TURION FORMATION AND GERMINATION IN SPIRODELA POLYRHIZA

Three clones of Spirodela polyrhiza L. (Schleid.) formed dormant bodies called turions. A clone from Puerto Rico did not form turions under all conditions tried. In those clones producing turions, formation was stimulated by the addition of sucrose (10–50 mM) to the nutrient solution. Increased levels of Ca(NO₃)₂ plus sucrose stimulated turion production. In the absence of NO₃^–, Ca⁺⁺ was more effective than K⁺ in stimulating turion formation. Turion buoyancy was not light dependent, nor was it promoted by sucrose. Normal turions required light for germination, whereas sucrose-induced turions germinated in the dark. Dark germination was not promoted by either Ca⁺⁺ or K⁺. Sucrose stimulation of turion formation and subsequent promotion of dark germination was attributed to metabolic rather than osmotic effects. One hundred mM sucrose concentrations inhibited turion buoyancy and germination. Turions formed one primary abscission layer which separated them from the stolon and the mother frond. Subepidermal idioblasts appeared to seal the stolon stump after separation.     

Lemna paucicostata Hegelm. 6746: LIFE CYCLE AND CHARACTERIZATION OF THE COLONY TYPES IN A POPULATION

The sequence of frond emergence and the intervals required for daughter colony separation have been determined for Lemna paucicostata Hegelm. 6746 growing under standardized conditions. After separation of a new mother colony, the first daughter colony is produced from the left meristematic pocket and separates after approximately 60 hours, the second daughter, produced from the right pocket, separates after a further 30 hours, and the third daughter, again from the left, after a further 40 hours. The pattern of alternating longer and shorter intervals for separation of daughters continues throughout the life of the mother colony.     

Cellular redox-status is associated with regulation of frond division in <Emphasis Type="Italic">Spirodela polyrrhiza</Emphasis>

We investigated a possible relationship between the levels of reactive oxygen species (ROS) and the stimulation of frond division of the aquatic plant Spirodela polyrrhiza (duckweed) during a 7-day experimental culture period. In particular, we monitored superoxide concentration using a state-of-the-art cell biosensor. A considerable reduction in ROS and superoxide concentration was observed during the first 2 days of culture, whereas duckweed cultures achieved near exponential growth rates after the second day. In addition, apoptotic markers such as the cytoplasmic concentration of cytochrome c, mitochondrial membrane depolarization and the activity of caspase-3 declined during the culture period and at least before daughter frond maturation. We suggest that S. polyrrhiza frond division may have been stimulated by the observed reduction of free radicals and the associated avoidance of cell apoptotic pathways in cultured plants.     

Morphogenesis of Asymmetry and Symmetry in Lemna perpusilla Torr

The determination of which of the two reproductive pockets ofLemna perpusilla will produce the first daughter frond undernon-flowering conditions, or the flower under inductive conditions,is related to asymmetric gradients within the mother frond whichmay be the result of the asymmetric position of the axillaryfrond. Treatments that encourage vigorous growth of axillaryfronds may overcome the normal situation of asymmetry and causea morphological symmetry which is qualitatively different fromthe symmetry induced by treatments (TIBA) which prevent thedevelopment of axillary fronds. The origin and maintenance ofasymmetry in Lemna are discussed. Lemna perpusilla Torr., morphogenesis, asymmetry of fronds, tri-iodobenzoic acid     

A novel mechanism of abscission in fronds of Lemna minor L. and the effect of silver ions

