Microgametogenesis in Plumbago zeylanica (Plumbaginaceae). 2. Quantitative cell and organelle dynamics of the male reproductive cell lineage

Abstract

Quantitative cell and organelle dynamics of the male gamete-producing lineage of Plumbago zeylanica were examined using serial transmission electron microscopic reconstruction at five stages of development from generative cell inception to sperm cell maturity. The founder population of generative cell organelles includes an average of 3.88 plastids, 54.9 mitochondria, and 3.7 vacuoles. During development the volume of the pollen grain increases from 6,200 lm 3 in early microspores to 115,000 lm 3 at anthesis, cell volume of the male germ lineage decreases more than 67% from 362.3 lm 3 to 118.4 lm 3 . By the time the generative cell separates from the intine, plastid numbers increase by >600%, mitochondria by 250%, and vesicles by 43 times. A cellular projection elongates toward and establishes an association with the vegetative nucleus; this leading edge contains plastids and numerous mitochondria. When the generative cell completes its separation from the intine, organellar polarity is reversed and plastids migrate to the opposite pole of the cell. Cytoplasmic microtubules are common in association with cellular organelles. Plastids accumulate at the distal end of the cell as a linked mass, apparently adhered by lateral electron dense regions. Before division of the highly polarized generative cell, plastids decrease in number by 16%, whereas mitochondria increase by $90% and vacuoles increase by $140% from the prior stage. After mitosis, the resultant sperm cells differ in size and organelle content. The sperm cell associated with the vegetative nucleus (S vn ) contains 62.7% of the cytoplasm volume, 87% of the mitochondria, 280.4 vesicles (79% of those in the generative cell), and 0.6% of the plastids. At maturity, the S vn mitochondria increase by 31% and the cell contains an average of 0.4 plastids, 158.9 vesicles, and 0.36 microbodies. The mature unassociated sperm (S ua ) contains 39.8 mitochondria (up 3.3%), 24.3 plastids (down 31%), 91.1 vesicles (up 54.9%), and 3.18 microbodies. The small number of organelles initially in the generative cell, followed by their rapid multiplication in a shrinking cytoplasm suggests a highly competitive cytoplasmic environment that would tend to eliminate residual organellar heterogeneity. Cell and cytoplasmic volumes vary as a consequence of fluctuations in the number and size of large vesicles or vacuoles, as well as loss of cytoplasmic volume by (1) formation of ''false cells'' involving amitotic cytokinesis, (2) ''pinching off'' of cytoplasm, and (3) dehydration of pollen contents prior to anthesis.

Proteome profile and functional classification of proteins in Arabidopsis thaliana (Landsberg erecta) mature pollen

“…Pollen proteins identified in this study are likely to be predominent in the vegetative cell, since sperm cells contain few organelles and a small cellular volume, e.g., less than 0.05% of the pollen volume in Plumbago (Russell and Strout 2005), and proportionately fewer transcript copies (Xu et al 1999;Engel et al 2003). Using the Gene Ontology annotation for Arabidopsis (Berardini et al 2004), the relative distribution of identified proteins (Table 1) associated with various cell components is presented in Fig.…”

Vegetative and Sperm Cell-Specific Aquaporins of Arabidopsis Highlight the Vacuolar Equipment of Pollen and Contribute to Plant Reproduction

“…By contrast, little is known about the morphology and role of vacuoles in sperm cells. Their existence has been documented using electron microscopy in a few plant species (Weber, 1988;Yu et al, 1992;Russell and Strout, 2005), and expression of two soluble E subunits of vacuolar H + -ATPase has been reported in the tonoplast of Arabidopsis (Arabidopsis thaliana) sperm cells (Strompen et al, 2005).…”

“…Pollen grains of angiosperms contain a large vegetative cell and one generative cell giving rise to one or two sperm cells (Brewbaker, 1967). Although the sperm shows an occasional absence of plastids, pollen grains harbor most of the typical organelles of plant cells (Van Aelst et al, 1993;Yamamoto et al, 2003;Russell and Strout, 2005;Matsushima et al, 2008;Tang et al, 2009). The maturation process of pollen grains associates multiple steps in the formation and differentiation of vacuoles (vacuolation; for review, see Pacini et al, 2011).…”

Sperm dimorphism in terms of nuclear shape and microtubule accumulation in Cyrtanthus mackenii

“…The difference in nuclear length between Svn and Sua increased with extended culture time from 12 to 24 h. The patterns of dimorphism in sperm cells in the MGUs, in terms of cell shape, have been reported to vary among species: the volume and surface area of Svn are greater than those of Sua in P. zeylanica (Russell 1984) and Torenia fournieri (Chen et al 2006), while the volume of the Sua is greater than that of Svn in N. tabacum (Tian et al 2001). Moreover, in the case of N. tabacum, dimorphism arises late during sperm development (Tian et al 2001), whereas in Plumbago zeylanica, it progressively advanced during development (Russell and Strout 2005). We propose that sperm dimorphism in C. mackenii follows the pattern of Nicotiana, as changes occur in Sua characteristics during sperm maturation.…”

“…4 Generative cells and sperm cells have many structural features that are distinct from somatic cells (Southworth and Cresti 1997). From an early stage, generative cells become spindle shaped and shed cytoplasm (Russell and Strout 2005). The microtubular cytoskeleton of generative cells and sperm cells differs from that of somatic cells: in maturing male germ cells, microtubules are typically arranged in prominent bundles that are aligned helically or longitudinally relative to the long axis of the cell (Palevitz and Tiezzi 1992;Southworth and Cresti 1997).…”