Pollen Wall Development in Gibasis (Commelinaceae)

The development of the one and-inline of the pollen wall aredescribed for Gibasis karwinsk yana and G. venustula. Duringthe tetrad stage the appearance of electron-opaque depositionsor tri-partite plates at discrete sites between the plasma membraneof the spore and the inward surface of the callose special wallare the first indications of exine development. The sulcus rapidlydifferentiates being composed of discrete exine granules ona thin foot layer. Probacula in non-apertural areas developin an electron-opaque granular layer situated between the plasmamembrane, which is highly convoluted, and the callose specialwall. A foot layer is formed from electron-opaque lamellae atthe plasma membrane. Exine pattern is clearly established withinthe tetrad. After release of the spores from the tetrad an intimate associationis rapidly developed between the plasma membrane of the periplasmodialtapetum and the newly-formed exine. Compacted electron-opaquematerial is found at the interface between membrane and theexine and vesicular material is added from the tapetum. Theincrease in volume that occurs in both spore and anther is accompaniedby considerable vacuolation. Intine development begins just prior to pollen grain mitosisand continues rapidly at the aperture. The thin foot layer becomesdiscontinuous. Further intine deposition takes place after mitosisand a bilayer is apparent in mature grains. The matrix of thislayer contains conspicuous electron-opaque platelets. The exineof the mature spore stains less intensely than in the youngspore and the interbacula spaces are filled with material fromthe degenerate tapetum. Gibasis karwinskyana, Gibasis venustula, Commelinaceae, exine, intine, tapetum, pollen wall, ultrastructure     

Development of Sporangiophores of Peronospora parasitica (Pers. ex Fr.) Fr.

The morphology of developing sporangiophores of Peronosporaparasitica from wallflower is described, and morphogenesis maybe divided into the following five stages: the sporangiophoreprimordium, unbranched sporangiophore, branched sporangiophore,spore formation and maturation, and formation of the cross wall.The growth of individual sporangiophores in a humidity chamberwas followed under the microscope, and increase in height andincrease in volume measured. The greatest increase in volumewas during spore formation, when the sporangiophore volume mightquadruple within an hour.     

Morphogenetic pathway of spore wall assembly in Saccharomyces cerevisiae

The Saccharomyces cerevisiae spore is protected from environmental damage by a multilaminar extracellular matrix, the spore wall, which is assembled de novo during spore formation. A set of mutants defective in spore wall assembly were identified in a screen for mutations causing sensitivity of spores to ether vapor. The spore wall defects in 10 of these mutants have been characterized in a variety of cytological and biochemical assays. Many of the individual mutants are defective in the assembly of specific layers within the spore wall, leading to arrests at discrete stages of assembly. The localization of several of these gene products has been determined and distinguishes between proteins that likely are involved directly in spore wall assembly and probable regulatory proteins. The results demonstrate that spore wall construction involves a series of dependent steps and provide the outline of a morphogenetic pathway for assembly of a complex extracellular structure.     

ULTRASTRUCTURAL STUDY ON SPORE WALL MORPHOGENESIS IN LYCOPODIUM CLAVATUM (LYCOPODIACEAE)

Spore wall morphogenesis of Lycopodium clavatum was observed by transmission electron microscopy. The spore plasma membrane indicates the reticulate spore sculpture shortly after meiosis. The mature spore wall of this species consists of two layers, inner endospore and outer exospore. There is no perispore in the sporoderm of this species. The exospore formation begins during the tetrad stage; and this layer is divided into two distinct sublayers, an outer lamellar layer and an inner granular layer. The lamellar layer is formed on the sculptured spore plasma membrane. Additional lamellae attach to this layer in a centripetal direction. For that reason, this layer may be derived from spore cytoplasm. The granular layer is formed only in the proximal region following lamellar layer formation, and it also may be derived from spore cytoplasm. The endospore is formed lastly and seems to be derived from spore cytoplasm as well. Accordingly, the spore sculpture of this species may be under the genetic control of the spore nucleus.     

