Are unusual ultrastructural features occurring in the pollen endomembrane system of Cyperaceae and other angiosperms?

Abstract

Cyperaceae representatives present peculiar microsporogenesis and microgametogenesis, which raises the question of how regular the sedge pollen is. In order to answer this question, preanthesis pollen grains of Cyperaceae individuals were analyzed under different tools such as light and transmission electron microscopy, which included cytochemistry and immunogold procedures. The results showed that maturing pollen in Cyperaceae presents some unusual endomembrane behaviors. Endoplasmic reticulum and dictyosomes are concerned in classic secretion pathways in vegetative cells, and possibly the late breakdown of degenerative microspores. However, cortical and concentric endoplasmic reticulum are also present and are possibly related to other functions aside the biosynthetic pathway. Unconventional secretion of large membrane‐bound bodies containing cell wall precursors was also observed and confirmed by immunogold. However, since these bodies most likely receive material from dictyosomes, as observed in silver nitrate reaction, the “unconventional” status of this secretion is debatable. Reports of the literature show that these unusual endomembrane formations are not exclusive of the sedge pollen, but little attention have been given to them so far. This could represent an opportunity to re‐examine our understanding on the endomembrane system in pollen cells in general.

Nuclear variations and tapetum polyploidy related to pollen grain development in Passiflora L. (Passifloraceae)

“…In Passiflora , a well‐developed ER has been observed from the perinuclear region to the border of tapetal cells, and it seems to be important in the secretion pathways of biomolecules during pollen grain development. The presence of this well‐developed network may also be related to the regulation of Ca 2+ in the cytosol (Costa et al, 2018), the storage and secretion of compounds directly to the locules (Amela‐García et al, 2002; Rocha et al, 2020), and as an anchoring point for the cytoskeleton (Hepler et al, 1990). Results in P. actinia and P. elegans suggest that all these changes occur in the endomembrane system until the end of stage 2, matching with the low cellulose deposition in tapetal cells.…”