Successive microsporogenesis affects pollen aperture pattern in the tam mutant of Arabidopsis thaliana

Morphological and Ultrastructural Features of Selected Epidendroideae Pollen Dispersal Units and New Insights into Their Chemical Nature

Orchidaceae display enormous diversity in their flower morphology, which is particularly evident in their pollen dispersal units (pollinia, pollinaria). The packaging of pollen by elastoviscin leads to a great diversity of these morphologically and structurally complex pollen units. Despite being one of the most diverse angiosperm families, the available palynological data on orchids remain limited and sometimes contradicting. This study provides new insights into the pollen morphology and ultrastructure of five orchid species from the subfamily Epidendroideae, using combined light, scanning electron, and transmission electron microscopy. The aim was to compare the morphology and ultrastructure of pollen dispersal units and to elucidate the chemical nature of the pollen wall layers and of elastoviscin. Our combined light and electron microscopy investigation demonstrated the presence of six tetrad types even within a single pollinium, which is unique for orchids. The application of different staining methods confirmed the assumed lipidic nature of elastoviscin and the differences in its contrast and ultrastructure suggest a mixture of sticky materials with dissimilar chemical compositions. This study affirmed that sporopollenin is mostly restricted to the outer pollen grains of peripheral tetrads in compact and sectile pollinia, while inner tetrads exhibit highly reduced non-sporopollenin pollen walls.

A possible pattern in the evolution of male meiotic cytokinesis in angiosperms

Evolution of cellular characteristics is a fundamental aspect of evolutionary biology, but knowledge about evolution at the cellular level is very limited. In particular, whether a certain intracellular characteristic evolved in angiosperms, and what significance of such evolution is to angiosperms, if it exists, are important and yet unanswered questions. We have found that bidirectional cytokinesis occurs or likely occurs in male meiosis in extant basal and near-basal angiosperm lineages, which differs from the unidirectional cytokinesis in male meiosis in monocots and eudicots. This pattern of cytokinesis in angiosperms seems to align with the distribution pattern of angiosperms with the lineages basal to monocots and eudicots living in tropical, subtropical or temperate environments and monocots and eudicots in an expanded range of environments including tropical, subtropical, temperate, subarctic and arctic environments. These two cytokinetic modes seem to result from two phragmoplast types, respectively. A phragmoplast in the bidirectional cytokinesis dynamically associates with the leading edge of a growing cell plate whereas a phragmoplast in the unidirectional cytokinesis is localized to an entire division plane. The large assembly of microtubules in the phragmoplast in unidirectional cytokinesis may be indicative of increased microtubule stability compared with that of the small microtubule assembly in the phragmoplast in bidirectional cytokinesis. Microtubules could conceivably increase their stability from evolutionary changes in tubulins and/or microtubule-associated proteins. Microtubules are very sensitive to low temperatures, which should be a reason for plants to be sensitive to low temperatures. If monocots and eudicots have more stable microtubules than other angiosperms, they will be expected to deal with low temperatures better than other angiosperms. Future investigations into the male meiotic cytokinetic directions, microtubule stability at low temperatures, and proteins affecting microtubule stability in more species may shed light on how plants evolved to inhabit cold environments.

