Pollen structure and development in Nymphaeales: insights into character evolution in an ancient angiosperm lineage

Mechanisms of pollen wall development in Lysimachia vulgaris

Exine, this complex sporopollenin-containing and highly variable among taxa envelope of the male gametophyte, consists of two layers, ectexine and endexine. We traced in detail the pollen wall development in Lysimachia vulgaris (Primulaceae), with emphasis on driving forces and critical ontogenetic time. By observation on the sequence of the emergent patterns and by analysis of their substructure with TEM, we intended to clarify the obvious and not-obvious ways of exine construction and to find out the common features in pattern development in other representatives in living nature. The ectexine and endexine ontogeny follows the main stages observed in many other species: first, the appearance of microspore plasma membrane invaginations with isotropic contents within, changed later to anisotropic state; then successive appearance of spherical, rod-like, and lamellate units in the periplasmic space. The lamellate endexine appears unusually early in the exine development. All these elements and their aggregations are manifestation of well-known physical phenomena: phase separation and micellar self-assembly. A consideration of similar surface patterns in very remote taxa suggests the participation in their development of some general nature phenomena as the lows of space-filling operations.

Pollen wall and tapetal development in Cymbalaria muralis: the role of physical processes, evidenced by in vitro modelling

Our aim was to unravel the underlying mechanisms of pollen wall development in Cymbalaria muralis. By determining the sequence of developing substructures with TEM, we intended to compare it with that of other taxa and clarify whether physical processes of self-assembly and phase separation were involved. In parallel, we tried to simulate in vitro the substructures observed in Cymbalaria muralis exine development, using colloidal mixtures, to determine whether purely physical self-assembly processes could replicate them. Exine ontogeny followed the main stages observed in many other species and was initiated by phase separation, resulting in heterogeneity of the homogeneous contents of the periplasmic space around the microspore which is filled with genome-determined substances. At every stage, phase separation and self-assembly come into force, gradually driving the substances through the sequence of mesophases: spherical micelles, columns of spherical micelles, cylindrical micelles arranged in a layer, laminate micelles. The final two of these mesophases define the structure of the columellate ectexine and lamellate endexine respectively. Structures obtained in vitro from colloidal mixtures simulated the developing exine structures. Striking columella-like surface of some abnormal tapetal cells and lamella-like structures in the anther medium confirm the conclusion that pattern generation is a feature of colloidal materials, after genomic control on material contents. Simulation experiments show the high pattern-generating capacity of colloidal interactions.

Morphology and Viability of Pollen from Three Hardy Water Lilies and Their Cross-Compatibility with Nymphaea hybrid

Owing to the wide variation in their morphological characteristics across diverse geographies, the identification and classification of plants in the Nymphaea genus are challenging. Therefore, the present study investigated the pollen morphological characteristics of hardy water lilies (N. ‘Rose Arey’, N. ‘Perry’s Fire Opal’, and N. ‘Peter Slocum’), their relationship with species classification and system evolution, and their cross-compatibility with three strains of Nymphaeahybrid (NH-1, NH-2, and NH-3), a tropical waterlily. Pollen of the hardy water lilies was single-grained, oblate, and 18.31–20.47 × 32.51–37.64 μm. The pollen apertures were of the ring-groove type, and the pollen exine ornamentation was rod- and tumour-shaped. Pollen grains of different species differed in size, the obviousness of tumour ornamentation, and the size and density of rod-like ornamentation; their germination rates also differed significantly. Viabilities of N. ‘Rose Arey’ and N. ‘Peter Slocum’ pollen were the highest and lowest, respectively. According to the artificial pollination results, all hybrid combinations except three (NH-1 × N. ‘Peter Slocum’, NH-2 × N. ‘Peter Slocum’, and NH-3 × N. ‘Peter Slocum’) bore seeds; combinations with NH-2 as the female parent and N. ‘Rose Arey’ as the male parent had the highest seed-setting rates.