Lemna minor L. (duckweed) forms colonies through vegetative propagation because mother fronds remain connected for some time with their daughter fronds by stipes. The colony size is controlled by abscission of stipes at a specific preformed abscission zone. Application of silver ions (Ag(+) ) enhances the rate of frond abscission, thus resulting in smaller colonies. The mechanism behind this process has not yet been identified. Silver caused an abscission response that saturated after 7 h of treatment. The half-maximal effective concentration was 0.72 μm Ag(+) for the standard clone, L. minor St. Other clones of the same species show sensitivities that differ by one order of magnitude. Transmission electron microscopy revealed: (i) large numbers of vesicles close to the plasmalemma in cells adjacent to the abscission zone, which proves a vesicular type secretory activity; and (ii) a moderately electron-dense secretion accumulated in the enlarging intercellular spaces, and seemed to flow from the adjacent cells towards the abscission zone. We assume that increasing pressure causes this material to push apart the cells, thereby causing the break in the abscission zone of the stipe. This is a novel mechanism of abscission that has not previously been described. The same mechanism occurs in stipes of both control and Ag(+) -treated samples. Silver ions only accelerate the process leading to abscission of stipes, without affecting the mechanism involved.     

VARIABILITY IN THE YEAST HANSENULA HOLSTII

In the heterothallic yeast Hansenula holstii morphological variants consisting of a mother cell, stalk, and terminal cell are formed during vegetative growth. Tetrad distribution ratios of clones derived from mother and terminal cells differ. Preceding stalk formation endospore-like structures form in many cells. A causal relationship may exist between formation of the modified nuclei and change in the segregational patterns in the variants.     

Allozymic and morphometric variation inLemna minor (Lemnaceae)

Allozymic and morphometric variation was studied in 28 clones ofLemna minor. This variation was compared with the corresponding variation in four clones ofLemna gibba and four clones ofSpirodela polyrrhiza. A high level of allozymic variation was observed among the clones, despite having been grown under uniform laboratory conditions for several years and despite its quasi-exclusive clonal means of propagation. Based on degree of allozymic similarity,Spirodela polyrrhiza was distinguished from the twoLemna species but the latter species were genetically indistinguishable. Allozymic similarity among clones ofLemna minor was not related to morphometric similarity, nor was it related to the degree of geographic separation or climatic similarity of their sites of origin. The results suggest that allozymic variation among these clones ofLemna minor may be largely neutral and not a consequence of differential selection.     

Trade-offs among growth, clonal, and sexual reproduction in an invasive plant Spartina alterniflora responding to inundation and clonal integration

The purpose of this article was to study the trade-offs among vegetative growth, clonal, and sexual reproduction in an aquatic invasive weed Spartina alterniflora that experienced different inundation depths and clonal integration. Here, the rhizome connections between mother and daughter ramets were either severed or left intact. Subsequently, these clones were flooded with water levels of 0, 9, and 18 cm above the soil surface. Severing rhizomes decreased growth and clonal reproduction of daughter ramets, and increased those of mother ramets grown in shallow and deep water. The daughter ramets disconnected from mother ramets did not flower, while sexual reproduction of mother ramets was not affected by severing. Clonal integration only benefited the total rhizome length, rhizome biomass, and number of rhizomes of the whole clones in non-inundation conditions. Furthermore, growth and clonal reproduction of mother, daughter ramets, and the whole clone decreased with inundation depth, whereas sexual reproduction of mother ramets and the whole clones increased. We concluded that the trade-offs among growth, clonal, and sexual reproduction of S. alterniflora would be affected by inundation depth, but not by clonal integration.     

Abscisic-acid-induced turion formation in Spirodela polyrrhiza L. II. Ultrastructure of the turion; a stereological analysis

Abstract The ultrastructural features of the abscisic-acid-induced turion of Spirodela polyrrhiza are briefly described and a comparison between turion and vegetative frond tissue was made by stereological analysis. The turion is characterized by its small size, reniform shape, and dark-brown coloration; the mesophyll is undifferentiated and totally lacking the substantial acrenchyma development found in the vegetative frond. The turion cells have a smaller vacuole and a denser cytoplasm than the cells of the vegetative frond. Stereological analysis showed that the tissues differed quantitatively only in three main respects: air space formation, vacuolation, and starch and cell wall material accumulation. During development, it is suggested that the cells of the turion, while reaching the same final size as the vegetative frond cells, accumulate numerous starch grains, thick cell walls, and large deposits of tannins and anthocyanin pigment at the expense of the vacuolar expansion characteristic of the normal maturity programme. Certain features of the turion ultrastructure indicate a differential cell sensitivity to ABA.     