Pollen Wall Ontogeny of Felicia muricata (Asteraceae: Astereae)

Pollen wall development in Felicia muricata Thunb. Nees (Asteraceae)was investigated by electron microscopy after addition of thelypophilic dye malachite green to the primary fixative. Pollenwall formation commences with the deposition of fibrillar electrondense units which represent future interbacular spaces. Radiallyarranged trilammelate structures develop between the fibrillarunits. Sporopollenin is deposited in the trilammelate structures.After release of the microspores from the tetrad, formationof interbacular spaces is accompanied by disintegration of fibrillarunits. Endexine formation occurs on white lines of unit membranedimensions. Intine formation follows the pattern described formost angiosperms. Observation of bead-like globules and pollenkittwith a membraneous structure is presumed to be related to theaddition of malachite green to the fixative.Copyright 1993,1999 Academic Press Felicia muricata (Thunb.) Nees, pollen, pollenkitt, ultrastructure, fixation     

Occurrence and Inheritance of 2 n Pollen in Musa

Sexual polyploidization explains the formation of most polyploidcrop species and provides means for their genetic improvement.This paper reports the occurrence and inheritance of 2n pollenin diploid and polyploidMusa . Pollen samples of male fertileaccessions available in the field gene bank of the InternationalInstitute of Tropical Agriculture were microscopically examinedthroughout a calendar year. Based on pollen diameter 2n pollenproducers were identified. The presence of 2n pollen in diploidspecies suggests that unilateral sexual polyploidization (2nxn) could have been involved in the origin of triploidMusa species.Segregation data suggests that at least one dominant gene controls2n pollen production inMusa . Further introgression of desirablealleles from diploid species to polyploids could be achievedeither through unilateral or bilateral (2nx2n ) sexual polyploidizationinMusa . Musa spp.; Musa hybrids; 2n gametes; evolution; ploidy manipulations; sexual polyploidization     

Tapetum-Specific Expression of the Gene for an Endo-{beta}-1,3-glucanase Causes Male Sterility in Transgenic Tobacco

A cDNA for a pathogenesis-related endo-ß-1,3-glucanaseisolated from soybean, was fused to an anther tapetum-specificpromoter (Osg6B promoter) isolated from rice and the resultingchimeric gene was introduced into tobacco. The Osg6B promoterbecame active in the anther tapetum during formation of tetradsand the tapetal glucanase activity in the transgenic plantscaused in a significant reduction in the number of fertile pollengrains. Most of the pollen grains were aberrant in shape, lackedgerminal apertures and aggregate of the pollen grains. Granulesof ß-1,3-glucan, which have not previously been reported,were often observed to adhere to the surface of the pollen grains.Further observations revealed that the callose wall was almostabsent in the pollen tetrads of transgenic plants. In wild-typeplants, by contrast, the tetrads were surrounded by callosethat was degraded soon after the tetrad stage to release freemicrospores. Thus, the introduced gene for endo-ß-1,3-endoglucanaseunder the control of the Osg6B promoter caused digestion ofthe callose wall at the beginning of the tetrad stage, a timethat was just a little earlier than the time at which endogenousglucanase activity normal appears. These results demonstratethat premature dissolution of the callose wall in pollen tetradscauses male sterility and suggest that the time at which tapetallyproduced glucanase is activate is critical for the normal developmentof microspores. (Received September 29, 1994; Accepted January 30, 1995)     

Pollen Tube Development and Characteristics of the Protein Emission in Conifers

When ripe, viable pollen of Pinus contorta, Pinus nigra, Piceasitchensis, Abies alba and Cedrus deodara is germinated on asuitable artificial substrate, the process of pollen tube growthbegins 12–36 h later, according to the species. The tubeemerges at the leptoma, a structurally specialized site in thepollen wall, and the early tube wall is continuous with thetwo microfibrillar intine strata within the grain. Later inpollen tube development, when cytoplasmic zonation has beenestablished, only the inner of these two wall layers is differentiated. Cytochemical methods show that, during hydration and throughoutthe period of tube growth in culture, proteins are releasedfrom the pollen grains; before germination most conspicuouslyfrom the leptoma, subsequently from the tube itself. The emissioncontains a number of hydrolytic enzymes and a non-catalyticmoiety. Gel-electrofocusing reveals that the hydrolases releasedfrom germinating Pinus contorta pollen include several acidphosphatase and esterase isozymes, and that there are differencesbetween the composition of the emission and the spectrum ofsoluble proteins extracted from the pollen grains before germination.Analysis by immunodiffusion demonstrates that two componentswith antigenic characteristics present in the quiescent pollengrains are represented in the emission. The possible utilityof the released components in the biology of conifer reproductionis discussed. Pinus contorta, Pinus nigra, Picea sitchensis, Abies alba, Cedrus deodara, lodgepole pine, Austrian pine, Sitka spruce, European silver fir, deodar, pollen tube, pollen germination, protein release, isoelectric focusing     

The polygalacturonase gene BcMF2 from Brassica campestris is associated with intine development

