, Lactuceae) with special attention paid to plasmodesmata

The aim of the paper is to determine what happens with plasmodesmata when mucilage is secreted into the periplasmic space in plant cells. Ultrastructural analysis of the periendothelial zone mucilage cells was performed on examples of the ovule tissues of several sexual and apomictic Taraxacum species. The cytoplasm of the periendothelial zone cells was dense, filled by numerous organelles and profiles of rough endoplasmic reticulum and active Golgi dictyosomes with vesicles that contained fibrillar material. At the beginning of the differentiation process of the periendothelial zone, the cells were connected by primary plasmodesmata. However, during the differentiation and the thickening of the cell walls (mucilage deposition), the plasmodesmata become elongated and associated with cytoplasmic bridges. The cytoplasmic bridges may connect the protoplast to the plasmodesmata through the mucilage layers in order to maintain cell-to-cell communication during the differentiation of the periendothelial zone cells.

A Different Pattern of Production and Scavenging of Reactive Oxygen Species in Halophytic Eutrema salsugineum (Thellungiella salsuginea) Plants in Comparison to Arabidopsis thaliana and Its Relation to Salt Stress Signaling

Isolated thylakoids from halophytic Eutrema salsugineum (Thellungiella salsuginea) produces more H2O2 in comparison to glycophytic Arabidopsis thaliana. The first objective of this study was to verify whether this feature is relevant also to the intact chloroplasts and leaves. Enhanced H2O2 levels in chloroplasts and leaves of E. salsugineum were positively verified with several methods (electron microscopy, staining with Amplex Red and with diaminobenzidine). This effect was associated with a decreased ratio of [Formula: see text]/H2O2 in E. salsugineum in comparison to A. thaliana as detected by electron paramagnetic resonance method. As a next step, we tested how this specific ROS signature of halophytic species affects the antioxidant status and down-stream components of ROS signaling. Comparison of enzymatic antioxidants revealed a decreased activity of ascorbate peroxidase (APX), enhanced activity of glutathione peroxidase, and the presence of thylakoid-bound forms of iron superoxide dismutase (FeSOD) and APX in E. salsugineum. These cues were, however, independent from application of salt stress. The typical H2O2-dependent cellular responses, namely the levels of glucosinolates and stress-related hormones were determined. The total glucosinolate content in E. salsugineum water-treated leaves was higher than in A. thaliana and increased after salinity treatment. Treatment with salinity up-regulated all of tested stress hormones, their precursors and catabolites [abscisic acid (ABA), dihydrophaseic acid, phaseic acid, 1-aminocyclopropane-1-carboxylic acid, salicylic acid, jasmonic acid, cis-(+)-12-oxo-phytodienoic acid and jasmonoyl-L-isoleucine] in A. thaliana, whereas in E. salsugineum only a stimulation in ethylene synthesis and ABA catabolism was noted. Obtained results suggest that constitutively enhanced H2O2 generation in chloroplasts of E. salsugineum might be a crucial component of stress-prepardeness of this halophytic species. It shapes a very efficient antioxidant protection (in which glucosinolates might play a specific role) and a fine tuning of hormonal signaling to suppress the cell death program directed by jasmonate pathway.

Plasmodesmata in integrated cell signalling: insights from development and environmental signals and stresses

To survive as sedentary organisms built of immobile cells, plants require an effective intercellular communication system, both locally between neighbouring cells within each tissue and systemically across distantly located organs. Such a system enables cells to coordinate their intracellular activities and produce concerted responses to internal and external stimuli. Plasmodesmata, membrane-lined intercellular channels, are essential for direct cell-to-cell communication involving exchange of diffusible factors, including signalling and information molecules. Recent advances corroborate that plasmodesmata are not passive but rather highly dynamic channels, in that their density in the cell walls and gating activities are tightly linked to developmental and physiological processes. Moreover, it is becoming clear that specific hormonal signalling pathways play crucial roles in relaying primary cellular signals to plasmodesmata. In this review, we examine a number of studies in which plasmodesmal structure, occurrence, and/or permeability responses are found to be altered upon given cellular or environmental signals, and discuss common themes illustrating how plasmodesmal regulation is integrated into specific cellular signalling pathways.

RNA-directed DNA methylation: an epigenetic pathway of increasing complexity

RNA-directed DNA methylation (RdDM) is the major small RNA-mediated epigenetic pathway in plants. RdDM requires a specialized transcriptional machinery that comprises two plant-specific RNA polymerases - Pol IV and Pol V - and a growing number of accessory proteins, the functions of which in the RdDM mechanism are only partially understood. Recent work has revealed variations in the canonical RdDM pathway and identified factors that recruit Pol IV and Pol V to specific target sequences. RdDM, which transcriptionally represses a subset of transposons and genes, is implicated in pathogen defence, stress responses and reproduction, as well as in interallelic and intercellular communication.