An integral insight into pollen wall development: involvement of physical processes in exine ontogeny in Calycanthus floridus L., with an experimental approach

We aimed to understand the underlying mechanisms of development in the sporopollenin-containing part of the pollen wall, the exine, one of the most complex cell walls in plants. Our hypothesis is that distinct physical processes, phase separation and micellar self-assembly, underpinexine development by taking the molecular building blocks, determined and synthesised by the genome, through several phase transitions. To test this hypothesis, we traced each stage of microspore development in Calycanthus floridus with transmission electron microscopy and then generated in vitro experimental simulations corresponding to every developmental stage. The sequence of structures observed within the periplasmic space around developing microspores starts with spherical units, which are rearranged into columns to then form rod-like units (the young columellae) and, finally, white line centred endexine lamellae. Phase separation precedes each developmental stage. The set of experimental simulations, obtained as self-assembled micellar mesophases formed at the interface between lipid and water compartments, was the same: spherical micelles; columns of spherical micelles; cylindrical micelles; and laminate micelles, separated by gaps, resembling white-lined lamellae. Thus, patterns simulating structures observed at the main stages of exine development in C. floridus were obtained from in vitro experiments, and hence purely physicochemical processes can construct exine-like patterns. This highlights the important part played by physical processes that are not under direct genomic control and share influence on the emerging ultrastructure with the genome during exine development. These findings suggest that a new approach to ontogenetic studies, including a consideration of physical factors, is required for a better understanding of developmental processes.

Pollen and microsporangium development in Ziziphus jujuba, Z. mucronata, Paliurus spina-christi and Gouania ulmifolia (Rhamnaceae)

The aim of this paper is to investigate the ultrastructural events that occur during pollen grains development, with emphasis in pollen grain wall and tapetum ontogeny in Ziziphus jujuba, Z. mucronata, Paliurus spina-christi (Paliureae) and Gouania ulmifolia (Gouanieae). Anthers at different developmental stages were processed according to classic techniques for transmission electron microscopy. Differences in the number of endothecium layers and in the number of tapetal cell nuclei were found. Tapetal cells present an anastomosing tubular network and large vesicles with fibrillar content in the cytoplasm. Pollen grain development and ontogeny of pollen grain wall are similar in the four species. The number of endothecium layers, the number of nuclei of the tapetal cells and tapetal cells ultrastructure of the four species support the phylogenetic relationships previously published for the Rhamnaceae family. Tapetal vesicles with fibrillar or polysaccharide content seem to be an exclusive characteristic of the tribes Paliureae and Gouanieae. Some ultrastructural characters of the pollen grain wall development are common to other species of Rhamnaceae, such as the primexine matrix present at the microspore mother cell stage, the aperture entirely built up during the tetrad stage, the thick and fibrillar intine, and the granular infractectum.

Artificial pollen walls simulated by the tandem processes of phase separation and self-assembly in vitro

Despite numerous attempts to elucidate the developmental mechanisms responsible for the observed diversity of pollen and spore walls, the processes involved remained obscure until the structures observed during exine development were recognized as a sequence of self-assembling micellar mesophases. To confirm this, a series of in vitro experiments was undertaken in which exine-like patterns were generated in colloidal mixtures by self-assembly, without any genomic participation. The intention was to test whether all the main types of exine structure could be simulated experimentally. Mixtures of substances, analogous to those involved in microspore development, were left undisturbed while water evaporated and self-assembly occurred. We varied the substances, their combinations and concentrations, and the physical constraints to make the experiments closer to the situation in nature. The resulting dry films were then examined using transmission electron microscopy. A variety of microstructures, simulating the full range of exine types, was obtained by micellar self-assembly. Moreover, the signs of related physicochemical process (i.e. phase separation) were also observed. Simple, energy-efficient, physical-chemical interactions, phase separation and self-assembly, are capable of generating exine-like patterns, providing evidence that these processes share control of exine formation with the well-documented program of gene expression.

