Megasporogenesis and programmed cell death in Tillandsia (Bromeliaceae)

Abstract

The degeneration of three of four meiotic products is a very common process in the female gender of oogamous eukaryotes. In Tillandsia (and many other angiosperms), the surviving megaspore has a callose-free wall in chalazal position while the other three megaspores are completely embedded in callose. Therefore, nutrients and signals can reach more easily the functional megaspore from the nucellus through the chalazal pole with respect to the other megaspores. The abortion of three of four megaspores was already recognized as the result of a programmed cell death (PCD) process. We investigated the process to understand the modality of this specific type of PCD and its relationship to the asymmetric callose deposition around the tetrad. The decision on which of the four megaspores will be the supernumerary megaspores in angiosperms, and hence destined to undergo programmed cell death, appears to be linked to the callose layer deposition around the tetrad. During supernumerary megaspores degeneration, events leading to the deletion of the cells do not appear to belong to a single type of cell death. The first morphological signs are typical of autophagy, including the formation of autophagosomes. The TUNEL positivity and a change in morphology of mitochondria and chloroplasts indicate the passage to an apoptotic-like PCD phase, while the cellular remnants undergo a final process resembling at least partially (ER swelling) necrotic morphological syndromes, eventually leading to a mainly lipidic cell corpse still separated from the functional megaspore by a callose layer.

The unusual tegumental tissues of the Lunaria annua (Brassicaceae) seed: A developmental study using light and electron microscopy

“…The many ribosomes, both free and associated within the ER, are likely involved in producing lytic enzymes. The dead cells often showed traces of partially lysed, osmiophilic cytoplasm and nuclei, hallmarks of PCD in plants (Brighigna et al, 2006;Papini et al, 1999Papini et al, , 2010cPapini et al, , 2011. At maturity, these dead cells are characterized by their walls, which possess a loose and filamentous matrix.…”

Ultrastructure of autophagy in plant cells

“…Later on these plastids fully degenerate and are hardly recognizable. 48,67 Plastids in tapetum cells of Lobivia rauschii show several concentric double-membrane structures, and many electron-dense plastoglobules. 67 No vesicles leaving the organelle and no role of autophagy have as yet been described during the degradation of these types of plastids.…”

“…They are bound by a membrane and contain many membranes, usually in concentric circles ( Fig. 10 in Papini et al 48 ). These bodies are often present at the interface of cytoplasm and vacuole, whereby they can protrude far into the vacuole.…”

Callose synthesis during reproductive development in monocotyledonous and dicotyledonous plants

“…9 In the females of oogamous eukaryotes, the megasporocyte develops into a functional megaspore with a callose-free wall in the chalazal position while the other 3 megaspores are surrounded by a callose layer; which of the 4 megaspores will develop into functional megaspores or supernumerary megaspores seems to be associated with callose deposition around the tetrad. 10 A study of dandelion ovules revealed the deposition of callose in both diplosporous and sexual dandelions; however, callose deposition was noted mostly at the micropylar pole of the megasporocyte in a diplosporous dandelion, and it was suggested to be correlated with abnormal asynaptic meiosis. 11 Despite these findings, knowledge of the role of callose deposition in embryo sac development is limited.…”