Abstract
Members of the genera Hieracium and Pilosella are model plants that are used to study the mechanisms of apomixis. In order to have a proper understanding of apomixis, knowledge about the relationship between the maternal tissue and the gametophyte is needed. In the genus Pilosella, previous authors have described the specific process of the “liquefaction” of the integument cells that surround the embryo sac. However, these observations were based on data only at the light microscopy level. The main aim of our paper was to investigate the changes in the integument cells at the ultrastructural level in Pilosella officinarum and Hieracium alpinum. We found that the integument peri-endothelial zone in both species consisted of mucilage cells. The mucilage was deposited as a thick layer between the plasma membrane and the cell wall. The mucilage pushed the protoplast to the centre of the cell, and cytoplasmic bridges connected the protoplast to the plasmodesmata through the mucilage layers. Moreover, an elongation of the plasmodesmata was observed in the mucilage cells. The protoplasts had an irregular shape and were finally degenerated. After the cell wall breakdown of the mucilage cells, lysigenous cavities that were filled with mucilage were formed.
Highlights
A breakdown of the cell wall between the adjacent mucilage cells occurred (Fig. 7b), after which lysigenous cavities that were filled with mucilage were formed (Figs. 1b, c and 2b)
We showed that the intensive Bliquefaction^ of the integument cells surrounding the embryo sac, which was previously described by Koltunow et al (1998), was in the fact gelatinisation: an accumulation of the mucilage in the cells and later the formation of lysigenous cavities that were filled with mucilage
Koltunow et al (1998) wrote that the material that was accumulated during the intensive changes of the integument cells surrounding the embryo sac was carbohydrate-rich
Summary
Members of the genera Hieracium L. and Pilosella Vaill. are important model plants for understanding the mechanisms of apomixis in angiosperms (e.g. Koltunow et al 2011a,b; Tucker et al 2012; Okada et al 2013; Hand and Koltunow 2014; Hand et al 2015; Shirasawa et al 2015; Rabiger et al 2016; Rotreklová and Krahulcová 2016).According to Koltunow et al (1998), the development of the embryo and endosperm in Hieracium aurantiacum L. [= Pilosella aurantiaca (L.) F. According to Koltunow et al (1998), this material may serve a nutritive role and they suggested that the accumulation of a large pool of nutrients around the embryo sac might have helped the evolution of the apomictic trait within the genus This suggestion about a nutritional function of these specific integumentary cells was accepted and repeated by other authors, e.g. Van Baarlen et al (1999) wrote that the ovules of Hieracium and Taraxacum contain a protein-rich storage tissue, which nourishes the embryo and reduces the importance of the endosperm function It was suggested by these authors that the presence of this tissue might explain the evolution of autonomous embryo development in most of the Asteraceae apomicts. During the differentiation of these cells and the deposition of mucilage, the plasmodesmata become elongated and are associated with structures called Bcytoplasmic bridges.^
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