Mimicking pollen and spore walls: self-assembly in action

BACKGROUND AND AIMS

Decades of research have attempted to elucidate the underlying developmental mechanisms that give rise to the enormous diversity of pollen and spore exines. The organization of the exine starts with the establishment of an elaborate glycocalyx within which the subsequent accumulation of sporopollenin occurs. Ontogenetic studies using transmission electron microscopy of over 30 species from many different groups have shown that the sequence of structures observed during development of the exine corresponds to the sequence of self-assembling micellar mesophases (including liquid crystals) observed at increasing concentrations of surfactants. This suggested that self-assembly plays an important part in exine pattern determination. Some patterns resembling separate layers of spore and pollen grain walls have been obtained experimentally, in vitro, by self-assembly. However, to firmly establish this idea, columellate and granulate exines, the most widespread forms, needed to be simulated experimentally.

METHODS

We used our original method, preparing mixtures of substances analogous to those known to occur in the periplasmic space of developing microspores, then leaving the mixtures undisturbed for specific periods of time to allow the process of self-assembly to occur. We developed our method further by using new substances analogous to those present in the periplasmic space and performing the experiments in a thin layer, more closely resembling the dimensions of the periplasmic space.

KEY RESULTS

The artificial microstructures obtained from our in vitro self-assembly experiments closely resembled the main types of exines, including tectate-columellate, granulate, alveolate and structureless, and permitted comparison with both developing and mature microspore walls. Compared with the previous attempts, we managed to simulate columellate and granulate exines, including lamellate endexine.

CONCLUSIONS

Our results show that simple physico-chemical interactions are able to generate patterns resembling those found in exines, supporting the idea that exine development in nature involves an interplay between the genome and self-assembly.

Suggested mechanisms underlying pollen wall development in Ambrosia trifida (Asteraceae: Heliantheae)

By a detailed ontogenetic study of Ambrosia trifida pollen, tracing each stage of development with TEM, we aim to understand the establishment of the pollen wall and to unravel the mechanisms underlying sporoderm development. The main steps of exine ontogeny in Ambrosia trifida, observed in the microspore periplasmic space, are as follows: spherical units, gradually transforming into columns, then to rod-like units; the appearance of the initial reticulate tectum; growth of columellae under the tectum and initial sporopollenin accumulation on them; the appearance of the endexine lamellae, first in fragments, then in long laminae; the cessation of the glycocalyx growth and its detachment from the plasma membrane, resulting in the appearance of gaps; massive accumulation of sporopollenin on the tectum, columellae, and endexine, and the appearance of the foot layer at the young post-tetrad stage, accompanied by establishment of caveae in sites of the former gaps; and final massive sporopollenin accumulation. This sequence of developmental events in all probability corresponds to the sequence of self-assembling micellar mesophases. This gives (together with earlier findings and experimental modeling of exine) strong evidence that the genome and self-assembly share control of exine formation. In this sense, self-assembly itself can be seen as an inherent mechanism of evolution.

Disentangling historical signal and pollinator selection on the micromorphology of flowers: an example from the floral epidermis of the Nymphaeaceae