Structural regulation and dynamic behaviour of organelles during plant meiosis

Eukaryotes use various mechanisms to maintain cell division stability during sporogenesis, and in particular during meiosis to achieve production of haploid spores. In addition to establishing even chromosome segregation in meiosis I and II, it is crucial for meiotic cells to guarantee balanced partitioning of organelles to the daughter cells, to properly inherit cellular functions. In plants, cytological studies in model systems have yielded insights into the meiotic behaviour of different organelles, i.e., clearly revealing a distinct organization at different stages throughout meiosis indicating for an active regulatory mechanism determining their subcellular dynamics. However, how, and why plant meiocytes organize synchronicity of these elements and whether this is conserved across all plant genera is still not fully elucidated. It is generally accepted that the highly programmed intracellular behaviour of organelles during meiosis serves to guarantee balanced cytoplasmic inheritance. However, recent studies also indicate that it contributes to the regulation of key meiotic processes, like the organization of cell polarity and spindle orientation, thus exhibiting different functionalities than those characterized in mitotic cell division. In this review paper, we will outline the current knowledge on organelle dynamics in plant meiosis and discuss the putative strategies that the plant cell uses to mediate this programmed spatio-temporal organization in order to safeguard balanced separation of organelles. Particular attention is thereby given to putative molecular mechanisms that underlie this dynamic organelle organization taken into account existing variations in the meiotic cell division program across different plant types. Furthermore, we will elaborate on the structural role of organelles in plant meiosis and discuss on organelle-based cellular mechanisms that contribute to the organization and molecular coordination of key meiotic processes, including spindle positioning, chromosome segregation and cell division. Overall, this review summarizes all relevant insights on the dynamic behaviour and inheritance of organelles during plant meiosis, and discusses on their functional role in the structural and molecular regulation of meiotic cell division.

A Review of the Developmental Processes and Selective Pressures Shaping Aperture Pattern in Angiosperms

Pollen grains of flowering plants display a fascinating diversity of forms. The observed diversity is determined by the developmental mechanisms involved in the establishment of pollen morphological features. Pollen grains are generally surrounded by an extremely resistant wall displaying apertures that play a key role in reproduction, being the places at which pollen tube growth is initiated. Aperture number, structure, and position (collectively termed ‘aperture pattern’) are determined during microsporogenesis, which is the earliest step of pollen ontogeny. Here, we review current knowledge about aperture pattern developmental mechanisms and adaptive significance with respect to plant reproduction and how advances in these fields shed light on our understanding of aperture pattern evolution in angiosperms.

Nymphaea colorata, another basal angiosperm species with bidirectional cytokinesis in microsporogenesis

Comparing cellular features in microsporogenesis across taxa may yield important clues to evolution of meiosis in plants. We previously provided evidence that bidirectional cytokinesis occurs in M. denudata and suggested that the same may also occur in P. trimera based on a published report. Both M. denudata and P. trimera are basal angiosperm species that belong to the order of Magnoliales. For comparison, only unidirectional cytokinesis, either centripetal or centrifugal cytokinesis, has been found in microsporogenesis in eudicots and monocots. These observations raise the possibility that bidirectional cytokinesis is a common feature of microsporogenesis in basal angiosperms but not in eudicots and monocots. In this report, we provide evidence that bidirectional cytokinesis also occurs in another basal angiosperm species, Nymphaea colorata. The new findings, together with the previous findings, indicate that bidirectional cytokinesis is a prominent feature of microsporogenesis in at least some basal angiosperm species, and it occurs independently of cytokinesis types with respect to the timing of cytokinesis and tetrad configurations.

Cell polarity, asynchronous nuclear divisions, and bidirectional cytokinesis in male meiosis in Magnolia denudata

Magnolia, a basal angiosperm genus important for evolutionary and phylogenetic studies, is known to have male meiotic features not seen in the vast majority of angiosperms. However, knowledge about male meiosis in Magnolia is still fragmentary. Here, we report findings from an extensive investigation into male meiosis in Magnolia denudata using a combination of light and electron microscopy methods. Male meiosis in M. denudata was synchronous in prophase I but asynchronous in subsequent nuclear divisions. The polarized microspore mother cells from late prophase I onward had an elongated cell shape and thickened callose wall areas at the two smaller ends of the cell. The first nuclear division occurred along the long axis of the cell and the first callose furrow formed at the equatorial plane of the first nuclear division at the late telophase I stage. The second equatorial callose furrow formed after telophase II in a plane perpendicular to the first callose furrow. While cytokinesis occurred centripetally from the two furrows, a central callose wall island (CWI) appeared in the center of the cell and dense assemblies of vesicles and short tubules decorated the cytoplasmic regions between the furrows and the CWI. This cytokinesis mode differs from either the centripetal or the centrifugal mode of cytokinesis in microsporogenesis in the vast majority of angiosperms. As a result of this unusual cytokinesis, a large central callose mass remains in the mature tetrads. These observations may be useful to studies of cytokinetic mechanisms, evolution of microsporogenesis, and phylogenetics of angiosperms.

