The syncytia formed by Utricularia, carnivorous plants which belong to the family Lentibulariaceae, Lamiales (Jobson et al. ²⁰⁰³; Müller et al. ²⁰⁰⁴), are extremely unusual because they arise from different progenitor cells; cells of maternal sporophytic nutritive tissue and the micropylar endosperm haustorium (both maternal and paternal genetic material). It was recently shown that these syncytia are characterized by hypertrophy of nuclei, abundant endoplasmic reticulum and organelles, and the occurrence of wall ingrowths forming an active organ which transport nutrients to the developing embryo (Płachno and Świątek ^2010a. Utricularia syncytia have structural and functional similarities in common with syncytia induced by parasitic cyst nematodes (Sobczak and Golinowski ²⁰⁰⁹ and literature cited therein) including the characteristics mentioned above. In both types of syncytia, their development is accompanied by nuclear changes—hypertrophy of nuclei. In nematode-induced syncytia, endoreduplication occurs. It has been shown that a nematode modifies the plant cell cycle into a process of genome multiplication (de Almeida Engler et al. ¹⁹⁹⁹; Goverse et al. ²⁰⁰⁰). However, these syncytia have different benefiters; in Utricularia—a new generation—embryos (Fig. 1a), while in syncytia induced by parasitic cyst nematodes—nematodes which treat the host as a slave and need plant nutrients for maturation (Davis et al. ²⁰⁰⁹; Gheysen and Mitchum ²⁰⁰⁹). It would seem that syncytia that are very active in the case of physiological activity should have a well-developed actin cytoskeleton, but unexpectedly, during the development of syncytia, no filamentous actin organization was observed. There was a diffuse signal of actin, which could be the result of cytoskeleton depolymerization (de Almeida Engler et al. ²⁰⁰⁴). Because the syncytia of Utricularia have a number of aspects in common with syncytia induced by parasitic cyst nematodes, we would like investigate how an actin cytoskeleton is arranged in these special syncytia.

The formation and ultrastructural organization of syncytia in Utricularia have recently been described (see Płachno and Świątek ^2010a). Actin microfilaments form a dense array in the entire cytoplasm of the syncytium (Fig. 1b–e). Within this actin network, thick actin cables running parallel to each other are also visible across the syncytium (Fig. 1c). The syncytial cytoplasm of the mature syncytium is stratified into two main zones—a central zone where two of the giant nuclei from the endosperm haustorium occur and a marginal zone where new protoplasts of the nutritive cells join the syncytium. Long actin bundles display a clear orientation running throughout the cytoplasm from the marginal to central part of the syncytium but also obliquely throughout the syncytium (Fig. 1d, e). Using an epifluorescence microscope, it was observed that a delicate actin meshwork invests both the giant nuclei from the endosperm haustorium and also nuclei from the nutritive tissue cells which were incorporated into the syncytium (Fig. 1d, e). When reconstructed using confocal laser scanning microscopy (CLSM), it is clear that actin filaments occur close to the nuclear envelope and also that the actin bundles penetrate the spaces between the lobes of giant nuclei from the former endosperm haustorium (Fig. 2a, b). One of these amoeboid-shaped nuclei is shown in Fig. 2a, b. It has one giant nucleolus in the central part, and moreover, smaller additional nucleoli which appear in the lobes of this nucleus. Cross-linked microfilaments form a dense actin array in the cytoplasm in this part of the syncytium. Near the giant nuclei, there are small nuclei, and their regular shape may suggest that they originate from the nutritive cells. These nuclei are also surrounded by the extensive actin cytoskeleton (Fig. 2a). In the marginal part of the syncytium, there is an extremely dense array of cross-linked actin microfilaments (Fig. 2c). An extensive network of actin microfilaments occurs in the area where the cell walls are digested and the protoplast of nutritive tissue cells fuses with the syncytium (Fig. 2c). In nutritive tissue cells, which have not yet fused with the syncytium, actin microfilaments form a randomly arranged network (Fig. 2c). In placental parenchyma cells, which border the nutritive tissue, there is delicate actin meshwork (Fig. 2c); actin microfilaments congregate around the nuclei and the amyloplast which contain a large starch grain.

We show that despite the many similarities in the ultrastructure, the syncytia of Utricularia, unlike syncytia induced by parasitic cyst nematodes, have a well-developed actin cytoskeleton. This difference may result from the specific mode of nutrition transport of parasitic nematodes. Depolymerization of actin microfilaments and also microtubules may decrease the viscosity of cytoplasm, and this may help nematodes in nutrient absorption (de Almeida Engler et al. ²⁰⁰⁴). A well-developed F-actin cytoskeleton, like in Utricularia, was also described in the endosperm in the micropylar endosperm chamber of Coronopus didymus (Brassicaceae) (Nguyen et al. ²⁰⁰²) and in the endosperm chalazal haustorium of Rhinanthus serotinus (Orobanchaceae) (Świerczyńska and Bohdanowicz ²⁰⁰³). In both cases, these structures play a nutritive role in relation to the developing embryo, thus they behave exactly like the syncytia in Utricularia. The long actin cables and bundles of F-actin, which run across the syncytium, probably form the transport routes for macromolecules and organelles; especially since F-actin is involved in the intracellular transport of organelles, auxin transport, and since it also regulates vacuolar structures and dynamics (e.g., Boevink et al. ¹⁹⁹⁸; Dhonukshe et al. ²⁰⁰⁸; Nick ¹⁹⁹⁹; Collings et al. ²⁰⁰²; Higaki et al. ²⁰⁰⁶; Kadota et al. ²⁰⁰⁹). In the Utricularia syncytia, the actin bundles are associated with mitochondria and endoplasmic reticulum; it should be underlined that the syncytium cytoplasm is rich in both of these organelles (Płachno and Świątek ^2010a, ^b), and it seems that actin bundles have fundamental role in the movement of these organelles inside such a large structure as syncytium. The endoplasmic reticulum is the largest reservoir of cellular membranes in eukaryotic cells. During formation of the syncytia, there are dynamic cell membrane changes due to progressive cell mergers. Recently, the participation of the F-actin cytoskeleton in fast endoplasmic reticulum motility has been well documented using a velocity-distribution map for the GFP-labeled endoplasmic reticulum in Arabidopsis (Ueda et al. ²⁰¹⁰).