The family Nymphaeaceae includes most of the diversity among the ANA-grade angiosperms. Among the species of this family, floral structures and pollination strategies vary. The genus Victoria, as well as subgenera Lotos and Hydrocallis in Nymphaea, present night-blooming, scented flowers pollinated by scarab beetles. Such similar pollination strategies have led to macromorphological similarities among the flowers of these species, which could be interpreted as homologies or convergences based on different phylogenetic hypotheses about the relationships of these groups. We employed scanning electron microscopy of floral epidermis for seven species of the Nymphaeaceae with contrasting pollination biology to identify the main characters of the floral organs and the potential homologous nature of the structures involved in pollinator attraction. Moreover, we used transmission electron microscopy to observe ultrastructure of papillate-conical epidermis in the stamen of Victoria cruziana. We then tested the phylogenetic or ecological distribution of these traits using both consensus network approaches and ancestral state reconstruction on fixed phylogenies. Our results show that the night-blooming flowers present different specialisations in their epidermis, with V. cruziana presenting the most elaborate floral anatomy. We also identify for the first time the presence of conical-papillate cells in the order Nymphaeales. The epidermal characters tend to reflect phylogenetic relationships more than convergence due to pollinator selection. These results point to an independent and parallel evolution of scarab pollination in Nymphaeaceae and demonstrate the promise of floral anatomy as a phylogenetic marker. Moreover, they indicate a degree of sophistication in the anatomical basis of cantharophilous flowers in the Nymphaeales that diverges from the most simplistic views of floral evolution in the angiosperms.

Sensitivity of sorghum pollen and pistil to high-temperature stress

High temperature (HT) decreases seed set percentage in sorghum (Sorghum bicolor [L.] Moench). The relative sensitivity of pollen and particularly pistil and the mechanistic response that induces tolerance or susceptibility to HT are not well known and hence are the major objectives of this research. The male sterile (ATx399) and fertile (RTx430) lines were exposed to 30/20 °C (optimum temperature), 36/26 °C (HT1 ), and 39/29 °C (HT2 ) from the start of booting to seed set in a controlled environment. Similarly, in the field, HT stress was imposed using heat tents. HT stress decreased pollen germination. Relatively high levels of reactive oxygen species and decreased antioxidant enzyme activity and phospholipid unsaturation were observed in pollen compared to pistil under HT. The severe cell organelle damage was observed in pollen and pistil at 36/26 and 39/29 °C, respectively. The seed set percentage was higher in HT-stressed pistil pollinated with optimum-temperature pollen. Direct and reciprocal crosses indicate that pollen was more sensitive with larger decreases in seed set percentage than pistil under HT stress. The negative impact was greater in pollen than pistil at lower temperatures. Overall, pollen was more sensitive than pistil to HT stress because it is more susceptible to oxidative damage than pistil.

Pollen wall ontogeny in Polemonium caeruleum (Polemoniaceae) and suggested underlying mechanisms of development

By a detailed ontogenetic study of Polemonium caeruleum pollen, tracing each stage of development at high TEM resolution, we aim to understand the establishment of the pollen wall and to unravel the mechanisms underlying sporoderm development. The main steps of exine ontogeny in Polemonium caeruleum, observed in the microspore periplasmic space, are spherical units, gradually transforming into columns, then to rod-like units (procolumellae), the appearance of the initial tectum, growth of columellae in height and tectum in thickness and initial sporopollenin accumulation on them, the appearance of the endexine lamellae and of dark-contrasted particles on the tectum, the appearance of a sponge-like layer and of the intine in aperture sites, the appearance of the foot layer on the base of the sponge-like layer and of spinules on the tectum, and massive sporopollenin accumulation. This sequence of developmental events fits well to the sequence of self-assembling micellar mesophases. This gives (together with earlier findings and experimental exine simulations) strong evidence that genome and self-assembly probably share control of exine formation. It is highly probable that self-assembly is an intrinsic instrument of evolution.

Self-assembly as the underlying mechanism for exine development in Larix decidua D. C

