Cotton embryogenesis: The tissues of the stigma and style and their relation to the pollen tube

Reproductive biology of the "Brazilian pine" (Araucaria angustifolia-Araucariaceae): the pollen tube growth and the seed cone development

KEY MESSAGE

In Araucaria angustifolia, the seed scale is part of the ovule, the female gametophyte presents a monosporic origin and arises from a coenocytic tetrad, and the pollen tube presents a single axis. The seed cone of conifers has many informative features, and its ontogenetic data may help interpret relationships among function, development patterns, and homology among seed plants. We reported the seed cone development, from pollination to pre-fertilization, including seed scale, ovule ontogeny, and pollen tube growth in Araucaria angustifolia. The study was performed using light microscopy, scanning electron microscopy, and X-ray microcomputed tomography (μCT). During the pollination period, the ovule arises right after the seed scale has emerged. From that event to the pre-fertilization period takes about 14 months. Megasporogenesis occurs three weeks after ovule formation, producing a coenocytic tetrad. At the same time as the female gametophyte's first nuclear division begins, the pollen tube grows through the seed scale adaxial face. Until maturity, the megagametophyte goes through the free nuclei stage, cellularization stage, and cellular growth stage. Along its development, many pollen tubes develop in the nucellar tissue extending straight toward the female gametophyte. Our observations show that the seed scale came out of the same primordia of the ovule, agreeing with past studies that this structure is part of the ovule itself. The formation of a female gametophyte with a monosporic origin that arises from a coenocytic tetrad was described for the first time in conifers, and the three-dimensional reconstruction of the ovule revealed the presence of pollen tubes with only one axis and no branches, highlighting a new pattern of pollen tube growth in Araucariaceae.

Structure and ultrastructure of the stigma and style of Luehea divaricata (Malvaceae-Grewioideae)

Transmitting tissue cells of Luehea divaricata present a thick middle lamella and prominent convex lens-shaped thickenings of the cell wall that act as reservoir of energy for pollen tube growth. Luehea Milld. is a Neotropical genus with 18 species. This paper reports the study of the structure and the ultrastructure of the stigma and the style of Luehea divaricata Milld. using bright-field microscope and transmission electron microscope. Multiseriate papillae are observed in the stigma. The papillae cells are large with a content that stains intensively and the sub-stigmatic tissue resembles the style's transmitting tissue. L. divaricata has a closed style with an epidermis that presents raised stomata and multicellular trichomes. Numerous organelles and a large nucleus are present in the dense cytoplasm of the transmitting tissue cells. A relevant feature is that the cells of this tissue in the species studied present a very thick middle lamella with two zones of different electron density in the angle of contact between cells, and convex lens-shaped thickenings of the cell wall are prominent mainly also in these angles of contact. The growth of the pollen tube is initiated on the stigmatic papillae surface and continues growing in the middle lamella of the sub-stigmatic tissue and the transmitting tissue, mainly at the contact angle between cells. The present work is the first contribution to the knowledge of structure and ultrastructure of the stigma and style in the genus Luehea, as well as to the subfamily Grewioideae (Malvaceae).

Style morphology and pollen tube pathway

The style morphology and anatomy vary among different species. Three basic types are: open, closed, and semi-closed. Cells involved in the pollen tube pathway in the different types of styles present abundant endoplasmic reticulum, dictyosomes, mitochondria, and ribosomes. These secretory characteristics are related to the secretion where pollen tube grows. This secretion can be represented by the substances either in the canal or in the intercellular matrix or in the cell wall. Most studies suggest that pollen tubes only grow through the secretion of the canal in open styles. However, some species present pollen tubes that penetrate the epithelial cells of the canal, or grow through the middle lamella between these cells and subepithelial cells. In species with a closed style, a pathway is provided by the presence of an extracellular matrix, or by the thickened cell walls of the stylar transmitting tissue. There are reports in some species where pollen tubes can also penetrate the transmitting tissue cells and continue their growth through the cell lumen. In this review, we define subtypes of styles according to the path of the pollen tube. Style types were mapped on an angiosperm phylogenetic tree following the maximum parsimony principle. In line with this, it could be hypothesized that: the open style appeared in the early divergent angiosperms; the closed type of style originated in Asparagales, Poales, and Eudicots; and the semi-closed style appeared in Rosids, Ericales, and Gentianales. The open style seems to have been lost in core Eudicots, with reversions in some Rosids and Asterids.

Many ways to exit? Cell death categories in plants

Programmed cell death (PCD) is an integral part of plant development and defence. It occurs at all stages of the life cycle, from fertilization of the ovule to death of the whole plant. Without it, tall trees would probably not be possible and plants would more easily succumb to invading microorganisms. Here, we have attempted to categorize plant PCD in relation to three established morphological types of metazoan cell death: apoptosis, autophagy and non-lysosomal PCD. We conclude that (i) no examples of plant PCD conform to the apoptotic type, (ii) many examples of PCD during plant development agree with the autophagic type, and (iii) that other examples are apparently neither apoptotic nor autophagic.