ALIX and ESCRT-III Coordinately Control Cytokinetic Abscission during Germline Stem Cell Division In Vivo

Abscission is the final step of cytokinesis that involves the cleavage of the intercellular bridge connecting the two daughter cells. Recent studies have given novel insight into the spatiotemporal regulation and molecular mechanisms controlling abscission in cultured yeast and human cells. The mechanisms of abscission in living metazoan tissues are however not well understood. Here we show that ALIX and the ESCRT-III component Shrub are required for completion of abscission during Drosophila female germline stem cell (fGSC) division. Loss of ALIX or Shrub function in fGSCs leads to delayed abscission and the consequent formation of stem cysts in which chains of daughter cells remain interconnected to the fGSC via midbody rings and fusome. We demonstrate that ALIX and Shrub interact and that they co-localize at midbody rings and midbodies during cytokinetic abscission in fGSCs. Mechanistically, we show that the direct interaction between ALIX and Shrub is required to ensure cytokinesis completion with normal kinetics in fGSCs. We conclude that ALIX and ESCRT-III coordinately control abscission in Drosophila fGSCs and that their complex formation is required for accurate abscission timing in GSCs in vivo.     

Anatomical Study on the Pedicels of Two Ferns

Pedicel is a stalk connecting sporangia with frond in ferns, and its structure and function are not clear. In this paper, we studied the pedicels of Dryopteris zhuweimingii and Pentarhizidium orientale. We found that the pedicels consist of three lines of cells, and the cell wall can be separated into two layers. The inner layer is secondary cell wall (S1) that spiral and cling to the outer layer. The outer layer is primary cell wall. And when we broke the pedicel into two sections, the inner layer of the cell wall could be pulled out like a spiral belt. This structure may be important to support and protect the sporangia.     

The cytoskeleton in the unique cell reproduction by conidiogenesis of the long-neck yeast Fellomyces (Sterigmatomyces) fuzhouensis

Summary. The morphology of conidiogenesis and associated changes in microtubules, actin distribution and ultrastructure were studied in the basidiomycetous yeast Fellomyces fuzhouensis by phase-contrast, fluorescence, and electron microscopy. The interphase cell showed a central nucleus with randomly distributed bundles of microtubules and actin, and actin patches in the cortex. The conidiogenous mother cell developed a slender projection, or stalk, that contained cytoplasmic microtubules and actin cables stretched parallel to the longitudinal axis and actin patches accumulated in the tip. The conidium was produced on this stalk. It contained dispersed cytoplasmic microtubules, actin cables, and patches concentrated in the cortex. Before mitosis, the nucleus migrated through the stalk into the conidium and cytoplasmic microtubules were replaced by a spindle. Mitosis started in the conidium, and one daughter nucleus then returned to the mother via an eccentrically elongated spindle. The cytoplasmic microtubules reappeared after mitosis. A strong fluorescence indicating accumulated actin appeared at the base of the conidium, where the cytoplasm cleaved eccentrically. Actin patches then moved from the stalk together with the retracting cytoplasm to the mother and conidium. No septum was detected in the long neck by electron microscopy, only a small amount of fine “wall material” between the conidium and mother cell. Both cells developed a new wall layer, separating them from the empty neck. The mature conidium disconnected from the empty neck at the end-break, which remained on the mother as a tubular outgrowth. Asexual reproduction by conidiogenesis in the long-neck yeast F. fuzhouensis has unique features distinguishing it from known asexual forms of reproduction in the budding and fission yeasts. Fellomyces fuzhouensis develops a unique long and narrow neck during conidiogenesis, through which the nucleus must migrate into the conidium for eccentric mitosis. This is followed by eccentric cytokinesis. We found neither an actin cytokinetic ring nor a septum in the long neck, from which cytoplasm retracted back to mother cell after cytokinesis. Both the conidium and mother were separated from the empty neck by the development of a new lateral wall (initiated as a wall plug). The cytoskeleton is clearly involved in all these processes. Correspondence and reprints: Department of Biology, Faculty of Medicine, Masaryk University, Tomešova 12, 602 00 Brno, Czech Republic.     

