Development and cellular organization of Pinus sylvestris pollen tubes

Abstract

The organization of Pinus sylvestris pollen tubes during growth was studied by video microscopy of living cells and by electron microscopy after freeze-fixation and freeze-substitution (FF-FS). Pollen germinated and the tubes grew slowly for a total period of about 7 days. Some of the grains formed two tubes, while 10-50% of the tubes ramified. These features are in accordance with development in vivo. The cytoplasmic hyaline cap at the tip disappeared during the 2nd or 3rd day of culture. Ag gregates of starch grains progressively migrated from the grain into the tube and later into the branches. Vacuoles first appeared at day 2 and eventually filled large parts of the tube. The tube nucleus was located at variable dis tances from the tip. Some of the organelles showed lin ear movements in a mostly circulatory pattern, but the majority of the organelles showed brownian-like move ments. Rhodamine-phalloidin-stained actin filaments had a gross axial orientation and were found throughout the tube including at the tip. The ultrastructure of pollen tubes was well preserved after FF-FS, but signs of shrinkage were visible. The secretory vesicles in grow ing tips were not organized in a vesicle cone, and coated pits had a low density with only local accumulations, which is in accordance with slow growth. The mitochon dria contained small cristae and a darkly stained matrix and were located more towards the periphery of the tube, indicating low respiratory activity and low oxygen lev els. The dictyosomes carried typical trans-Golgi net works, but some contained less than the normal number o f cisternae. Other elements of the cytoplasm were irreg ularly spaced rough endoplasmic reticulum, many multivesicular bodies, lipid droplets and two types of vacu oles. The typical organization associated with tip growth in angiosperm pollen tubes, e.g. Ni.coti

Microtubules and microfilaments are both responsible for pollen tube elongation in the coniferPicea abies (Norway spruce)

“…Pinus pollen tubes are extremely branched both in vivo and in vitro (Singh 1978). This branching occurs by bifurcation of the tip (de Win et al 1996) and is increased by colchicine treatment (Terasaka and Niitsu 1994). The threshold concentrations for significant branching in P. abies were similar to the thresholds for inhibition of tube elongation caused by APM, oryzalin, and propyzamide.…”

“…Myosin inhibition with NEM stops organelle motility in Lilium longiflorum pollen tubes (Kohno and Shimmen 1988), and myosin is detected throughout angiosperm pollen tubes (Tang et al 1989, Miller et al 1995, Yokota et al 1995. In conifers, microfilaments form an axial array in the pollen tube (Terasaka and Niitsu 1994, de Win et al 1996, Lazzaro 1996. In chemically fixed material labeled with rhodamine-phalloidin (Lazzaro 1996) or immunolabeled with actin antibodies (Fig.…”

Effects of Brefeldin A on Pollen Germination and Tube Growth. Antagonistic Effects on Endocytosis and Secretion

“…1, D-F) that was observed in our experiment was possibly caused by the improper delivery to and depletion of secretory vesicles from the clear zone of the pollen tubes. Nevertheless, we did not detect BFA-induced vesicular aggregation, as was found in BFA-treated lily pollen tubes (Parton et al, 2003), probably because of the different type of cytoplasmic streaming found in conifer and angiosperm pollen tubes (De Win et al, 1996;Lazzaro et al, 2003).…”

“…Outside the tip, the mitochondrial density increases toward the periphery, giving rise to rows of mitochondria along the tube wall. Unlike in angiosperms, the Golgi does not show any specific zonation or accumulation in the gymnosperm tube (De Win et al, 1996). In addition, the initiation of germination and the maintenance of pollen tube elongation in gymnosperms depend on continuous protein synthesis (Fernando et al, 2001;Hao et al, 2005).…”

Overexpression of the Tomato Pollen Receptor Kinase LePRK1 Rewires Pollen Tube Growth to a Blebbing Mode

“…Using pollen tubes to deliver sperm cells for fertilization (siphonogamy) is an evolutionary innovation of seed plants (Rounds and Bezanilla, 2013). From the haustorial pollen tubes of cycads and ginkgo (Ginkgo biloba), to the sometimes branched pollen tubes of conifers (deWin et al, 1996;Fernando et al, 2005), to the nonbranched pollen tubes of flowering plants, the shape of the growing pollen tube became more and more restricted to a hemispherical tip capping a cylinder with a uniform diameter (Kroeger and Geitmann, 2012). An alternative method of extending a cellular leading edge, by continuously producing blebs (i.e., rounded protrusions), has been reported for Dictyostelium (Zatulovskiy et al, 2014) and Drosophila melanogaster stem cells (Charras and Paluch, 2008) but not in pollen tubes.…”