Preferential degradation of plastid DNA with preservation of mitochondrial DNA in the sperm cells ofPelargonium zonale during pollen development

Summary In the present study, we studied changes in organellar DNA in the sperm cells of maturing pollen ofPelargonium zonale, a plant typical to exhibit biparental inheritance, by fluorescence microscopy after staining with 4,6-diamidino-2-phenylindole (DAPI) and by immunogold electron microscopy using anti-DNA antibody. Fluorescence intensities of DAPI-stained plastid nuclei in generative and sperm cells at various developmental stages were quantified with a video-intensified microscope photon counting system (VIMPCS). Results indicated that the amount of DNA per plastid in generative cells increased gradually during pollen development and reached a maximum value (about 70 T per plastid; 1 T represents the amount of DNA in a particle of T4 phage) in young sperm cells at 5 days before flowering. However, the DNA content of plastids was subsequently reduced to about 20% of the maximum value on the day of flowering. Moreover, the DNA content of the plastid further decreased to 4% of the maximum value when pollen grains were cultured for 6 h in germination medium. In contrast, the amount of DNA per mitochondrion did not decrease significantly around the flowering day. Similar results were also obtained by immunogold electron microscopy using anti-DNA antibody. The density of gold particles on plastids decreased during pollen maturation whereas labelling density on mitochondria remained relatively constant. The number of plastids and mitochondria per generative cell or per pair of sperm cells did not change significantly, indicating that the segregation of DNA by plastid division was not responsible for the decrease in the amount of DNA per plastid. These results indicate that the plastid DNA is preferentially degraded, but the mitochondrial DNA is preserved, in the sperm cells ofP. zonale. While the plastid DNA of the sperm cells decreased before fertilization, it was also suggested that the low DNA contents that remain in the plastids of the sperm cells are enough to account for the biparental inheritance of plastids inP. zonale.Abbreviations DAPI 4,6-diamidino-2-phenylindole - VIMPCS video-intensified microscope photon counting system     

INTERMEDIATE FEATURES OF CYANELLE DIVISION OF CYANOPHORA PARADOXA (GLAUCOCYSTOPHYTA) BETWEEN CYANOBACTERIAL AND PLASTID DIVISION1

Cyanelles of glaucocystophytes may be the most primitive of the known plastids based on their peptidoglycan content and the sequence phylogeny of cyanelle DNA. In this study, EM observations have been made to characterize the cyanelle division of Cyanophora paradoxa Korshikov and to gain insights into the evolution of plastid division. Constriction of cyanelles involves ingrowth of the septum at the cleavage site with the inner envelope membrane invaginating at the leading edge and the outer envelope membrane invaginating behind the septum. This means the inner and outer envelope membranes do not constrict simultaneously as they do in plastid division in other plants. The septum and the cyanelle envelope became stained after a silver‐methenamine staining was applied for in situ detection of polysaccharides. Septum formation was inhibited by β‐lactams and vancomycin, which are potent inhibitors of bacterial peptidoglycan biosynthesis. These results suggest the presence of peptidoglycan at the septum and the cyanelle envelope. In dividing cyanelles, a single electron‐dense ring (cyanelle ring) was observed on the stromal face of the inner envelope membrane at the isthmus, but no ring‐like structures were detected on the outer envelope membrane. Thus a single, stromal cyanelle ring such as this is quite unique and also distinct from FtsZ rings, which are not detectable by TEM. These features suggest that the cyanelle division of glaucocystophytes represents an intermediate stage between cyanobacterial and plastid division. If monophyly of all plastids is true, the cyanelle ring and the homologous inner plastid dividing ring might have evolved earlier than the outer plastid dividing ring.     

