The Epidermal Cell Structure of the Secondary Pollen Presenter in Vangueria infausta (Rubiaceae: Vanguerieae) Suggests a Functional Association with Protruding Onci in Pollen Grains

Abstract

Secondary pollen presentation is a well-known phenomenon in the Rubiaceae with particularly conspicuous pollen presenters occurring in the tribe Vanguerieae. These knob-like structures are formed by a modification of the upper portion of the style and stigma, together known as the stylar head complex. In the flower bud and shortly before anthesis, the anthers surrounding the stylar head complex dehisce and release pollen grains which adhere to the pollen presenter. The epidermal cells of the pollen presenter facing the anthers are radially elongated with a characteristic wall thickening encircling the anticlinal walls of each cell towards the distal end. These cells were studied in the pollen presenter of Vangueria infausta using electron and light microscopy in conjunction with histochemical tests and immunohistochemical methods. Other prominent thickenings of the cell wall were also observed on the distal and proximal walls. All these thickenings were found to be rich in pectin and possibly xyloglucan. The terms “thickenings of Igersheim” and “bands of Igersheim” are proposed to refer, respectively, to these wall structures in general and those encircling the anticlinal walls of each cell near the distal end. The epidermal cells have an intricate ultrastructure with an abundance of organelles, including smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria and secretory vesicles. This indicates that these cells are likely to have an active physiological role. The pollen grains possess prominent protruding onci and observations were made on their structure and development. Walls of the protruding onci are also rich in pectin. Pectins are hydrophilic and known to be involved in the dehydration and rehydration of pollen grains. We hypothesise that the thickenings of Igersheim, as well as the protruding onci of the pollen grains, are functionally associated and part of the adaptive syndrome of secondary pollen presentation, at least in the Vanguerieae.

A protocol for combining fluorescent proteins with histological stains for diverse cell wall components

“…Importantly, Auramine O also showed broader staining in the upper part of the root, suggesting that it also has the ability to bind to suberin or suberin‐like compounds (Figure d). As Auramine O has been reported to stain exine‐like molecules and cutin (Edlund et al ., ; Nishikawa et al ., ; Tilney et al ., ; Yadav et al ., ; Gimenez et al ., ; Li et al ., ), our observation supports the idea that Arabidopsis cutin and suberin are very similar in structure and that Auramine O could indiscriminately bind both compounds. Besides Basic Fuchsin and Auramine O, we tested Safranine O as a potential lignin stain (Sibout et al ., ; Ramirez et al ., ).…”

Anther structure and pollen development in species of Rubiaceae and anatomical evidence of pathway to morphological dioecy

“…An Acad Bras Cienc (2022) 94(4) e20191362 17 | 24 between the outstanding oncus and the cellular structure of the epidermal stigmata that would facilitate the secondary presentation of pollen thanks to the connections provided by the protruding onci between the pollen grain and the epidermis of the pollen presenter (Tilney et al 2014). Considering that the species analysed have a secondary presentation of pollen, future ultrastructural and histochemical studies of stigma and pollen may reveal whether a similar relationship exists.…”