Abstract
The aim of the paper is to determine what happens with plasmodesmata when mucilage is secreted into the periplasmic space in plant cells. Ultrastructural analysis of the periendothelial zone mucilage cells was performed on examples of the ovule tissues of several sexual and apomictic Taraxacum species. The cytoplasm of the periendothelial zone cells was dense, filled by numerous organelles and profiles of rough endoplasmic reticulum and active Golgi dictyosomes with vesicles that contained fibrillar material. At the beginning of the differentiation process of the periendothelial zone, the cells were connected by primary plasmodesmata. However, during the differentiation and the thickening of the cell walls (mucilage deposition), the plasmodesmata become elongated and associated with cytoplasmic bridges. The cytoplasmic bridges may connect the protoplast to the plasmodesmata through the mucilage layers in order to maintain cell-to-cell communication during the differentiation of the periendothelial zone cells.
Highlights
In some members of the Asteraceae family, there is a specific cell differentiation in the integument
A thick layer of an amorphous wall matrix with dense fibrillar material in a reticulate arrangement (Fig. 2a, b) and the primary cell wall, which was present before the deposition of the mucilage material, were distinguished
A middle lamella was seen as an electron-dense layer, especially between the periendothelial zone cells and other integument cell types (Fig. 2c)
Summary
In some members of the Asteraceae family, there is a specific cell differentiation in the integument. Depending on the changes in this integument zone, three types of ovules were proposed in Asteraceae: BTaraxacum^, BGalinsoga^ and BRatibida^. BTaraxacum^ type cells have very swollen walls with a spongy structure (Kolczyk et al 2014). Some characteristics of these cells such as a reduction in protoplast size and thickening of the cell wall are similar to mucilage cells. According to Pandey et al (1978), the cells of periendothelial zone undergo gelatinisation in Youngia japonica. A liquefaction of the integument cells was observed in Hieracium by Koltunow et al (1998). Musiał et al (2013) claimed that these cells undergo progressive degeneration in Taraxacum, which is probably by programmed cell death (PCD)
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