CDKD-dependent activation of CDKA;1 controls microtubule dynamics and cytokinesis during meiosis

Precise control of cytoskeleton dynamics and its tight coordination with chromosomal events are key to cell division. This is exemplified by formation of the spindle and execution of cytokinesis after nuclear division. Here, we reveal that the central cell cycle regulator CYCLIN DEPENDENT KINASE A;1 (CDKA;1), the Arabidopsis homologue of Cdk1 and Cdk2, partially in conjunction with CYCLIN B3;1 (CYCB3;1), is a key regulator of the microtubule cytoskeleton in meiosis. For full CDKA;1 activity, the function of three redundantly acting CDK-activating kinases (CAKs), CDKD;1, CDKD;2, and CDKD;3, is necessary. Progressive loss of these genes in combination with a weak loss-of-function mutant in CDKA;1 allowed a fine-grained dissection of the requirement of cell-cycle kinase activity for meiosis. Notably, a moderate reduction of CDKA;1 activity converts the simultaneous cytokinesis in Arabidopsis, i.e., one cytokinesis separating all four meiotic products concurrently into two successive cytokineses with cell wall formation after the first and second meiotic division, as found in many monocotyledonous species.

Evolutionary constraints on disparity of ericaceous pollen grains

BACKGROUND AND AIMS

Flowering plants show a high diversity of pollen morphology, assumed to reflect not only variations in the underlying design, but also stress imposed by ecological conditions related to pollen survival and germination. Both components are expected to constrain the accumulation of pollen disparity. However, this assumption has rarely been tested using empirical data.

METHODS

This study is designed to test this hypothesis by inferring the accumulation of pollen disparity in Ericaceae, a large eudicot family with recent, ongoing radiations, with focus on three functionally significant pollen characters using a dated phylogeny.

KEY RESULTS

Multiple lines of evidence supported the hypothesis that pollen disparity in Ericaceae did not evolve steadily but rather pulsed over time, clearly decoupling from the relative constant rate pattern of species diversification inferred. In a 3-D pollen morphospace, most major clades appear to occupy distinct neighbouring regions, whereas the subfamily Epacridoideae overlaps extensively with other subfamilies. No evidence for correlations was found between dimension of pollen disparity and species diversity at either the subfamily or generic level. Furthermore, the distribution of species in present pollen morphospace showed a strong central tendency, with the core compartment containing a large number of species from species-rich genera.

CONCLUSIONS

The recovered evidence fits well with the expectations of limitations on available pollen morphological disparity, and suggests that innovation of pollen germination traits may have little effect on species diversification.

A Ploidy-Sensitive Mechanism Regulates Aperture Formation on the Arabidopsis Pollen Surface and Guides Localization of the Aperture Factor INP1

Pollen presents a powerful model for studying mechanisms of precise formation and deposition of extracellular structures. Deposition of the pollen wall exine leads to the generation of species-specific patterns on pollen surface. In most species, exine does not develop uniformly across the pollen surface, resulting in the formation of apertures-openings in the exine that are species-specific in number, morphology and location. A long time ago, it was proposed that number and positions of apertures might be determined by the geometry of tetrads of microspores-the precursors of pollen grains arising via meiotic cytokinesis, and by the number of last-contact points between sister microspores. We have tested this model by characterizing Arabidopsis mutants with ectopic apertures and/or abnormal geometry of meiotic products. Here we demonstrate that contact points per se do not act as aperture number determinants and that a correct geometric conformation of a tetrad is neither necessary nor sufficient to generate a correct number of apertures. A mechanism sensitive to pollen ploidy, however, is very important for aperture number and positions and for guiding the aperture factor INP1 to future aperture sites. In the mutants with ectopic apertures, the number and positions of INP1 localization sites change depending on ploidy or ploidy-related cell size and not on INP1 levels, suggesting that sites for aperture formation are specified before INP1 is brought to them.