Autotomy in a polychaete: Abscission zone at the base of the tentacular crown of Sabella penicillus

Summary Under various circumstances the tentacular crown of some sabellid polychaetes becomes detached from the body. Separation occurs always at a preestablished zone of abscission at the base of the crown. We used electron microscopy to study the abscission zone of Sabella penicillus, both in specimens whose crown was intact and in those whose crown had separated.The abscission zone is within the intermediate layer, between the crown skeleton and the body wall musculature, and only structures supported by the crown skeleton separate from the animal's body. Abscission involves a rupture of the paramyosin muscle cells which form bridges connecting extensions from the epimysium of the body wall musculature and from the cartilage matrix of the crown. After abscission the anterior and posterior ends of the cells remain in place on the crown and body respectively. Sabella penicillus appears able to control the loss of its tentacular crown, so this abscission is a kind of autotomy. Under some circumstances autotomy of the crown may permit escape or confer some surgical benefit to the animal. Using standard histology we found the same anatomical provision for crown abscission in a variety of sabellids. We conclude that differences in their capacities to autotomize the crown have a behavioral/physiological basis rather than an anatomical one.     

Redistribution of phosphate deposits in the algaScenedesmus quadricauda deprived of exogenous phosphate—an ultra-cytochemical study

Summary The green algaScenedesmus quadricauda (Turp.) Bréb. was cultivated in the presence or absence of orthophosphate and synchronized daughter or mother cells were cytochemically stained. Forin situ capturing of water soluble phosphates Ca²⁺ and Mg²⁺ ions were added to the ice-cold glutaraldehyde fixative to form a polymeric metal-phosphate complex which was equivalent to the energy-rich condensed polyphosphates in staining by alkaline lead acetate. The X-ray microanalysis of the extensive stained deposits proved the presence of phosphorus. In orthophosphate-supplied daughter cells cytoplasmic vacuoles contained round stained bodies; a layer of phosphate-containing paracrystals encompassing some starch grains and a fine stained layer delineating the chloroplast envelope were also observed. In the equivalent mother cells only the material inside theloculi of stacked thylakoids was stained. In orthophosphate starved daughter cells filamentous phosphate-containing paracrystals filled extensive cytoplasmic vacuoles. A stained layer covered the chloroplast envelope and continuous stained layers appeared inside theloculi of stacked thylakoids. Mother cells that develop from these daughter cells were filled with starch grains and showed only peripheral stained deposits. The results are compared with the biochemical evidence of phosphate turnover in algal cells.Abbreviations ADP adenosine diphosphate - ATP adenosine triphosphate - ATPase adenosine triphosphatase - EDAX energy dispersive analysis of X-rays - Pi orthophosphate - PPi pyrophosphate - PP polyphosphate - PhAR photosynthetic active radiation - TCA trichloroacetic acid     

The Developmental Anatomy of the Monkshood-Tuber of Aconitum kusnezoffii

The monkshood-tuber of Aconitum kusnezoffii is a specialized structure attached to the root of the plant and is composed of apical bm4, axillary bud and its tuberous adventitious root. After the axillary bud emerges at the end of March of the following year, the primordium of the adventitious root develops from the procambium-like cells of the bud just below the first node at the abaxial part. With the active proliferation of the secondary phloem parenchyma ceils, the root increases its size very rapidly to form a “tuber”, When the adventitious root begins to develop, the first internode of the axillary bud extends horizontally to form a "bridge" connecting the daughter monkshood-tuber to the mother plant. In the third spring, each “tuber” sprouts one shoot giving rise to a new plant and in likewise produces a new “tuber”. It might be concluded that the monkshood-tuber is a renewal bud developing from a axillary bud. The so called “bridge” which connect the “tuber” and the mother plant is a special kind of subterraneous stem which is actually the first internode of the axillary bud.     