Our findings suggest a new approach to pollen ontogenetic investigations, entailing consideration of physical factors, which enable a better understanding of exine developmental processes. The sporopollenin-containing part of the pollen wall-the exine-is one of the most complex cell walls in plants. By tracing each stage of microspore development in Larix decidua with TEM, we aimed to understand the underlying mechanisms of its exine establishment. Our hypothesis is that self-assembly interferes with exine development. Our specific aim is to generate experimental simulations of the exine developmental pattern. The sequence of events leading to exine development includes the appearance of spherical units in the periplasmic space, their rearrangement into radial columns, and the appearance of white-lined endexine lamellae. The final accumulation of sporopollenin proceeds in the post-tetrad period. The sequence of self-assembling micellar mesophases corresponds with that of the developmental events: spherical micelles; columns of spherical micelles; and laminate micelles separated by strata of water and visible as white-lined lamellae in TEM. Several patterns, simulating structures at different stages of exine development in Larix, were obtained from in vitro experiments. Purely physicochemical processes of self-assembly, which are not under direct genetic control, play an important role in exine development and share control with the genome. These findings suggest that a new approach to ontogenetic investigations, entailing consideration of physical factors (e.g., cell tensegrity), is required for a better understanding of developmental processes.

Developmental and ultrastructural characters of the pollen grains and tapetum in species of Nymphaea subgenus Hydrocallis

Variations in pollen characters and tapetum behavior were recently acknowledged in the early-divergent family Nymphaeaceae and even within the genus Nymphaea, which probably is not monophyletic; some traits such as infratectum and tapetum type are also a matter of different interpretations. In this study, developmental characters of the pollen grains and tapetum in Nymphaea subgenus Hydrocallis are provided for the first time. Observations were made in N. amazonum, N. gardneriana, and N. prolifera using light, scanning, and transmission electron microscopy. Tapetum is of the secretory type and produces orbicules. At microspore and pollen grain stages, the distal and proximal walls differ considerably. This result supports the operculate condition of the aperture in Hydrocallis, and such aperture might be plesiomorphic for Nymphaeoideae. The infratectum is intermediate, composed of inter-columellae granular elements, robust columellae consisting of agglomerated granules, complete columellae, and fused columellae. Narrow microchannels are present and persist until the mature pollen grain stage. The membranous granular layer is often present in the pollen grains of Nymphaeaceae. In N. gardneriana, this layer is most probably a component of the intine because it is lost after acetolysis. Orbicules in the Nymphaeaceae are characterized as spherical or subspherical, with a smooth sporopolleninic wall that surrounds an electron-lucent core and with individual orbicules that usually merge to give irregular aggregations. The aperture, pollen wall ultrastructure, and the tapetum of the studied species are discussed in an evolutionary and systematic context, and these characters are also compared with those of other angiosperm lineages.

Comparative exine development from the post-tetrad stage in the early-divergent lineages of Ranunculales: the genera Euptelea and Pteridophyllum

Studies of pollen wall development produce a great deal of morphological data that supplies useful information regarding taxonomy and systematics. We present the exine development of Euptelea and Pteridophyllum, two taxa whose pollen wall development has never previously been studied using transmission electron microscopy. Both genera are representatives of the two earliest-diverging families of the order Ranunculales and their pollen data are important for the diagnosis of the ancestral pollen features in eudicots. Our observations show these genera are defined by having microechinate microreticulate exine ornamentation, perforate tectum, columellate morphology of the infratectum and the existence of a foot layer and endexine. The presence of lamellations is detected during the early stages of development in the nexine of both genera, especially in the apertures. Euptelea presents remains of the primexine layer during the whole maturation process, a very thin foot layer, and a laminate exinous oncus in the apertural region formed by ectexine and endexine elements. Pteridophyllum has a thicker tectum than Euptelea, a continuous foot layer and a thicker endexine. In the apertures, the exinous oncus is formed by islets and granules of endexine, in contrast to the Euptelea apertures. The secretory tapetum produces orbicules in both genera, but they have different morphology and electron-density. Comparisons with pollen data from related orders and families confirm the ancestral states for the pollen of eudicots proposed in previous studies: reticulate and echinate surfaces, columellate infractectum and a thin foot layer relative to the thickness of the ectexine. According to our observations, we propose considering the possibility of a polymorphic state for the aperture number in the ancestor of Ranunculales, and suggest the development of orbicules as the ancestral state in this order.