CHLOROPLAST DEVELOPMENT DURING SPOROGENESIS IN SIX SPECIES OF MOSSES

Chloroplast development during sporogenesis in Mnium cuspidatum, M. medium, M. rostratum, Aulacomnium heterostichum, Bartramia pomiformis, and Timmia megapolitana is as follows: During the early mitotic divisions in the sporogenous area of the capsule the number of plastids is reduced from many to one cup-shaped plastid per sporogenous cell. This single plastid divides during the early spore-mother-cell stage. A second division of plastids produces four plastids within each spore-mother-cell. A massive accumulation of starch occurs within each of the four plastids. Following meiosis, the single plastid allocated to each spore produces distinct lobes that are “blebbed” off as proplastids. A photosynthetic membrane system is established within the many proplastids as each spore matures.     

MONOPLASTIDIC CELL DIVISION IN LOWER LAND PLANTS

In many bryophytes and vascular cryptogams mitosis and/or meiosis takes place in cells containing a single plastid. In monoplastidic cell division plastid polarity assures that nuclear and plastid division are infallibly coordinated. The two major components of plastid polarity are morphogenetic plastid migration and microtubule organization at the plastids. Before nuclear division the plastid migrates to a position intersecting the future division plane. This morphogenetic migration is a reliable marker of division polarity in cells with and without a preprophase band of microtubules (PPB). The PPB, which predicts the future division plane before mitosis, is a characteristic feature of land plants and its insertion into the cytokinetic apparatus marks the evolution of a cortical microtubule system and a commitment to meristematic growth. Microtubule systems associated with plastid division, the axial microtubule system (AMS) in mitosis and the quadripolar microtubule system (QMS) in meiosis, contribute to predictive positioning of plastids and participate directly in spindle ontogeny. Division polarity in monoplastidic sporocytes is remarkable in that division sites are selected prior to the two successive nuclear divisions of meiosis. Plastid arrangement prior to meiosis determines the future spore domains in monoplastidic sporocytes, whereas in polyplastidic sporocytes the spore nuclei play a major role in claiming cytoplasmic domains. It is hypothesized that predivision microtubule systems associated with monoplastidic cell division are early forming components of the mitotic apparatus that serve to orient the spindle and insure equal apportionment of nucleus and plastids. “Can it be supposed that cytoplasm would be intrusted with so important a task as the preparation of a chloroplast for each of the four nuclei that are later to preside over the spores before there is any indication that such nuclear division is to take place?” Bradley Moore Davis, 1899     

The genetic control of plastid division in higher plants

The division of plastids is an important part of plastid differentiation and development and in distinct cell types, such as leaf mesophyll cells, results in large populations of chloroplasts. The morphology and population dynamics of plastid division have been well documented, but the molecular controls underlying plastid division are largely unknown. With the isolation of Arabidopsis mutants in which specific aspects of plastid and proplastid division have been disrupted, the potential exists for a detailed knowledge of how plastids divide and what factors control the rate of division in different cell types. It is likely that knowledge of plant homologues of bacterial cell division genes will be essential for understanding this process in full. The processes of plastid division and expansion appear to be mutually independent processes, which are compensatory when either division or expansion are disrupted genetically. The rate of cell expansion appears to be an important factor in initiating plastid division and several systems involving rapid cell expansion show high levels of plastid division activity. In addition, observation of plastids in different cell types in higher plants shows that cell-specific signals are also important in the overall process in determining not only the differentiation pathway of plastids but also the extent of plastid division. It appears likely that with the exploitation of molecular techniques and mutants, a detailed understanding of the molecular basis of plastid division may soon be a reality.     

DIFFERENTIATION OF INTRACAPSULAR CELLS IN THE SPOROPHYTE OF SPHAEROCARPOS DONNELLII

Ultrastructural and histochemical changes during intracapsular cell differentiation in the premeiotic sporophyte of the liverwort Sphaerocarpos donnellii Austin were studied. From an initially undifferentiated meristematic tissue, spore mother cells and nutritive cells become differentiated. The first indications of ultrastructural differentiation into two cell types are the accumulation of lipid within spherosomes and the occurrence of plastid tubules in the presumptive spore mother cells. Once differentiated the two cell types are clearly distinguishable on the basis of cytoplasmic vacuolation, stored food reserve, and cell and nuclear size. The mature spore mother cell contains many spherosomes, small vacuoles, starch-containing plastids, and a large central nucleus. The mature nutritive cell, on the other hand, is extremely vacuolate and contains large, starch-filled plastids, a few spherosomes, and a small nucleus. A previously undescribed type of cell was observed in developing sporophyte capsules. This cell is located peripherally in the capsule and degenerates during differentiation of spore mother cells and nutritive cells.     