Embryo and endosperm, partners in seed development

Angiosperm seeds are the major source of human calories, generating a pressing need to understand the underlying processes governing seed growth and development. They are composed of the two fertilization products, embryo and endosperm surrounded by the maternally derived seed coat. The successful interaction of all three seed components is a requirement for seeds to complete their development and to produce viable embryos that are competent to establish a new sporophytic generation. Here, we review recent reports investigating signal exchange between embryo and endosperm, focusing in particular on the transport of metabolites and small RNAs between both fertilization products.

Genomic imprinting in the Arabidopsis embryo is partly regulated by PRC2

Genomic imprinting results in monoallelic gene expression in a parent-of-origin-dependent manner and is regulated by the differential epigenetic marking of the parental alleles. In plants, genomic imprinting has been primarily described for genes expressed in the endosperm, a tissue nourishing the developing embryo that does not contribute to the next generation. In Arabidopsis, the genes MEDEA (MEA) and PHERES1 (PHE1), which are imprinted in the endosperm, are also expressed in the embryo; whether their embryonic expression is regulated by imprinting or not, however, remains controversial. In contrast, the maternally expressed in embryo 1 (mee1) gene of maize is clearly imprinted in the embryo. We identified several imprinted candidate genes in an allele-specific transcriptome of hybrid Arabidopsis embryos and confirmed parent-of-origin-dependent, monoallelic expression for eleven maternally expressed genes (MEGs) and one paternally expressed gene (PEG) in the embryo, using allele-specific expression analyses and reporter gene assays. Genetic studies indicate that the Polycomb Repressive Complex 2 (PRC2) but not the DNA METHYLTRANSFERASE1 (MET1) is involved in regulating imprinted expression in the embryo. In the seedling, all embryonic MEGs and the PEG are expressed from both parents, suggesting that the imprint is erased during late embryogenesis or early vegetative development. Our finding that several genes are regulated by genomic imprinting in the Arabidopsis embryo clearly demonstrates that this epigenetic phenomenon is not a unique feature of the endosperm in both monocots and dicots.

In most cells nuclear genes are present in two copies, with one maternal and one paternal allele. Usually, the two alleles share the same fate regarding their activity, with both copies being active or both being silent. An exception to this rule are genes that are regulated by genomic imprinting, where only one allele is expressed and the other one remains silent depending on the parent it was inherited from. The two alleles are equal in terms of their DNA sequence but carry different epigenetic marks distinguishing them. Genomic imprinting evolved independently in mammals and flowering plants. In mammals, genes regulated by genomic imprinting are expressed in a wide range of tissues including the embryo and the placenta. In plants, genomic imprinting has been primarily described for genes expressed in the endosperm, a nutritive tissue in the seed with a function similar to that of the mammalian placenta. Here, we describe that some genes are also regulated by genomic imprinting in the embryo of the model plant Arabidopsis thaliana. An epigenetic silencing complex, the Polycomb Repressive Complex 2 (PRC2), partly regulates genomic imprinting in the embryo. Interestingly, embryonic imprints seem to be erased during late embryo or early seedling development.

Organisation of the endosperm and endosperm-placenta syncytia in bladderworts (Utricularia, Lentibulariaceae) with emphasis on the microtubule arrangement

Multinucleate cells play an important role in higher plants, especially during reproduction; however, the configurations of their cytoskeletons, which are formed as a result of mitosis without cytokinesis, have mainly been studied in coenocytes. Previous authors have proposed that in spite of their developmental origin (cell fusion or mitosis without cytokinesis), in multinucleate plant cells, radiating microtubules determine the regular spacing of individual nuclei. However, with the exception of specific syncytia induced by parasitic nematodes, there is no information about the microtubular cytoskeleton in plant heterokaryotic syncytia, i.e. when the nuclei of fused cells come from different cell pools. In this paper, we describe the arrangement of microtubules in the endosperm and special endosperm-placenta syncytia in two Utricularia species. These syncytia arise from different progenitor cells, i.e. cells of the maternal sporophytic nutritive tissue and the micropylar endosperm haustorium (both maternal and paternal genetic material). The development of the endosperm in the two species studied was very similar. We describe microtubule configurations in the three functional endosperm domains: the micropylar syncytium, the endosperm proper and the chalazal haustorium. In contrast to plant syncytia that are induced by parasitic nematodes, the syncytia of Utricularia had an extensive microtubular network. Within each syncytium, two giant nuclei, coming from endosperm cells, were surrounded by a three-dimensional cage of microtubules, which formed a huge cytoplasmic domain. At the periphery of the syncytium, where new protoplasts of the nutritive cells join the syncytium, the microtubules formed a network which surrounded small nuclei from nutritive tissue cells and were also distributed through the cytoplasm. Thus, in the Utricularia syncytium, there were different sized cytoplasmic domains, whose architecture depended on the source and size of the nuclei. The endosperm proper was isolated from maternal (ovule) tissues by a cuticle layer, so the syncytium and chalazal haustorium were the only way for nutrients to be transported from the maternal tissue towards the developing embryo.