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

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.

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.

Three-dimensional reconstruction of Picea wilsonii Mast. pollen grains using automated electron microscopy

Three-dimensional electron microscopy (3D-EM) has attracted considerable attention because of its ability to provide detailed information with respect to developmental analysis. However, large-scale high-resolution 3D reconstruction of biological samples remains challenging. Herein, we present a 3D view of a Picea wilsonii Mast. pollen grain with 100 nm axial and 38.57 nm lateral resolution using AutoCUTS-SEM (automatic collector of ultrathin sections-scanning electron microscopy). We established a library of 3,127 100 nm thick serial sections of pollen grains for preservation and observation, demonstrating that the protocol can be used to analyze large-volume samples. After obtaining the SEM images, we reconstructed an entire pollen grain comprising 734 serial sections. The images produced by 3D reconstruction clearly revealed the main components of the P. wilsonii pollen grain, i.e., two sacci and pollen corpus, tube cell, generative cell, and two degenerated prothallial cells, and their internal organization. In addition, we performed a quantitative analysis of the different pollen grain cells, including sacci, and found that there were 202 connections within a saccus SEM image. Thus, for the first time, this study provided a global 3D view of the entire pollen grain, which will be useful for analyzing pollen development and growth.

Pollen grain development and male sterility in the perfect flowers of Maytenus obtusifolia Mart. (Celastraceae)

Perfect flowers of Maytenus obtusifolia have partial sterility of pollen grains, resulting in collapsed and developed free microspores. However, the cellular events resulting in partial male sterility have not been determined. In pistillate flowers of this species, male sterility is related to the premature programmed cell death (PCD) in tapetum and sporogenic cells. The process occurs through autophagy via macroautophagy and massive autophagy and is associated with sporophytic cytoplasmic male sterility (CMS). Here, we characterised the development of pollen grains and investigated the cellular events that result in tapetum cells and free microspores PCD in perfect flowers, using light and transmission electron microscopy combined with the TUNEL (Terminal deoxynucleotidyl transferase mediated dUDP end-Labeling) assay and the ZIO (Zinc iodide-osmium tetroxide) method. Pollen grain development in perfect flowers was divided into eight developmental stages based on the characteristics of the pollen grains. Tapetum cells undergo PCD at the free microspore stage, through a macroautophagic process, by formation of autophagosomes and by autophagosomes giving rise to lytic vacuoles at maturity. In the final stage of PCD, massive autophagy occurs by rupture of the tonoplast. The development of viable and inviable microspores diverges at the vacuolated microspore stage, when PCD occurs in some free microspores, causing interruption of pollen development through necrosis. These events result in the observed partial male sterility. Viable microspores undergo mitosis and develop into tricellular pollen grains. Male sterility in hermaphrodite individuals is here interpreted as gametophytic CMS.

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.

Assembling the thickest plant cell wall: exine development in Echinops (Asteraceae, Cynareae)

The exceptionally complex exine of Echinops, representing a significant investment of energy, develops from an elaborate glycocalyx which establishes, by self-assembly, a multi-layered system of micelles upon which sporopollenin polymerizes. We report on pollen development in two species of Echinops (Asteraceae, Cynareae) studied using transmission and scanning electron microscopy with an emphasis on the organisation and development of the massive sporoderm (maximum thickness 18 μm). The major events of exine deposition during the tetrad stage follow the now familiar sequence of self-assembling micellar mesophases and the subsequent incorporation of sporopollenin, observed here as: (1) spherical units with light cores; (2) columns of spherical units with dark cores; (3) large branched macromolecules arranged in a dendritic, three-dimensional network of long alveoli; and (4) alveoli with electron-transparent cores and dense walls. Later, (5) the primexine exhibits an elongated-alveolate pattern in which the alveoli have electron-dense cores and lighter exteriors. When (6) the thick inner columellae make contact with the outer primexine, sporopollenin accumulation in the cores of the primexine alveolae establishes continuity between the inner and outer columellae. In the free microspore stage, (7) the foot layer and first lamellae of the endexine appear (8). The endexine lamellae then increase in number and massive accumulation of sporopollenin occurs on all exine elements, making individual elements such as tectal spines, more pronounced. These and earlier findings, as well as experimental simulations of exine development, show that pollen wall morphogenesis involves a subtle interplay of gene-driven biological processes and physico-chemical factors offering abundant opportunities for the generation of complex, taxon-specific patterns.