In situ plastid and mitochondrial DNA determination: Implication of the observed MINIMAL PLASTID GENOME NUMBER

The total loss of plastid DNA has never been reported for any alga or plant cell line, with the sole exception of the protozoan Euglena, yet plastid distribution at mitosis is apparently stochastric (Birky and Skavaril, Journal of Theoretical Biology, vol. 106, pp. 441–447, 1984) and accidental loss might be expected. It is not obvious how stem cells of photosynthetic eukaryotes avoid this problem. The chrysophyte alga Ochromonas danica, described as having but one or two plastids, can proliferate indefinitely without the benefit of photosynthesis. Under such conditions its plastid genome copy number per cell might drop to the absolute minimum compatible with maintaining its inheritance. In situ quantitation of Ochromonas plastid DNA in both photosynthetic and enriched mixotrophic growth, and in heterotrophic growth in prolonged darkness, suggests that plastids are capable of very wide variation (7 to >;200 genomes/plastid) in their DNA content, and likewise, cells can vary from one to >;8 plastids per cell, with total genomes numbers from 7 to >;1,000 per cell. Among many growth conditions tested, the smallest plastids were found in rapidly dividing cells grown in the dark, many of which contained but one plastid. The inability to find plastids with fewer than seven plastid genome equivalents of DNA, even in these rapidly multiplying cells grown in total darkness for months, suggests that multiple copies of the plastid genome may be very carefully maintained, even in the prolonged absence of photosynthesis. This implies that multiple copies are important for reasons other than photosynthetic capability; two possibilities are the biosynthetic steps necessary for eukaryote cell survival known to occur solely within a plastid, and/or the potential that multiple plastid genome copies provide to escape the effects of Muller's ratchet.     

The ultrastructural features and division of secondary plastids

Plastids in heterokonts, cryptophytes, haptophytes, dinoflagellates, chlorarachniophytes, euglenoids, and apicomplexan parasites derive from secondary symbiogenesis. These plastids are surrounded by one or two additional membranes covering the plastid-envelope double membranes. Consequently, nuclear-encoded plastid division proteins have to be targeted into the division site through the additional surrounding membranes. Electron microscopic observations suggest that the additional surrounding membranes are severed by mechanisms distinct from those for the division of the plastid envelope. In heterokonts, cryptophytes and haptophytes, the outermost surrounding membrane (epiplastid rough endoplasmic reticulum, EPrER) is studded with cytoplasmic ribosomes and connected to the rER and the outer nuclear envelope. In monoplastidic species belonging to these three groups, the EPrER and the outer nuclear envelope are directly connected to form a sac enclosing the plastid and the nucleus. This nuclear-plastid connection, referred to as the nucleus-plastid consortium (NPC), may be significant to ensure the transmission of the plastids during cell division. The plastid dividing-ring (PD-ring) is a conserved component of the division machinery for both primary and secondary plastids. Also, homologues of the bacterial cell division protein, FtsZ, may be involved in the division of secondary plastids as well as primary plastids, though in secondary plastids they have not yet been localized to the division site. It remains to be examined whether or not dynamin-like proteins and other protein components known to function in the division of primary plastids are used also in secondary plastids. The nearly completed sequencing of the nuclear genome of the diatom Thalassiosira pseudonana will give impetus to molecular and cell biological studies on the division of secondary plastids.     