The well-developed cytoskeleton in syncytia is probably also connected with the unique and complex development of these syncytia because an extensive network of F-actin occurs in the area where cell walls are digested and the protoplast of nutritive tissue cells fuses with the syncytium. In plant cells, F-actin is responsible for proper cell wall dynamic (cell wall growth and expansion, Staiger et al. ²⁰⁰⁰), e.g., transport of cell wall pectins is F-actin dependent (Baluška et al. ²⁰⁰²). We think that F-actin is also connected with remodeling of cell walls in the syncytia. There are two opposing processes related to the dynamics of the cell wall which occur in the syncytium; the first is digestion of cell walls of the nutritive tissue cells, which fuse with the syncytium, the second—formation of the cell wall ingrowths on the cell wall surface in the some marginal parts of the syncytium (Płachno and Świątek ^2010a), thus F-actin is involved here in both cell wall dissolution and deposition. In the places where wall ingrowths occur, there is an intensive selective transport over short distance (transfer cells; Gunning and Pate ¹⁹⁶⁹; ¹⁹⁷⁴; Offler et al. ²⁰⁰³), in which F-actin also takes part.

It is known that F-actin is responsible for the movement and position of cell nuclei (Ketelaar et al. ²⁰⁰²; Chytilova et al. ²⁰⁰⁰). In Utricularia syncytia, the giant endosperm nuclei move until they finally occupy a more or less central position within the syncytium (Fig. 3 in Płachno and Świątek ^2010a). The nuclei from the former nutritive tissue cells also change their position within the syncytium. The motility of nuclei in the syncytium is another reason for the presence bundles of actin in the syncytium cytoplasm. The giant nuclei also changes shape forming lobes; actin may be responsible for this especially since recently it has been proved that the perinuclear actin cap regulates the nuclear shape in animal cells (Khatau et al. ²⁰⁰⁹). Actin is encoded by large gene families in plants; numbers of family member vary from 10 in Arabidopsis to more than 100 in Petunia (Baird and Meagher ¹⁹⁸⁷; McDowell et al. ¹⁹⁹⁶). So there are several actin isoform which occur in the different tissues and time during plant development (Meagher et al. ¹⁹⁹⁹). In Arabidopsis, there occur two classes of actin isoform, which are preferentially expressed in either generative or vegetative organs (McDowell et al. ¹⁹⁹⁶; Kandasamy et al. ²⁰⁰²). In connection with this knowledge, the important questions should be asked. Utricularia syncytia arise from different progenitor cells; cells of maternal sporophytic nutritive tissue and the micropylar endosperm haustorium. Are different isoforms of actin present in the two kinds of cells and what happened after cells' fusing? Are there different actin-binding proteins from these two kinds of cells that can interact in the merged cytoplasm in some unique way? Unfortunately, there is now a lack of knowledge about isoforms of actin in Utricularia. However, Lentibulariaceae to which Utricularia genus belongs, are anomalous plants in various subjects (Albert et al. ²⁰¹⁰), e.g., specific body plan (Rutishauser and Isler ²⁰⁰¹), embryo reduction (Płachno and Świątek ^2010b), variation in genome size from approximately 60 to 1,500 Mb (Greilhuber et al. ²⁰⁰⁶), and an adaptive changes in cytochrome C oxidase, which provide respiratory power (Laakkonen et al. ²⁰⁰⁶). Therefore, it can be assumed that the Utricularia will be also characterized by the unusuality in terms of actin and proteins associated with it. In case of Utricularia syncytia, results of Binarova et al. ¹⁹⁹⁶ and Schwarzerova et al. ²⁰¹⁰ who described that in spruce, meristematic cells contain a distinctly different F-actin pattern (dense, randomly oriented actin filaments) than suspensor cells (thick actin cables oriented longitudinally and transversely oriented thinner actin filaments) should be mentioned, and this is connected with different expression of actin isoforms. Protoplasts which arise from these different classes of cells have F-actin distribution in their cells of origin. The actin pattern with thick actin cables oriented longitudinally, occur also in large highly vacuolated and polyploid cells of angiosperms such as endosperm haustoria (Świerczyńska and Bohdanowicz ²⁰⁰³) and embryo suspensor (Kozieradzka-Kiszkurno et al. ²⁰⁰⁶). We observed that in Utricularia nutritive tissue cells, actin forms a randomly arranged network of F-actin, and later in syncytium, two patterns of F-actin were observed, one characteristic for nutritive cells and second—actin bundles—characteristic for haustoria and suspensors, thus syncytia inherit their F-actin patterns from their progenitors.

In conclusion, an explanation for the presence of an extensive F-actin network in the Utricularia syncytia is probably connected with the movement of nuclei and other organelles and also the transport of nutrients in these physiological activity organs which are necessary for the development of embryos in these unique carnivorous plants.

This study was funded by grant N N304 002536 from the Polish Ministry of Science and Higher Education. The first author gratefully acknowledges the support of an award from the Foundation for Polish Sciences (Start Programme). We especially thank both Małgorzata Ignatowska (University of Gdańsk) and Hanna Sas-Nowosielska (University of Silesia) for their expert technical help.

Conflict of interest statement The authors declare that they have no conflict of interest.

Open Access This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.