Style morphology and pollen tube pathway

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).

Pollen-Pistil Interactions as Reproductive Barriers

Pollen-pistil interactions serve as important prezygotic reproductive barriers that play a critical role in mate selection in plants. Here, we highlight recent progress toward understanding the molecular basis of pollen-pistil interactions as reproductive isolating barriers. These barriers can be active systems of pollen rejection, or they can result from a mismatch of required male and female factors. In some cases, the barriers are mechanistically linked to self-incompatibility systems, while others represent completely independent processes. Pollen-pistil reproductive barriers can act as soon as pollen is deposited on a stigma, where penetration of heterospecific pollen tubes is blocked by the stigma papillae. As pollen tubes extend, the female transmitting tissue can selectively limit growth by producing cell wall-modifying enzymes and cytotoxins that interact with the growing pollen tube. At ovules, differential pollen tube attraction and inhibition of sperm cell release can act as barriers to heterospecific pollen tubes.

Arabinogalactan glycoprotein dynamics during the progamic phase in the tomato pistil

Pistil AGPs display dynamic localization patterns in response to fertilization in tomato. SlyFLA9 (Solyc07g065540.1) is a chimeric Fasciclin-like AGP with enriched expression in the ovary, suggesting a potential function during pollen-pistil interaction. During fertilization, the male gametes are delivered by pollen tubes to receptive ovules, deeply embedded in the sporophytic tissues of the pistil. Arabinogalactan glycoproteins (AGPs) are a diverse family of highly glycosylated, secreted proteins which have been widely implicated in plant reproduction, particularly within the pistil. Though tomato (Solanum lycopersicum) is an important crop requiring successful fertilization for production, the molecular basis of this event remains understudied. Here we explore the spatiotemporal localization of AGPs in the mature tomato pistil before and after fertilization. Using histological techniques to detect AGP sugar moieties, we found that accumulation of AGPs correlated with the maturation of the stigma and we identified an AGP subpopulation restricted to the micropyle that was no longer visible upon fertilization. To identify candidate pistil AGP genes, we used an RNA-sequencing approach to catalog gene expression in functionally distinct subsections of the mature tomato pistil (the stigma, apical and basal style and ovary) as well as pollen and pollen tubes. Of 161 predicted AGP and AGP-like proteins encoded in the tomato genome, we identified four genes with specifically enriched expression in reproductive tissues. We further validated expression of two of these, a Fasciclin-like AGP (SlyFLA9, Solyc07g065540.1) and a novel hybrid AGP (SlyHAE, Solyc09g075580.1). Using in situ hybridization, we also found SlyFLA9 was expressed in the integuments of the ovule and the pericarp. Additionally, differential expression analyses of the pistil transcriptome revealed previously unreported genes with enriched expression in each subsection of the mature pistil, setting the foundation for future functional studies.

Spatio-Temporal Distribution of Cell Wall Components in the Placentas, Ovules and Female Gametophytes of Utricularia during Pollination

In most angiosperms, the female gametophyte is hidden in the mother tissues and the pollen tube enters the ovule via a micropylar canal. The mother tissues play an essential role in the pollen tube guidance. However, in Utricularia, the female gametophyte surpasses the entire micropylar canal and extends beyond the limit of the integument. The female gametophyte then invades the placenta and a part of the central cell has direct contact with the ovary chamber. To date, information about the role of the placenta and integument in pollen tube guidance in Utricularia, which have extra-ovular female gametophytes, has been lacking. The aim of this study was to evaluate the role of the placenta, central cell and integument in pollen tube pollen tube guidance in Utricularia nelumbifolia Gardner and Utricularia humboldtii R.H. Schomb. by studying the production of arabinogalactan proteins. It was also determined whether the production of the arabinogalactan proteins is dependent on pollination in Utricularia. In both of the examined species, arabinogalactan proteins (AGPs) were observed in the placenta (epidermis and nutritive tissue), ovule (integument, chalaza), and female gametophyte of both pollinated and unpollinated flowers, which means that the production of AGPs is independent of pollination; however, the production of some AGPs was lower after fertilization. There were some differences in the production of AGPs between the examined species. The occurrence of AGPs in the placental epidermis and nutritive tissue suggests that they function as an obturator. The production of some AGPs in the ovular tissues (nucellus, integument) was independent of the presence of a mature embryo sac.

Ovary Signals for Pollen Tube Guidance in Chalazogamous Mangifera indica L

Most flowering plants show porogamy in which the pollen tubes reach the egg apparatus through the micropyle. However, several species show chalazogamy, an unusual pollen tube growth, in which the pollen tubes reach the embryo sac through the chalaza. While ovary signals for pollen tube growth and guidance have been extensively studied in porogamous species, few studies have addressed the process in chalazogamous species such as mango (Mangifera indica L.), one of the five most important fruit crops worldwide in terms of production. In this study, we characterize pollen-pistil interaction in mango, paying special attention to three key players known to be involved in the directional pollen tube growth of porogamous species such as starch, arabinogalactan proteins (AGPs), and γ-aminobutyric acid (GABA). Starch grains were observed in the style and in the ponticulus at anthesis, but their number decreased 1 day after anthesis. AGPs, revealed by JIM8 and JIM13 antibodies, were homogenously observed in the style and ovary, but were more conspicuous in the nucellus around the egg apparatus. GABA, revealed by anti-GABA antibodies, was specifically observed in the transmitting tissue, including the ponticulus. Moreover, GABA was shown to stimulate in vitro mango pollen tube elongation. The results support the heterotrophic growth of mango pollen tubes in the style at the expense of starch, similarly to the observations in porogamous species. However, unlike porogamous species, the micropyle of mango does not show high levels of GABA and starch, although they were observed in the ponticulus and could play a role in supporting the unusual pollen tube growth in chalazogamous species.

Structure of the stigma and style of Callaeum psilophyllum (Malpighiaceae) and its relation with potential pollinators

The family Malpighiaceae, particularly in the Neotropic, shows a similar floral morphology. Although floral attraction and rewards to pollinators are alike, stigmas and styles show more diversity. The stigmas were described covered with a thin and impermeable cuticle that needs to be ruptured by the mechanical action of the pollinators. However, this characteristic was only mentioned for a few species and the anatomy and ultrastructure of the stigmas were not explored. In this work, we analyze the morphology, anatomy, and ultrastructure of the stigma and style of Callaeum psilophyllum. Moreover, we identify the potential pollinators in order to evaluate how the disposition of the stigmas is related with their size and its role in the exposure of the receptive stigmatic surface. Our observations indicate that Centris flavifrons, C. fuscata, C. tarsata, and C. trigonoides are probably efficient pollinators of C. psilophyllum. The three stigmas are covered by a cuticle that remained intact in bagged flowers. The flowers exposed to visitors show the cuticle broken, more secretion in the intercellular spaces between sub-stigmatic cells and abundant electron-dense components inside vacuoles in stigmatic papillae. This indicates that the stigmas prepares in similar ways to receive pollen grains, but the pollinator action is required to break the cuticle, and once pollen tubes start growing, stigmatic and sub-stigmatic cells release more secretion by a granulocrine process.