A comparative ultrastructural study of pit membranes with plasmodesmata associated thickenings in four angiosperm species

Abstract

Recent micromorphological observations of angiosperm pit membranes have extended the number and range of taxa with pseudo-tori in tracheary elements. This study investigates at ultrastructural level (TEM) the development of pseudo-tori in the unrelated Malus yunnanensis, Ligustrum vulgare, Pittosporum tenuifolium, and Vaccinium myrtillus in order to determine whether these plasmodesmata associated thickenings have a similar developmental pattern across flowering plants. At early ontogenetic stages, the formation of a primary thickening was observed, resulting from swelling of the pit membrane in fibre-tracheids and vessel elements. Since plasmodesmata appear to be frequently, but not always, associated with these primary pit membrane thickenings, it remains unclear which ultrastructural characteristics control the formation of pseudo-tori. At a very late stage during xylem differentiation, a secondary thickening is deposited on the primary pit membrane thickening. Plasmodesmata are always associated with pseudo-tori at these final developmental stages. After autolysis, the secondary thickening becomes electron-dense and persistent, while the primary thickening turns transparent and partially or entirely dissolves. The developmental patterns observed in the species studied are similar and agree with former ontogenetic studies in Rosaceae, suggesting that pseudo-tori might be homologous features across angiosperms.

Plasmodesmatal pores in the torus of bordered pit membranes affect cavitation resistance of conifer xylem

“…The plasmodesmatal nature of the pores can be confirmed based on two arguments: (1) the average pore size corresponds with the ca. 50 nm diameter of plasmodesmata (Murmanis & Sachs 1969;Roberts 2005;Bell & Oparka 2011); and (2) their grouped distribution and branched pattern reflect the characteristically uneven and branched arrangement of secondary plasmodesmata (Rabaey et al 2008). Variation in average pore size between SEM and TEM (51 and 64 nm, respectively) is likely due to differences in sample preparation and observation.…”

Heterogeneous distribution of pectin epitopes and calcium in different pit types of four angiosperm species

“…Pseudotori occur in tracheary elements of several woody taxa, including Rosaceae, Ericaceae, and Oleaceae (Jansen et al. , 2007; Rabaey et al. , 2008).…”

“…Pseudotori develop as secondary thickenings following the formation of plasmodesmata‐associated primary thickenings. Later in development, autolytic enzymes remove the primary thickenings leaving the pseudotori intact, and the tracheary cell undergoes programmed cell death (Rabaey et al. , 2008).…”

“…The function, if any, of these peculiar structures is unknown. Pseudotori occur in tracheary elements of several woody taxa, including Rosaceae, Ericaceae, and Oleaceae (Jansen et al, 2007;Rabaey et al, 2008). Pseudotori develop as secondary thickenings following the formation of plasmodesmata-associated primary thickenings.…”