Aperture number influences pollen survival in Arabidopsis mutants

PREMISE OF THE STUDY

Pollen grains are subject to intense dehydration before dispersal. They rehydrate after landing on a stigma or when placed in humid environment by absorbing water from the stigma or surroundings. Resulting fluctuations in water content cause pollen grains to undergo significant changes in volume. Thus, morphological or structural adaptations might exist to help pollen adjust to sudden volume changes, though little is known about the correlation between pollen morphology and its ability to accommodate volume changes. We studied the effect of one morphological feature of pollen grains, the aperture number, on pollen wall resistance to water inflow in Arabidopsis thaliana.

METHODS

We used three Arabidopsis thaliana mutants that differ in the number of apertures in their pollen (zero, four, or a mix of four to eight, respectively) and the wild type with pollen with three apertures. We tested pollen survival in solutions with various mannitol concentrations.

KEY RESULTS

The number of intact pollen grains increased with increasing mannitol concentration for all pollen morphs tested. At a given mannitol concentration, however, an increase in aperture number was associated with an increase in pollen breakage.

CONCLUSIONS

Aperture patterns, i.e., number, shape, and position, influence the capacity to accommodate volume variations in pollen grains. When subjected to water inflow, pollen grains with few apertures survive better than pollen with many apertures. Trade-offs between survival and germination are likely to be involved in the evolution of pollen morphology.

Evolutionary stasis in pollen morphogenesis due to natural selection

The contribution of developmental constraints and selective forces to the determination of evolutionary patterns is an important and unsolved question. We test whether the long-term evolutionary stasis observed for pollen morphogenesis (microsporogenesis) in eudicots is due to developmental constraints or to selection on a morphological trait shaped by microsporogenesis: the equatorial aperture pattern. Most eudicots have three equatorial apertures but several taxa have independently lost the equatorial pattern and have microsporogenesis decoupled from aperture pattern determination. If selection on the equatorial pattern limits variation, we expect to see increased variation in microsporogenesis in the nonequatorial clades. Variation of microsporogenesis was studied using phylogenetic comparative analyses in 83 species dispersed throughout eudicots including species with and without equatorial apertures. The species that have lost the equatorial pattern have highly variable microsporogenesis at the intra-individual and inter-specific levels regardless of their pollen morphology, whereas microsporogenesis remains stable in species with the equatorial pattern. The observed burst of variation upon loss of equatorial apertures shows that there are no strong developmental constraints precluding variation in microsporogenesis, and that the stasis is likely to be due principally to selective pressure acting on pollen morphogenesis because of its implication in the determination of the equatorial aperture pattern.

Sexual polyploidization in plants--cytological mechanisms and molecular regulation

In the plant kingdom, events of whole genome duplication or polyploidization are generally believed to occur via alterations of the sexual reproduction process. Thereby, diploid pollen and eggs are formed that contain the somatic number of chromosomes rather than the gametophytic number. By participating in fertilization, these so-called 2n gametes generate polyploid offspring and therefore constitute the basis for the establishment of polyploidy in plants. In addition, diplogamete formation, through meiotic restitution, is an essential component of apomixis and also serves as an important mechanism for the restoration of F1 hybrid fertility. Characterization of the cytological mechanisms and molecular factors underlying 2n gamete formation is therefore not only relevant for basic plant biology and evolution, but may also provide valuable cues for agricultural and biotechnological applications (e.g. reverse breeding, clonal seeds). Recent data have provided novel insights into the process of 2n pollen and egg formation and have revealed multiple means to the same end. Here, we summarize the cytological mechanisms and molecular regulatory networks underlying 2n gamete formation, and outline important mitotic and meiotic processes involved in the ectopic induction of sexual polyploidization.