Brassica campestris Male Fertility 2 (BcMF2) is a putative polygalacturonase(PG) gene previously isolated from the flower bud of Chinesecabbage (Brassica campestris L. ssp. chinensis Makino, syn.B. rapa ssp. chinensis). This gene was found to be expressedspecifically in tapetum and pollen after the tetrad stage ofanther development. Antisense RNA technology was used to studythe function of BcMF2 in Chinese cabbage. Scanning and transmissionelectron microscopy revealed that there were deformities inthe transgenic mature pollen grains such as abnormal locationof germinal furrows. In addition, the homogeneous pectic exintinelayer facing the exterior seemed to be overdeveloped and predominantlyoccupied the intine, thus reversing the normal proportionaldistribution of the internal endintine layer and the externalexintine layer. Since it is a continuation of the intine layer,the pollen tube wall could not grow normally. This resultedin the formation of a balloon-like swelling structure in thepollen tube tip in nearly 80% of the transgenic pollen grains.Premature degradation of tapetum was also found in these transgenicplants, which displayed decreased expression of the BcMF2 gene.BcMF2 might therefore encode a new PG with an important rolein pollen wall development, possibly via regulation of pectin'sdynamic metabolism. Key words: Brassica campestris, Brassica rapa, Chinese cabbage, intine, PG, polygalacturonase, pollen wall Received 28 August 2008; Revised 14 October 2008 Accepted 20 October 2008     

Spore Germination Patterns in the Ferns, Cyathea and Dicksonia

Cell division patterns during germination of spores of Cyatheaaustralis, C. cooperi and Dicksonia antarctica were examinedby light microscopy of sectioned materials and by the scanningelectron microscope. In C. australis and C. cooperi the rhizoidwas traced to a small cell formed by an asymmetric divisionof the spore by a wall parallel to its equatorial plane. Incontrast, the rhizoid was formed by a division of the sporeparallel to its polar axis in D. antarctica. In spores of bothgenera, a second division wall oriented in a plane perpendicularto the first gave rise to the protonemal cell. Certain aspectsof germination described here in spores of Cyathea and Dicksoniaare in conflict with the published accounts of spore germinationin these genera. Cyathea, Dicksonia, spore germination, cell division pattern     

The Microfibrillar Component of the Pollen Intine Some Structural Features

The microfibrillar polysaccharide component of the pollen intinecan be isolated by progressive chemical digestion of the exineand the cellular contents and the extraction of the matrix materials.The resulting intine ‘ghosts’ reveal various characteristicstructural features. The microfibrils have apparent individualdiameters in the range of 5–15 nm, but they are commonlyassociated laterally to form ribbons, or aggregated in strandsor cables of dimensions great enough to be resolved with theoptical microscope. These often show preferred orientations,which can be associated with pollen grain shape and with thedisposition-of the germination apertures. The apertural intinemay be structurally complex, as in Abutilon hybridum, where,after the removal of the exine, the polysaccharide caps whichoverlie the protein storage sites of the pollen grain wall retainthe elaborate patterning of the original cytoplasmic evaginationsfrom the vegetative cell. Secale cereale, Narcissus pseudonarcissus, Abutilon hybridum, Crocus vernus, pollen grain, intine, exine, wall pattern, germination apertures, polysaccharide microfibrils, fluorescence microscopy     

A comparison of the pollen wall ultrastructure of aureoid and non-aureoidSenecio species (Asteraceae) in North America

The structure of the pollen wall as revealed by transmission electron microscopy is presented for 29 speciesSenecio including 19 aureoid and ten non-aureoid species. The previous report of a helianthoid wall structure (exine with internal foramina) is confirmed for all the aureoid species examined while all of the non-aureoid taxa showed a senecioid pattern. The data provide support for the treatment of the aureoid complex as a distinct taxon.     

Germination of Monocolpate Angiosperm Pollen: Evolution of the Actin Cytoskeleton and Wall during Hydration, Activation and Tube Emergence