The nuclei of cellular organelles and the formation of daughter organelles by the “plastid-dividing ring”

It has been established that organelles, such as mitochondria and plastids, contain organelle-specific DNA and arise from the division of pre-existing organelles (e.g., Possingham and Lawrence, 1983). We propose that organelle DNAs, such as mitochondrial DNA and plastid DNA are not naked in organellesin situ but are organized in each case to form an “organelle nucleus” with basic proteins (Kuroiwa, 1982). The concept of organelle nuclei has changed our ideas about the division of organelles. Thus, the process of organelle division must be composed of two main events: division of the organelle nucleus and organellekinesis (division of the other components of the mitochondrion or plastid). The latter term has been adopted as an appropriate analogue of cytokinesis. We were the first to identify the plastid-dividing ring (PD-ring), which is located in the cytoplasm close to the outer envelope membrane at the constricted isthmus of dividing chloroplasts in the red algaCyanidium caldirum. The PD-ring is about 60 nm in width and 25 nm in thickness, and is a circular bundle of actin-like, fine filaments, each about 4–5 nm in diameter. Since cytochalasin B, an inhibitor of polymerization of actin filaments, inhibits the formation of the PD-ring and, thus, prevents subsequent division of chloroplasts, the PD-ring is thought to be a structure that is essential for the division of plastids (plastidkinesis). The behavior of the PD-ring during a cycle of chloroplast division can be classified into the following four stages on the basis of morphological and temporal differences. The chloroplast growth stage: the small, spherical chloroplast increases in volume and becomes a football-like structure, while the PD-ring from the previous division disappears. Formation of the PD-ring: the somewhat electron-dense body (see below) is fragmented into many, somewhat electron-dense granules, which are aligned along the equatorial region of the chloroplast and fine filaments are formed from the somewhat electron-dense granules in the equatorial region. The fine filaments of the PD-ring align themselves according to the longest axis of their overall domain, i.e., circumferentially. Contraction stage: a bundle of fine filaments begins to contract and generates a deep furrow. Conversion stage: after chloroplast division, the remnants of the PD-ring are converted into somewhat electron-dense bodies. Similar events occur during the second cycle of chloroplast division. Since similar structures are observed extensively in the plastids of algae, moss and higher plants, the PD-ring appears to be an essential structure for the division of plastids in plants.     

Observations on dividing plastids in the protonema of the moss Funaria hygrometrica Sibth.

The process of division was investigated in the different types of plastids found in the tip cell of the protonema of Funaria hygrometrica Sibth. There were no structural changes in the envelope membranes of any of the plastid types during the initial stage of division. As the process of constriction advanced, thylakoids were locally disintegrated and sometimes starch grains in the isthmus were locally dissolved. In the isthmus, tightly constricted plastids were characterized by an undulating envelope and an increasing number of vesicles. After three-dimensional reconstruction of electronmicrographs a distinct filamentous structure was observed in the plane of division outside the plastid but close to the envelope. At different stages of division the constricted regions were partly surrounded by one or a few filaments. The roundish plastids in the apical zone were accompanied by single microtubule bundles, and the spindle-shaped plastids in the cell base were surrounded by single microtubules and microtubule bundles. A model of co-operation between microtubules and the filamentous structure in the division process is discussed.A preliminary report was presented at the Tagung der Deutschen Botanischen Gesellschaft und der Vereinigung für Angewandte Botanik, Hamburg, September 1986     

Isolation of mitochondrial and plastid ftsZ genes and analysis of the organelle targeting sequence in the diatom Chaetoceros neogracile (Diatoms,Bacillariophyceae)

