Cell-wall development in freeze-fixed pollen: Intine formation of Ledebouria socialis (Hyacinthaceae)

Abstract

Abstract. The structure and development of the inner pectocellulosic pollen wall, the intine, was re-examined using high-pressure freezing with subsequent freeze substitution in Ledebouria socialis Roth, a monocotyledonous angiosperm. The bilayered intine is formed immediately after differentiation of the endexine. Similar to somatic cell walls, intine matrix substances originate from the Golgi apparatus and leave the cytoplasm via exocytosis. Exintine development starts with the apposition of intine matrix substances to the inner polysaccharide layer of the endexine (termed inner endexine), leading to irregular cell-wall ingrowths. Subsequently the inner endexine becomes intensely infiltrated with intine matrix substances; this process is interpreted as transformation of the inner endexine into intine. Along the aperture region, cell-wall matrix substances are unevenly deposited to such an extent that more or less radially oriented tubules filled with cytoplasm remain within the growing exintine. These tubules subsequently become cut off from the microspore cytoplasm by selective membrane fusions, leading to the incorporation of ground cytoplasm and ribosomes into the exintine. Exintine formation is completed prior to the first mitotic division of the pollen grain whereas the endintine is formed as a homogeneous thin layer after mitosis. Both transformation of the inner endexine by infiltration and passive incorporation of cytoplasm and ribosomes into the exintine by membrane fusions are novel features and are only observed in optimally freeze-fixed, freeze-substituted samples; general aspects of ultrastructure preservation in high-pressure-frozen, freeze-substituted plant cells are discussed as well. Modifications of the Golgi apparatus and post-Golgi-apparatus structures during pollen wall development are correlated with increasing and decreasing polysaccharide exocytosis, respectively. These events Abbreviations: ER=endoplasmic reticulum; FS=freeze substitution; HPF=high-pressure freezing; MS=microspore(s); PATAg = periodic acid-thiocarbohydrazine-silver proteinate; PGS=post-Golgi-apparatus structures; UA-Pb=uranyl acetatelead strictly coincide with the formation of morphologically and chemically different pollen wall layers and therefore seem to reflect the different deposition patterns of the predominant cell-wall polysaccharides.

Ultrastructural observations on anther tapetum development of freeze-fixed Ledebouria socialis Roth (Hyacinthaceae)

“…3). Microtubules were observed during all developmental stages but we failed in detecting microfilaments (for detailed discussions see Ding et al 1992;McCauley and Hepler 1992;Meindl et al 1992;Hess 1993). (Fig.…”

“…High-pressure freezing and FS were performed according to Studer et al (1989) as previously reported (Hess 1993;Hess and Glaser 1993). Briefly, intact anthers were cryoimmobilized using a highpressure freezer (HPM 010; Balzers Union, Liechtenstein).…”

“…High-pressure freezing (HPF) is at present the only method available for freeze-fixation of large samples (Craig and Staehelin 1988;Studer et al 1989;for review: Moor 1987) and has already been used for freezing intact anthers (Hess 1992(Hess , 1993. Extensive specimen preparation (e.g.…”

“…For practical reasons tapetum development is divided into the same "developmental stages" as microspore development (Hess 1993). The complex process of Ubisch-body formation is not dealt with in the present paper.…”

Electron microscopy of high pressure frozen samples: bridging the gap between cellular ultrastructure and atomic resolution

“…Last but not least, specimens with an almost impermeable coat such as C. elegans or plant pollen are practically impossible to preserve by direct aldehyde Wxation. After high pressure freezing and freeze substitution, however, their ultrastructure is beautifully preserved (Hess 1993;Hohenberg et al 1994;McDonald and Morphew 1993).…”