The monocolpate pollen grain of Narcissus pseudonarcissus L.has two preferred sites for tube emergence, one at each endof the colpus. While the cellulosic microfibnls of the innerlayer of the intine are disposed circumferentially in the centreof the grain, the microfibrils in these terminal sites are shorterand randomly oriented Soon after the beginning of hydration,inclusions of the vegetative cell begin movement, firstly ina rotatory manner, and then in a pattern focused on one or bothgermination sites, where the intine bulges as hydration progresses.These changes are associated with the evolution of the actincytoskeleton. Actin is present in the unactivated grain in theform of fusiform bodies. During hydration these dissociate toform finer fibrils, initially randomly disposed. Then, correlatedwith the change of the pattern of movement in the vegetativecell, the actin fibril system becomes polarized towards thegermination sites, where shorter fibrils accumulate. Callose,absent from the ungerminated grain, is deposited within thecellulosic wall in these locations, forming a shallow dome whicheventually develops into an annulus subtending the inner calloselining of the emerging tube. The transition to cylindrical growthis associated firstly with the development of zonation in thecytoplasm of the vegetative cell, with the tip occupied by apopulation of wall precursor bodies (P-particles) and a denseaggregate of short actin fibrils; and then with the establishmentof the ‘inverse fountain’ pattern of movement characteristicof the apical part of the extending tube. Narcissus pseudonarcissus L, pollen activation, pollen germination, actin cytoskeleton, tip-growth system, pollen-tube wall development     

Genetic Control of Early Embryogenesis in the Red Flour Beetle, Tribolium castaneum

SYNOPSIS. The power of genetic analysis possible with the fruitfly, Drosophila melanogaster, has yielded a detailed understandingof pattern formation controlled by homeotic and segmentationgenes in early embryogenesis. We are studying the genetic regulationof embryogenesis in the red flour beetle, Tribolium castaneum.The dynamic process of germ rudiment formation and sequentialsegmentation exhibited by Tribolium provides a context differentthan Drosophila within which to assess the function of homeoticand segmentation gene homologs. Our analyses of the genes inthe HOM-C suggest many similarities in structure and functionwith the well-characterized Drosophila genes. Abdominal resemblesits Drosophila homolog abdominal-A in functioning to establishsegmental identities in the abdomen, such that in each casemutations result in homeotic transformations to PS6. Althoughthe anterior functional boundary of abdominal-A homologs isprecisely conserved, the domain within which Abdominal is importantextends more posterior than that of abdominal-A. The final expression pattern of the segmentation gene engrailedin Tribolium is identical to Drosophila, suggesting that thesehomologs are involved in a conserved developmental process.However, as expected the development of that pattern is different;engrailed stripes anticipate the formation of each new segmentas they appear sequentially in the elongating germ band. Althoughthe grasshopper even-skipped and fushi tarazu homologs are notapparently important in segmentation, the expression patternsof the Tribolium homologs strongly suggest that they have gaineda role in segmentation in the lineage leading to beetles andflies. Nevertheless, differences between Tribolium and Drosophilain the dynamics of even-skipped expression and the fushi tarazumutant phenotype indicate divergence in the regulation and rolesof these genes.     

Pollen wall development in Vigna vexillata I. Characterization of wall layers

Light and electron microscope observations characterized the layers that comprise Vigna vexillata L. pollen walls, and identified the timing of their development. Exine sculpturings form an unusually coarse ektexinous reticulum. The structure of the ektexine is granular; this differs from the columellate/tectate type of structure typical of most angiosperm pollen. The ektexine overlies a homogeneous-to-lamellar, electron-dense endexine, which in turn surrounds a thick, microfibrillar intine. Pollen grains are triporate and operculate, with Zwischenkörper and thickened intine underlying the apertures. The ektexine forms during the tetrad period of microspore development, the endexine and Zwischenkörper during the free microspore stage, and the intine during the bicelled (pollen) stage. Coarsely reticulate exine sculpturings and the granular structure of the patterned exine wall of the pollen grains are features that make this species suitable for detailed studies of pollen wall pattern formation.     

Amorphophallus: New insights into pollen morphology and the chemical nature of the pollen wall

In pollen characters, Amorphophallus is one of the most diverse genera in the Araceae. The present work is a critical survey of contradicting reports on the impact of acetolysis treatment on Amorphophallus pollen, on the chemical nature of the outer pollen wall layer and of electron-dense (dark) granules found within it. Furthermore, we wanted to clarify the pollen polarity and to test conclusions based on different preparation techniques. Pollen morphology of 25 species is investigated by light microscopy, scanning electron microscopy and transmission electron microscopy. Our results show that Amorphophallus pollen is not resistant to acetolysis treatment. The use of different transmission electron microscopy staining methods proved the polysaccharide nature of the outer pollen wall layer and of the granules within it. Moreover, an additional thin surface layer was found in all investigated species. Microspores in early and late tetrad stages show that the less convex side of the microspore is the proximal face and the more convex side the distal face. The extrusion of pollen in strands is illustrated for the first time by light microscopy and scanning electron microscopy. Furthermore, observations of pollen in water showed that in some of the investigated species the pollen wall is shed immediately before pollen tube formation.     