Mitochondria and plastids multiply by division in eukaryotic cells. Recently, the eukaryotic homolog of the bacterial cell division protein FtsZ was identified and shown to play an important role in the organelle division process inside the inner membrane. To explore the evolution of FtsZ proteins, and to accumulate data on the protein import system in mitochondria and plastids of the red algal lineage, one mitochondrial and three plastid ftsZ genes were isolated from the diatom Chaetoceros neogracile, whose plastids were acquired by secondary endosymbiotic uptake of a red alga. Protein import into organelles depends on the N‐terminal organelle targeting sequences. N‐terminal bipartite presequences consisting of an endoplasmic reticulum signal peptide and a plastid transit peptide are required for protein import into diatom plastids. To characterize the organelle targeting peptides of C. neogracile, we observed the localization of each green fluorescent protein‐tagged predicted organelle targeting peptide in cultured tobacco cells and diatom cells. Our data suggested that each targeting sequences functioned both in tobacco cultured cells and diatom cells.     

A mutation of the CRUMPLED LEAF gene that encodes a protein localized in the outer envelope membrane of plastids affects the pattern of cell division, cell differentiation, and plastid division in Arabidopsis

We identified a novel mutation of a nuclear-encoded gene, designated as CRUMPLED LEAF (CRL), of Arabidopsis thaliana that affects the morphogenesis of all plant organs and division of plastids. Histological analysis revealed that planes of cell division were distorted in shoot apical meristems (SAMs), root tips, and embryos in plants that possess the crl mutation. Furthermore, we observed that differentiation patterns of cortex and endodermis cells in inflorescence stems and root endodermis cells were disturbed in the crl mutant. These results suggest that morphological abnormalities observed in the crl mutant were because of aberrant cell division and differentiation. In addition, cells of the crl mutant contained a reduced number of enlarged plastids, indicating that the division of plastids was inhibited in the crl. The CRL gene encodes a novel protein with a molecular mass of 30 kDa that is localized in the plastid envelope. The CRL protein is conserved in various plant species, including a fern, and in cyanobacteria, but not in other organisms. These data suggest that the CRL protein is required for plastid division, and it also plays an important role in cell differentiation and the regulation of the cell division plane in plants. A possible function of the CRL protein is discussed.     

Chloroplast division in cultured cells of the hornwortAnthoceros punctatus

Using cultured cells of the hornwortAnthoceros punctatus, the change in the relative chloroplast DNA content in each stage of chloroplast division was investigated to clarify the relationship between the division cycle of a chloroplast and a cell nucleus. Samples of cultured cells were stained with 4′,6-diamidino-2-phenylindole (DAPI) and then observed with an epifluorescence microscope and a chromosome image analyzing system (CHIAS). A chloropiast in cultured cells duplicated DNA with an increase in size. When a chloroplast began to divide, it was constricted in the middle, taking a dumbbell shape, and then divided into two daughter chloroplasts. In cultured cells of this species, the pattern of quantitative change of chloroplast DNA, that is, the DNA replication pattern of chloroplasts, corresponded to that of cell nuclear DNA in mitosis.     

Morphogenetic plastid migration and microtubule organization during megasporogenesis inIsoetes

Summary The large megasporocytes ofIsoetes provide an exceptional system for studying microtubule dynamics in monoplastidic meiosis where plastid polarity assures coordination of plastid and nuclear division by the intimate association of MTOCs with plastids. Division and migration of the plastid in prophase establishes the tetrahedrally arranged cytoplasmic domains of the future spore tetrad and the four plastid-MTOCs serve as focal points of a unique quadripolar microtubule system (QMS). The QMS is a dynamic structure which functions in plastid deployment and contributes directly to development of both first and second division spindles. The nucleation of microtubules at discrete plastid-MTOCs is compared with centrosomal nucleation of microtubules in animal cells where growth of microtubules involves dynamic instability.Abbreviations AMS axial microtubule system - MTOC microtubule organizing center - N nucleus - QMS quadripolar microtubule system - P plastid - PPB preprophase band of microtubules     

Preferential digestion of chloroplast nuclei (nucleoids) during senescence of the coleoptile ofOryza sativa

