Nuclear differentiation in the filamentous caulonema of the moss Funaria hygrometrica

A single homeobox gene triggers phase transition, embryogenesis and asexual reproduction

Plants characteristically alternate between haploid gametophytic and diploid sporophytic stages. Meiosis and fertilization respectively initiate these two different ontogenies(1). Genes triggering ectopic embryo development on vegetative sporophytic tissues are well described(2,3); however, a genetic control of embryo development from gametophytic tissues remains elusive. Here, in the moss Physcomitrella patens we show that ectopic overexpression of the homeobox gene BELL1 induces embryo formation and subsequently reproductive diploid sporophytes from specific gametophytic cells without fertilization. In line with this, BELL1 loss-of-function mutants have a wild-type phenotype, except that their egg cells are bigger and unable to form embryos. Our results identify BELL1 as a master regulator for the gametophyte-to-sporophyte transition in P. patens and provide mechanistic insights into the evolution of embryos that can generate multicellular diploid sporophytes. This developmental innovation facilitated the colonization of land by plants about 500 million years ago(4) and thus shaped our current ecosystems.

Endopolyploidy in Bryophytes: Widespread in Mosses and Absent in Liverworts

Endopolyploidy occurs when DNA replication is not followed by mitotic nuclear division, resulting in tissues or organisms with nuclei of varying ploidy levels. Endopolyploidy appears to be a common phenomenon in plants, though the prevalence of endopolyploidy has not been determined in bryophytes (including mosses and liverworts). Forty moss species and six liverwort species were analyzed for the degree of endopolyploidy using flow cytometry. Nuclei were extracted in LB01 buffer and stained with propidium iodide. Of the forty moss species, all exhibited endopolyploid nuclei (mean cycle value =0.65±0.038) except for the Sphagnum mosses (mean cycle value =0). None of the liverwort species had endopolyploid nuclei (mean cycle value = 0.04 ± 0.014). As bryophytes form a paraphyletic grade leading to the tracheophytes, understanding the prevalence and role of endopolyploidy in this group is important.

Cellular differentiation in moss protonemata: a morphological and experimental study

BACKGROUND AND AIMS

Previous studies of protonemal morphogenesis in mosses have focused on the cytoskeletal basis of tip growth and the production of asexual propagules. This study provides the first comprehensive description of the differentiation of caulonemata and rhizoids, which share the same cytology, and the roles of the cytoskeleton in organelle shaping and spatial arrangement.

METHODS

Light and electron microscope observations were carried out on in vitro cultured and wild protonemata from over 200 moss species. Oryzalin and cytochalasin D were used to investigate the role of the cytoskeleton in the cytological organization of fully differentiated protonemal cells; time-lapse photography was employed to monitor organelle positions.

KEY RESULTS

The onset of differentiation in initially highly vacuolate subapical cells is marked by the appearance of tubular endoplasmic reticulum (ER) profiles with crystalline inclusions, closely followed by an increase in rough endoplasmic reticulum (RER). The tonoplast disintegrates and the original vacuole is replaced by a population of vesicles and small vacuoles originating de novo from RER. The cytoplasm then becomes distributed throughout the cell lumen, an event closely followed by the appearance of endoplasmic microtubules (MTs) in association with sheets of ER, stacks of vesicles that subsequently disperse, elongate mitochondria and chloroplasts and long tubular extensions at both poles of the nucleus. The production of large vesicles by previously inactive dictysomes coincides with the deposition of additional cell wall layers. At maturity, the numbers of endoplasmic microtubules decline, dictyosomes become inactive and the ER is predominantly smooth. Fully developed cells remain largely unaffected by cytochalasin; oryzalin elicits profound cytological changes. Both inhibitors elicit the formation of giant plastids. The plastids and other organelles in fully developed cells are largely stationary.

CONCLUSIONS

Differentiation of caulonemata and rhizoids involves a remarkable series of cytological changes, some of which closely recall major events in sieve element ontogeny in tracheophytes. The cytology of fully differentiated cells is remarkably similar to that of moss food-conducting cells and, in both, is dependent on an intact microtubule cytoskeleton. The disappearance of the major vacuolar apparatus is probably related to the function of caulonema and rhizoids in solute transport. Failure of fully differentiated caulonema and rhizoid cells to regenerate is attributed to a combination of endo-reduplication and irreversible tonoplast fragmentation. The formation of giant plastids, most likely by fusion, following both oryzalin and cytochalasin treatments, suggests key roles for both microtubules and microfilaments in the spatial arrangement and replication of plastids.