Tapetum: regulation and role in sporopollenin biosynthesis in Arabidopsis

Pollen acts as a biological protector for protecting male sperm from various harsh conditions and is covered by an outer cell wall polymer called the exine, a major constituent of which is sporopollenin. The tapetum is in direct contact with the developing gametophytes and plays an essential role in pollen wall and pollen coat formation. The precise molecular mechanisms underlying tapetal development remain highly elusive, but molecular genetic studies have identified a number of genes that control the formation, differentiation, and programmed cell death of tapetum and interactions of genes in tapetal development. Herein, several lines of evidence suggest that sporopollenin is built up via catalytic enzyme reactions in the tapetum. Furthermore, as based on genetic evidence, we review the currently accepted understanding of the molecular regulation of sporopollenin biosynthesis and examine unanswered questions regarding the requirements underpinning proper exine pattern formation.     

The influence of tetrad shape and intersporal callose wall formation on pollen aperture pattern ontogeny in two eudicot species

Background and Aims

In flowering plants, microsporogenesis is accompanied by various types of cytoplasmic partitioning (cytokinesis). Patterns of male cytokinesis are suspected to play a role in the diversity of aperture patterns found in pollen grains of angiosperms. The relationships between intersporal wall formation, tetrad shape and pollen aperture pattern ontogeny are studied.

Methods

A comparative analysis of meiosis and aperture distribution was performed within tetrads in two triporate eudicot species with contrasting aperture arrangements within their tetrads [Epilobium roseum (Onagraceae) and Paranomus reflexus (Proteaceae)].

Key Results and Conclusions

Intersporal wall formation is a two-step process in both species. Cytokinesis is first achieved by the formation of naked centripetal cell plates. These naked cell plates are then covered by additional thick, localized callose deposits that differ in location between the two species. Apertures are finally formed in areas in which additional callose is deposited on the cell plates. The recorded variation in tetrad shape is correlated with variations in aperture pattern, demonstrating the role of cell partitioning in aperture pattern ontogeny.     

Successional processes induced by fires on the northern offshore islands of New Zealand

A modern pollen-vegetation data set of 46 samples is presented from subantarctic Campbell Island, 600 km south of the New Zealand mainland. The sampled vegetation includes all major community types: maritime turf and grassland, sedge flushes, dwarf forest, scrub, cushion bog, tussock grassland, and high altitude graminoid turfs and tundra. Macrophyllous forbs—characteristic plants of subantarctic islands—are common throughout. Most taxa have highly restricted pollen dispersal, largely due to the short stature of the vegetation and the high proportion of insect-pollinated species. Percentages of pollen or spores of the dominant taxa have a significant positive correlation with the percent vegetation cover of the corresponding species, the exceptions being the widespread ferns Polystichum vestitum and Blechnum spp., and the ubiquitous macrophyllous forb, Bulbinella rossii .The relationship between the vegetation cover of a given taxon and its pollen representation was usually not strong enough to give confidence in a quantitative reconstruction based on pollen frequency alone. However, the broad vegetation groupings have characteristic pollen and spore spectra clearly related to the abundance of their dominant plant species. Detrended correspondence analysis of the pollen spectra grouped most sites according to their source vegetation type and generated a pattern similar to that of vegetation data analysed in a similar fashion in previous studies of the island. This study, together with recent work on Auckland Island pollen and spore representation, has resulted in a combined modem palynological data base of more than 100 sites for the New Zealand subantarctic islands.     

Pollen wall and tapetum development in Plantago major L. (Plantaginaceae): assisting self-assembly

We have attempted to elucidate the underlying mechanisms of sporoderm development and pattern determination in Plantago major through a detailed ontogenetic study, using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). We aim to compare our observations and interpretation with those on other species. Our study of sporoderm development in Plantago from the early tetrad stage to mature pollen grains has shown that pure physical processes, including self-assembly, which are not under direct genetic control, play an important role and represent evidently one of the instruments of evolution. Our observations fit well with the sequence of self-assembling micellar mesophases and show reiteration of some of them, confirming our self-assembly hypothesis. Some attention was also paid to the possible role of rough and smooth endoplasmic reticulum in the cortical cytoplasm of the developing microspores. The tapetum and Ubisch bodies development are also traced. The importance of detailed ontogenetic studies for understanding the establishment of complex pollen walls in any species and for understanding mechanisms underlying sporoderm development was demonstrated. We also present a simulation, obtained in vitro experiments by self-assembly, mimicking pollen grain of Plantago major. It is clear that, in pollen wall development, biological processes and purely physical factors work in tandem.