Summary The coleoptile ofOryza sativa develops, grows and ages within 4 days that follow imbibition. It is, thus, a very useful system for experimental analysis of the life cycle of organelles, for example, the development, growth and aging of plastids in higher plants. We examined the behavior and levels of DNA and chlorophyll in the plastid by epifluorescence microscopy after staining with 4-6-diamidino-2-phenylindole (DAPI), and by fluorimetry with a video-intensified-photon counting system (VIMPCS). The whitish yellow coleoptile appeared soon after imbibition and, between the first 24 and 60 h that followed imbibition, it grew markedly in a longitudinal direction, with concomitant elongation of the cells, and an increase in the volume of plastids and in the amount of DNA in the plastids. The chlorophyll content per plastid began to increase when the coleoptile turned green, 48 h after imbibition, and reached a plateau value when the coleoptile was 3.5 mm in length, 72 h after imbibition. More than 12 h later, the chlorophyll disappeared just before the breakdown of chloroplasts was initiated. Proplastids in young coleoptiles, contained a plastid nucleus which was located in the central area of the plastids and each nucleus consisted of approximately 6 copies of plastid DNA (ptDNA). The number of copies of ptDNA per plastid increased gradually, with a concomitant increase in the volume of the plastids after imbibition, and reached approximately 130 times the value in the young proplastids, 60 h after imbibition, when the plastid developed into a chloroplast. However, each plastid nucleus did not scatter throughout the entire interior region of each chloroplast. The disappearance of each plastid nucleus occurred more than 12 h before the degeneration of the chloroplasts. The number of plastids per cell increased from 10 to 15 in young coleoptiles within 12 h after imbibition. Yet the number remained constant throughout subsequent growth and aging of the coleoptile. Thus the preferential reduction in the amount of chloroplast DNA was not due to the division of the plastid but could, perhaps, be associated directly with the aging of the cells of the coleoptile which precedes senescence of the coleoptiles.     

PLASTID DIVISION IN MALLOMONAS (SYNUROPHYCEAE,HETEROKONTA)1

Laser scanning confocal microscopy and TEM were used to study the morphology of secondary plastids in algae of the genus Mallomonas (Synurophyceae). At interphase, Mallomonas splendens (G. S. West) Playfair, M. rasilis Dürrschm., M. striata Asmund, and M. adamas K. Harris et W. H. Bradley contained a single H‐shaped plastid consisting of two large lobes connected by a narrow isthmus. Labeling of DNA revealed a necklace‐like arrangement of plastid nucleoids at the periphery of the M. splendens plastid and a less‐patterned array in M. rasilis. The TEM of M. splendens and M. rasilis showed an electron‐dense belt surrounding the plastid isthmus in interphase cells; this putative plastid‐dividing ring (PD ring) was adpressed to the inner pair of the four plastid membranes, suggesting that it is homologous to the PD ring of green and red plastids. The PD ring did not contain actin (indicated by lack of staining with phalloidin) and displayed filaments or tubules of 5–10 nm in diameter that may be homologous to the tubules described in red algal PD rings. Confocal microscopy of chl autofluorescence from M. splendens showed that the plastid isthmus was severed as mitosis began, giving rise to two single‐lobed daughter plastids, which, as mitosis and cell division progressed, separated from one another and then each constricted to form the H‐shaped plastids of daughter cells. Similar plastid division cycles were observed in M. rasilis and M. adamas; however, the plastid isthmus of M. striata was retained throughout most of cell division and was eventually severed by the cell cleavage furrow.     

