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

Syncytia in Utricularia: Origin and Structure

In animals and plants, multinucleate cells (syncytia and coenocytes) are essential in ontogeny and reproduction. Fuso-morphogenesis is the formation of multinucleated syncytia by cell-cell fusion, but coenocytes are formed as a result of mitosis without cytokinesis. However, in plants, coenocytes are more widespread than true syncytia. Except for articulated laticifers, most plant syncytia have a trophic function. Here, we summarize the results of histological, histochemical, and ultrastructural analyses of syncytia in the Utricularia species from the Lentibulariaceae family. Utricularia syncytia, known only from a few species, are heterokaryotic because the syncytium possesses nuclei from two different sources: cells of maternal sporophytic nutritive tissue (placenta) and endosperm haustorium. Thus, syncytium contains both maternal and paternal genetic material. In species from section Utricularia, syncytia are highly active structures (with hypertrophied nuclei, cell wall ingrowths, and extensive cytoskeleton) that exist only during embryo development. They serve as an example of evolutionary unique trophic structures in the plant kingdom.

Spatiotemporal Distribution of Homogalacturonans and Hemicelluloses in the Placentas, Ovules and Female Gametophytes of Utricularia nelumbifolia during Pollination

Utricularia nelumbifolia is a large carnivorous plant that is endemic to Brazil. It forms an extra-ovular female gametophyte, which surpasses the entire micropylar canal and extends beyond the limit of the integument and invades the placenta tissues. Due to the atypical behavior of the female gametophyte, it is interesting to determine the interaction between the gametophyte and sporophytic tissue. Therefore, the aim of this study was to evaluate the role of the placenta, the ovular tissues, the hypertrophied central cell and the integument in guiding the pollen tube in Utricularia nelumbifolia Gardner by studying the distribution of homogalacturonans and hemicelluloses. It was also determined whether the distribution of the homogalacturonans (HG) and hemicelluloses in Utricularia are dependent on pollination. The antibodies directed against the wall components (anti-pectin: JIM5, JIM7, LM19, LM20 and the anti-hemicelluloses: LM25, LM11, LM15, LM20, LM21) were used. Because both low- and high-esterified HG and xyloglucan were observed in the placenta, ovule (integument, chalaza) and female gametophyte of both pollinated and unpollinated flowers, the occurrence of these cell-wall components was not dependent on pollination. After fertilization, low methyl-esterified HGs were still observed in the cell walls of somatic cells and female gametophyte. However, in the case of high-esterified HG, the signal was weak and occurred only in the cell walls of the somatic cells. Because xyloglucans were observed in the cell walls of the synergids and egg cells, this suggests that they play a role in sexual reproduction. Utricularia nelumbifolia with an extra ovule-female gametophyte is presented as an attractive model for studying the male-female dialogue in plants.

Syncytia in Fungi

Filamentous fungi typically grow as interconnected multinucleate syncytia that can be microscopic to many hectares in size. Mechanistic details and rules that govern the formation and function of these multinucleate syncytia are largely unexplored, including details on syncytial morphology and the regulatory controls of cellular and molecular processes. Recent discoveries have revealed various adaptations that enable fungal syncytia to accomplish coordinated behaviors, including cell growth, nuclear division, secretion, communication, and adaptation of the hyphal network for mixing nuclear and cytoplasmic organelles. In this review, we highlight recent studies using advanced technologies to define rules that govern organizing principles of hyphal and colony differentiation, including various aspects of nuclear and mitochondrial cooperation versus competition. We place these findings into context with previous foundational literature and present still unanswered questions on mechanistic aspects, function, and morphological diversity of fungal syncytia across the fungal kingdom.

Delayed selfing ensures reproductive assurance in Utricularia praeterita and Utricularia babui in Western Ghats

Numerous bladderwort (Utricularia) species are distributed worldwide, but their reproductive biology is rarely investigated. Bladderworts are known to depend on tiny organisms to meet a significant proportion of their energy requirement by trapping them in bladders. However, information on the extent of their reliance on insects for pollination success is limited. We examined the reproductive strategy of two Utricularia species viz. Utricularia praeterita and U. babui, endemic to Western Ghats, India. The main aspects of the investigation involved floral biology, breeding system, pollination mechanism, and reproductive success. Flowers of both the species are structured for outbreeding through entomophilous floral suites, herkogamy, protandrous dichogamy and sensitive lobes of the stigma. With nearly 65% natural fruit-set, both the species appeared to be sufficiently open-pollinated. However, pollinators failed to show in plants of U. praeterita while in U. babui there was an apparent mismatch between the extent of fruit-set and pollinator visits. The study demonstrated that in the absence/insufficient visits of pollinators, the two species resort to autonomous selfing. In U. babui, denser patches of plants appeared to be crucial for attracting the pollinators. Both species are self-compatible, and reproductive success is predominantly achieved by delayed autonomous selfing. The sensitive stigma in the species fails to prevent selfing due to diminished herkogamy during the late anthetic stages. It is inferred that in the pollinator-limited environment, delayed selfing contributes to absolute natural fecundity in U. praeterita, while it produces a mixed progeny in U. babui.