Callose and determination of pistil viability and incompatibility

Callose provides a useful phenotypic bioassay in plant breeding to determine: incompatibility system; gametophytic competition; and stigma and ovule viability. Callose appearance in ovules may be associated with senescence, and used to determine the effective pollination period. In incompatible matings, callose formation is specific and related to rejection phenomena. The stigma callose response is induced by informational molecules carried by the germinal line, i.e. self or interspecific pollen, but not by the somatic line. Several methods of visualizing callose are reviewed. The role of callose in pollen-stigma interactions has many analogies with host-parasite interactions, and a model is proposed based on relationships between callose, boron and inhibitor (phytoalexin-like) synthesis. The callose response provides a useful tool for the biotechnology of seed production.

Ultrastructure and histochemistry of Citrus limon (L.) stigma

Citrus limon has a "wet" stigma which can be divided in two zones: a glandular superficial one formed by papillae, and a non-glandular one formed by parenchymatic cells. The stigmatic exudate is produced by the papillae after the latter have reached their ultimate size. The papillae of the mature pistil are of varying size and composition. Both the unicellular and multicellular ones are present. The cells at the base of the papillae are rich in cytoplasm, whereas the tip cells are vacuolated. Histochemical analysis has shown that the exudate of Citrus is composed of lipids, polysaccharides, and proteins. Our results indicate that the lipidic component is produced and secreted first, followed by production and secretion of the polysaccharidic component. The lipidic component of the exudate is produced in the basal papillae cells and accumulates as droplets in dilated parts of the smooth endoplasmic reticulum (SER). Subsequently the lipid droplets are transported to the plasma membrane, and transferred by the latter into the cell walls. Then the exudate component is accumulated in the intercellular spaces and in the middle lamellar regions of the walls. Subsequently, the polysaccharidic component of the exudate is produced and secreted by the tip cells of the papillae.

Ultrastructure of transmitting tissue of Lycopersicon peruvianum style: Development and histochemistry

The development of the transmitting tissue of the style of Lycopersicon peruvianum goes, after the completion of cell division and cell wall formation, through two distinct phases. During the first phase, the cells enlarge and the main part of the intercellular substance, consisting of pectins, is formed. During the second phase, the cells form an extensive rough endoplasmic reticulum (RER) and proteins are incorporated in the intercellular substance. A possible role of these proteins in the incompatibility reaction is proposed.

Transmitting tissue in the pistil of tobacco: Light and electron microscopic observations

The pistil of tobacco (Nicotiana tabacum L. cv. Wisconsin 38) is comprised of two fused carpels. The stigma is bilobed, papillose, and at maturity is covered with a sticky exudate. The style is solid. Both stigma and style are made up of four tissue elements-epidermis, cortex, vascular, and transmitting tissue. Transmitting tissue in this species is chlorophyllous. Transmitting cells have thin primary walls and are separated by massive deposits of denselystaining amorphous material. The cells contain numerous mitochondria, dictyosomes, RER, amyloplasts, ribosomes, as well as crystal-containing microbodies and myelin-like formations. Observations are discussed in relation to other reports dealing with similar cell populations.

Cotton embryogenesis: The tissues of the stigma and style and their relation to the pollen tube

The stigma of cotton (Gossypium hirsutum) is covered by unicellular hairs. The cytoplasm of these hairs degenerates before the stigma becomes receptive. The vacuole remains intact, but the hair cytoplasm becomes a mass of dark, amorphous material with only a few organelles still being visible. The rest of the stigma consists of thin-walled parenchyma cells with large vacuoles and large amounts of starch. The cells of the style are differentiated into a uniseriate epidermis, vascular tissue, a cortex of thin-walled, vacuolate parenchyma cells, and the transmitting tissue. This latter tissue occupies the center of the style and consists of thick-walled cells with few vacuoles. The cells are rich in starch, ribosomes, endoplasmic reticulum and dictyosomes. They also contain deposits of calcium salts in the form of druses. The pollen germinates on the stigmatic hairs, grows down the outside of the hair and between the cells of the stigma to the transmitting tissue of the style. There the tubes grow between the walls of the cells but do not enter the cells themselves. Some transmitting cells adjacent to the pollen tube degenerate after the tip of the pollen tube has grown past them. However, not all degenerate, and those that do show no fixed spatial relationship to one another. The cells which do degenerate follow a characteristic pattern of breakdown. No ultrastructural evidence was found for the secretion of hydrolytic enzymes by the pollen tube.