Characterization and structural analysis of wild type and a non-abscission mutant at the development funiculus (Def) locus in Pisum sativum L

Background  

In pea seeds (Pisum sativum L.), the Def locus defines an abscission event where the seed separates from the funicle through the intervening hilum region at maturity. A spontaneous mutation at this locus results in the seed failing to abscise from the funicle as occurs in wild type peas. In this work, structural differences between wild type peas that developed a distinct abscission zone (AZ) between the funicle and the seed coat and non-abscission def mutant were characterized.     

EFFECTS OF INDOLE-3-ACETIC ACID AND PARACHLOROPHENOXY-ISOBUTYRIC ACID ON ABSCISSION IN PETIOLES OF DEBLADED LEAVES OF PHASEOLUS VULGARIS

The effects of indole-3-acetic acid (IAA) and p-chlorophenoxyisobutyric acid (PCIB) on rates of abscission layer formation and abscission were investigated. The primary leaves of Phaseolus vulgaris were used as test material. Treatment at the distal end of one petiole of the pair from debladed primary leaves with 1% IAA inhibited the abscission of that petiole and accelerated the abscission of its opposite untreated partner. PCIB applied simultaneously with IAA counteracted the accelerating effect of IAA on the opposite untreated petiole. This influence increased with increasing concentrations of PCIB. Anatomical studies revealed that PCIB, although it counteracted the effect of IAA on the rate of abscission, had no effect on abscission layer formation. In other words abscission layer formation takes place under the influence of the auxin despite the presence of the antiauxin. The centripetal sequence of abscission layer formation was found in all cases.     

Induction of Secondary Abscission in Apple Pedicels in vitro

In vitro cultivated apple pedicels without a primary abscission layer can form a secondary (adventitious) abscission layer, especially under the influence of auxins. In the apple cv. Cox's Orange Pippin abscission can only be induced by auxins, while the site of the abscission layer, a few millimetres from the basal ends of the pedicels, is fixed and independent of the auxin concentration. The auxin treatment has to last at least 5–6 days to induce abscission, which is not affected by the presence of a flower. A secondary layer does not occur when pedicels are placed inverted on the media. Although abscission occurs both in light and in darkness, it is strongly promoted by light. Abscission is also accelerated by raising the temperature from 9°C to 21–25°C. High concentrations of 2,3,5-triiodobenzoic acid reduce the percentage of auxin-induced abscission. Sugar is required, but the presence of macro-elements is not essential.     

Protein, nucleic acid and starch metabolism in the duckweed Spirodela oligorrhiza, treated with cytokinins

Bacteria-free cultures of Spirodela oligorrhiza continue to increase in frond number for 2 to 3 days after transfer to darkness. There is then no further increase in frond number for 3 to 4 weeks, although DNA, RNA and protein synthesis continue at decreased rates and starch accumulates in the plants. We refer to such ;non-growing' plants in darkness as dormant. Adding kinetin to dormant Spirodela initiated increased DNA, RNA and protein synthesis within 1h, although new fronds were not detected until 24h after the addition of kinetin. The frond number then continued to increase. Starch accumulated in dormant plants. Accumulation of starch appeared to be a consequence of inhibition of growth rather than the converse. No evidence was obtained for a block in [(14)C]glucose metabolism that might explain the lack of growth in darkness in the absence of kinetin. In darkness, more ribosomes were membrane-bound in dormant Spirodela than in Spirodela growing with kinetin. Similarities between the response of Spirodela to darkness, stringent control in bacteria and pleiotypic controls in animal cells are discussed. It is suggested that all three processes are ultimately controlled by specific protein kinases that are individually sensitive to different effectors.