Serial block face SEM visualization of unusual plant nuclear tubular extensions in a carnivorous plant (Utricularia, Lentibulariaceae)

Background and Aims

In Utricularia nelumbifolia , the nuclei of placental nutritive tissue possess unusually shaped projections not known to occur in any other flowering plant. The main aim of the study was to document the morphology and ultrastructure of these unusual nuclei. In addition, the literature was searched to find examples of nuclear tubular projections in other plant groups, and the nuclei of closely related species of Utricularia (i.e. sects Iperua , Orchidioides , Foliosa and Utricularia ) were examined.

Methods

To visualize the complexity of the nuclear structures, transmission electron microscopy (TEM) was used, and 3-D ultrastructural reconstructions were made using the serial block face scanning electron microscopy (SBEM) technique. The nuclei of 11 Utricularia species, i.e. U. nelumbifolia , U. reniformis , U. cornigera , U. nephrophylla (sect. Iperua ), U. asplundii , U. alpina , U. quelchii (sect. Orchidioides ), U. longifolia (sect. Foliosa ), U. intermedia , U. minor and U. gibba (sect. Utricularia ) were examined.

Key Results

Of the 11 Utricularia species examined, the spindle-like tubular projections (approx. 5 μm long) emanating from resident nuclei located in placental nutritive tissues were observed only in U. nelumbifolia . These tubular nuclear extensions contained chromatin distributed along hexagonally shaped tubules. The apices of the projections extended into the cell plasma membrane, and in many cases also made contact at the two opposing cellular poles, and with plasmodesmata via a short cisterna of the cortical endoplasmic reticulum. Images from the SBEM provide some evidence that the nuclear projections are making contact with those of neighbouring cells.

Conclusions

The term chromatubules (chromatin-filled tubules) for the nuclear projections of U. nelumbifolia placental tissue was proposed here. Due to the apparent association with the plasma membrane and plasmodesmata, it was also speculated that chromatubules are involved in nucleus-cell-cell communication. However, further experimental evidence is required before any functional hypothesis can be entertained.

Disruption of endosperm development is a major cause of hybrid seed inviability between Mimulus guttatus and Mimulus nudatus

Divergence of developmental mechanisms within populations could lead to hybrid developmental failure, and might be a factor driving speciation in angiosperms. We investigate patterns of endosperm and embryo development in Mimulus guttatus and the closely related, serpentine endemic Mimulus nudatus, and compare them to those of reciprocal hybrid seed. We address whether disruption in hybrid seed development is the primary source of reproductive isolation between these sympatric taxa. M. guttatus and M. nudatus differ in the pattern and timing of endosperm and embryo development. Some hybrid seeds exhibit early disruption of endosperm development and are completely inviable, while others develop relatively normally at first, but later exhibit impaired endosperm proliferation and low germination success. These developmental patterns are reflected in mature hybrid seeds, which are either small and flat (indicating little to no endosperm) or shriveled (indicating reduced endosperm volume). Hybrid seed inviability forms a potent reproductive barrier between M. guttatus and M. nudatus. We shed light on the extent of developmental variation between closely related species within the M. guttatus species complex, an important ecological model system, and provide a partial mechanism for the hybrid barrier between M. guttatus and M. nudatus.

Rhinanthus serotinus (Schönheit) Oborny (Scrophulariaceae): immunohistochemical and ultrastructural studies of endosperm chalazal haustorium development

Chalazal endosperm haustorium in Rhinanthus serotinus consists of a single large binucleate cell. It originates from the primary endosperm cell dividing transversely into two unequal cells: a smaller micropylar cell and a larger chalazal cell. The chalazal cell undergoes a single mitotic division, then lengthens significantly during development and functions as a chalazal endosperm haustorium. In this paper, immunofluorescent techniques, rhodamine phalloidin assay, and electron microscopy were used to examine the actin and tubulin cytoskeleton during the development of the chalazal haustorium. During the differentiation stage, numerous longitudinally oriented bundles of microfilaments ran along the axis of transvacuolar strands in haustorium. Microtubules formed intensely fluorescent areas near the nuclear envelope and also formed radial perinuclear microtubule arrays. In the fully differentiated haustorium cell, the actin cytoskeleton formed dense clusters of microfilaments on the chalazal and micropylar poles of the haustorium. Numerous microfilament bundles occurred near wall ingrowths on the chalazal wall. There were numerous clusters of microfilaments and microtubules around the huge lobed polytenic haustorial nuclei. The microfilaments were oriented longitudinally to the long axis of the haustorium cell and surrounded both nuclei. The microtubules formed radial perinuclear systems which were appeared to radiate from the surface of the nuclear envelope. The early stage of degeneration of the chalazal haustorium was accompanied by the degradation of microtubules and disruption of the parallel orientation of microtubules in the chalazal area of the cell. The degree of vacuolization increased, autophagous vacuoles appeared and the number of vesicles decreased.