Distribution of splicing proteins and putative coiled bodies during pollen development and androgenesis in Brassica napus L

Small nuclear ribonucleoprotein particles (snRNPs) are subunits of splicing complexes, which show a transcription-dependent localization pattern. We have analyzed the labelling pattern of snRNPs during pollen development and microspore and pollen embryogenesis in Brassica napus with an antibody which recognizes protein D of U1, U2, U4, U5, and U6 snRNPs. It was found that nuclei were labelled almost uniformly for snRNPs in microspores and young bicellular pollen. In the generative nuclei of late-bicellular pollen and in the vegetative nuclei and sperm nuclei of mature pollen no snRNPs could be detected. The snRNP-positive nuclei contained mostly one or two brightly labelled nuclear bodies, most likely coiled bodies, often closely related to the nucleolus. These nuclear bodies increased in size from 0.5 micron in nuclei of young microspores up to 2 microns in nuclei of late microspores and the vegetative nucleus of early-bicellular pollen. Also their number increased during these developmental stages. After induction of embryogenesis the size of the coiled bodies decreased to about 0.5 micron and in several occasions the coiled body was found free in the nucleoplasm, away from the nucleolus. The results support the idea that the size and number of coiled bodies coincide with changes in general nuclear activity. They also indicate that, in nuclei of Brassica napus, at least assembly and disassembly of coiled bodies takes place in the nucleoplasm, whereas mature coiled bodies are located adjacent to the nucleolus.

Conducting tissues and phyletic relationships of bryophytes

Internal specialized conducting tissues, if present, are restricted to the gametophytic generation in liverworts while they may occur in both generations in mosses. Conducting tissues are unknown in the anthocerotes. Water-conducting cells (WCCs) with walls perforated by plasmodesma-derived pores occur in the Calobryales and Pallaviciniaceae (Metzgeriales among liverworts and in Takakia among mosses. Imperforate WCCs (hydroids) are present in bryoid mosses. A polarized cytoplasmic organization and a distinctive axial system of microtubules is present in the highly specialized food-conducting cells of polytrichaceous mosses (leptoids) and in less specialized parenchyma cells of the leafy stem and seta in other mosses including Sphagnumn. A similar organization, suggested to reflect specialization in long-distance symplasmic transport of nutrients, also occurs in other parts of the plant in mosses, including rhizoids and caulonemata, and may be observed in thallus parenchyma cells of liverworts. Perforate WCCs in the Calobryales, Metzgeriales and Takakia, and hydroids in bryoid mosses, probably evolved independently Because of fundamental differences in developmental design, homology of any of these cells with tracheids is highly unlikely. Likewise, putative food-conducting of bryophytes present highly distinctive characteristics and cannot be considered homologous with the sieve cells of tracheophytes.

Higher levels of organization in the interphase nucleus of cycling and differentiated cells

The review examines the structured organization of interphase nuclei using a range of examples from the plants, animals, and fungi. Nuclear organization is shown to be an important phenomenon in cell differentiation and development. The review commences by examining nuclei in dividing cells and shows that the organization patterns can be dynamic within the time frame of the cell cycle. When cells stop dividing, derived differentiated cells often show quite different nuclear organizations. The developmental fate of nuclei is divided into three categories. (i) The first includes nuclei that undergo one of several forms of polyploidy and can themselves change in structure during the course of development. Possible function roles of polyploidy is given. (ii) The second is nuclear reorganization without polyploidy, where nuclei reorganize their structure to form novel arrangements of proteins and chromosomes. (iii) The third is nuclear disintegration linked to programmed cell death. The role of the nucleus in this process is described. The review demonstrates that recent methods to probe nuclei for nucleic acids and proteins, as well as to examine their intranuclear distribution in vivo, has revealed much about nuclear structure. It is clear that nuclear organization can influence or be influenced by cell activity and development. However, the full functional role of many of the observed phenomena has still to be fully realized.