THE QUADRIPOLAR MICROTUBULE SYSTEM AND MEIOTIC SPINDLE ONTOGENY IN HORNWORTS (BRYOPHYTA: ANTHOCEROTAE)

Ontogeny of the meiotic spindle in hornworts was studied by light microscopy of live materials, transmission electron microscopy, and indirect immunofluorescence microscopy. As in monoplastidic meiosis of mosses and Isoetes, the single plastid divides twice, and the four resultant plastids migrate into the future spore domains where they organize a quadripolar microtubule system (QMS). Additionally, a unique axial microtubule system (AMS) was found to parallel the plastid isthmus at each division in meiosis, much as in the single plastid division of mitosis. This finding is used to make a novel comparison of mitotic and meiotic spindle development. The AMS contributes directly to development of the mitotic spindle, whereas ontogeny of the meiotic spindle is more complex. Nuclear division in meiosis is delayed until after the second plastid division; the first AMS disappears without spindle formation, and the two AMSs of the second plastid division contribute to development of the QMS. Proliferation of microtubules at each plastid results in the QMS consisting of four cones of microtubules interconnecting the plastids and surrounding the nucleus. The QMS contributes to the development of a functionally bipolar spindle. The meiotic spindle is comparable to a merger of two mitotic spindles. However, the first division spindle does not terminate in what would be the poles of mitosis; instead the poles converge to orient the spindle axis midway between pairs of non-sister plastids.     

Control of plastid division by means of nuclear DNA amount

Summary For a given cell type and genotype a close positive correlation exists between the number of plastids in a cell and the amount of DNA in the nucleus. Comprehensive evidence is presented. The duplication of the DNA amount entails an increase of the plastid number in differentiating cells by about 70%. Exceptions reported in the literature are critically examined. The odds are in favour of the assumption that exceptions to the rule which are not due to special circumstances do not exist. In meristematic cells even a duplication of the plastid number will occur, for cells without plastids are not to be found. The plastids are always ready to divide, the interpretation goes, but the size of their populations is limited by the amount of nuclear DNA. Thus meristematic cells manage to control their plastid populations by releasing once in a cell cycle the brakes imposed upon plastid division, whereupon the plastids make use of their newly won freedom, dividing until the old ratio between plastid number and nuclear DNA amount is established again. As a shorter time is needed for plastid division than for mitosis, there is no danger of cells arising without plastids; no distributing mechanism is required if at least three to four plastids are present in a cell. The findings are consistent with and would appear to be best explained by the theory of the symbiotic origin of the plastids.     

Diversification of ftsZ During Early Land Plant Evolution

The plastid division proteins FtsZ are encoded by a small nuclear gene family in land plants. Although it has been shown for some of the gene products that they are imported into plastids and function in plastid division, the evolution and function of this gene family and their products remain to be unraveled. Here we present two new ftsZ genes from the moss Physcomitrella patens and compare the genomic structure of members of the two plant ftsZ gene families. Comparison of sequence features and phylogenetic analyses confirm the presence of two clusters of paralogues in land plants and demonstrate that these genes were duplicated before the divergence of mosses, ferns and seed plants.     

Exceptional inheritance of plastids via pollen in Nicotiana sylvestris with no detectable paternal mitochondrial DNA in the progeny

Plastids and mitochondria, the DNA‐containing cytoplasmic organelles, are maternally inherited in the majority of angiosperm species. Even in plants with strict maternal inheritance, exceptional paternal transmission of plastids has been observed. Our objective was to detect rare leakage of plastids via pollen in Nicotiana sylvestris and to determine if pollen transmission of plastids results in co‐transmission of paternal mitochondria. As father plants, we used N. sylvestris plants with transgenic, selectable plastids and wild‐type mitochondria. As mother plants, we used N. sylvestris plants with Nicotiana undulata cytoplasm, including the CMS‐92 mitochondria that cause cytoplasmic male sterility (CMS) by homeotic transformation of the stamens. We report here exceptional paternal plastid DNA in approximately 0.002% of N. sylvestris seedlings. However, we did not detect paternal mitochondrial DNA in any of the six plastid‐transmission lines, suggesting independent transmission of the cytoplasmic organelles via pollen. When we used fertile N. sylvestris as mothers, we obtained eight fertile plastid transmission lines, which did not transmit their plastids via pollen at higher frequencies than their fathers. We discuss the implications for transgene containment and plant evolutionary histories inferred from cytoplasmic phylogenies.