Deposition of callose in young ovules of two Taraxacum species varying in the mode of reproduction

Abstract

Although callose occurs during megasporogenesis in most flowering plants, the knowledge about its general function and the mechanisms by which the callose layer is formed in particular places is still not sufficient. The results of previous studies suggest a total lack of callose in the ovules of diplosporous plants in which meiosis is omitted or disturbed. This report is the first documentation of callose events in dandelions ovules. We demonstrated the pattern of callose deposition during the formation of megaspores through diplospory of Taraxacum type and during normal meiotic megasporogenesis in apomictic triploid Taraxacum atricapillum and amphimictic diploid Taraxacum linearisquameum. We found the presence of callose in the megasporocyte wall of both diplosporous and sexual dandelions. However, in a diplosporous dandelion, callose predominated at the micropylar pole of megaspore mother cell (MMC) which may be correlated with abnormal asynaptic meiosis and may indicate diplospory of the Taraxacum type. After meiotic division, callose is mainly deposited in the walls between megaspores in tetrads and in diplodyads. In subsequent stages, callose gradually disappears around the chalazal functional megaspore. However, some variations in the pattern of callose deposition within tetrad may reflect variable positioning of the functional megaspore (FM) observed in the ovules of T. linearisquameum.

Insights into developmental processes in anthers, ovaries, and ovules of Taraxacum belorussicum (Asteraceae-Cichorioideae) using DIC optics

“…Regular meiotic division and linear tetrads of megaspores has been documented in the ovules of diploid dandelions, for example, in T. pieninicum Pawł. (Małecka 1961), diploid sexuals of T. officinale (Van Baarlen et al 2000, 2002, and most recently in diploid T. linearisquameum Soest (Musiał et al 2015).…”

“…Whole florets, ovaries, and ovules were then isolated and dehydrated for 1 h in ethanol series: 70, 80 (one change) and 100 % (three changes). Dehydrated samples were cleared in methyl salicylate (SigmaAldrich) according to a procedure described by Musiał et al (2015). Cleared material was placed in a drop of pure methyl salicylate on a Rajs' slides (Herr 2000) and examined using a Nikon Eclipse 80i microscope fitted with Nomarski's Interference Contrast (DIC optics).…”

“…This method gives a possibility to investigate in toto non-stained flowers, anthers, ovaries or dissected ovules and is less time-consuming, compared to other standard techniques used in embryological studies, e.g., technique of paraffin or Technovit sections (Jensen 1962;O'Brien and McCully 1981). It also allows for a detailed cyto-embryological analysis of both microsporogenesis and megasporogenesis in Taraxacum species (Van Baarlen et al 2000, 2002Musiał et al 2015).…”

“…In the sexual pathway of reproduction, regular meiosis and double fertilization are indispensable for seeds formation, whereas these processes are omitted during seeds' production in agamospermous dandelions, resulting in offspring that are genetically identical to the mother plant. For this reason, the genus Taraxacum is often used as a model system not only in research on molecular mechanisms underlying apomixis (Vijverberg et al 2010 and references therein), but also for comparative embryological analysis of the sexual and apomictic reproductive modes (Małecka 1971a(Małecka , 1973Van Baarlen et al 2000, 2002Musiał and Koś-cińska-Pająk 2013;Musiał et al 2013bMusiał et al , 2015Płachno et al 2014Płachno et al , 2015.…”

Callose synthesis during reproductive development in monocotyledonous and dicotyledonous plants

“…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.…”