Wall architecture with high porosity is established at the tip and maintained in growing pollen tubes of Nicotiana tabacum

Abstract

SUMMARYA major question in pollen tube growth in planta remains: do the pollen tube walls form a barrier to interaction with the environment? Using cryo-FESEM, we directly assessed the 3D construction and porosity of tobacco pollen tube walls. Fractured mature primary walls showed a 40-50 nm spaced lattice of continuous fibers interconnected by short rods in the primary wall. These observations agree with TEM observations of sectioned walls. In the secondary callose wall, for which no structure is visible using TEM, cryo-FESEM also revealed a 50 nm lattice consisting of longer fibers, approximately 10-15 nm wide, with rod-like, thinner interconnections at angles of approximately 90°with the longer fibers. Such architecture may reflect functional needs with respect to porosity and mechanical strength. The wall does not form a mechanical barrier to interaction with the environment and is gained at low cost. Cryo-FESEM additionally revealed another special feature of the wall: the tubes were tiled with scales or rings that were highly conspicuous after pectin extraction with EDTA. These rings cause the typical banding patterns of pectin that are commonly seen in pollen tubes during oscillatory growth, as confirmed by staining with toluidine blue as well as by DIC microscopy. Growth analysis by VEC-LM showed that the ring-or scale-like structures of the primary wall consist of material deposited prior to the growth pulses. The alternating band pattern seen in the callose wall is probably imposed by constrictions resulting from the rings of the primary wall.

The Cell Wall of the Arabidopsis Pollen Tube—Spatial Distribution, Recycling, and Network Formation of Polysaccharides

“…Microindentation combined with enzymatic treatments have shown that in the cylindrical distal part of the cell, callose plays a role of reinforcement against compression and tension stresses (Parre and Geitmann, 2005b). The spatial profile of our callose label is consistent with previous observations in other species (Ferguson et al, 1998;Geitmann and Parre, 2004;Parre and Geitmann, 2005b;Fayant et al, 2010;Derksen et al, 2011). Callose deposition in Arabidopsis only began after the first 10 mm and reached a plateau in the distal region.…”

“…This layer was formed between the pectic fibrillar layer and the plasma membrane, and appeared electron transparent (Fig. 2I), similar to observations made in tobacco pollen tubes (Derksen et al, 2011). In pollen grains, the callose label was only found in the intine of the grain (Fig.…”

“…1, B, B', C, and C'). Low esterified pectins showed a somewhat irregularly patterned deposition, a phenomenon that has been linked to changes in the growth rate (Li et al, 1994(Li et al, , 1996Derksen et al, 2011). These irregularities are not reflected in the graph (Fig.…”

“…3C). Temporal changes in growth rate are associated with modifications in cell wall thickness at the pole (Li et al, 1996;McKenna et al, 2009;Derksen et al, 2011).…”

“…The biochemical composition of the pollen tube cell wall has been well characterized in many species such as Lilium longiflorum (Lancelle and Hepler, 1992;Jauh and Lord, 1996), tobacco (Nicotiana tabacum; Kroh and Knuiman, 1982;Geitmann et al, 1995;Ferguson et al, 1998;Derksen et al, 2011), Petunia hybrida (Derksen et al, 1999), Pinus sylvestris (Derksen et al, 1999), and Solanum chacoense (Parre and Geitmann, 2005a). But for Arabidopsis (Arabidopsis thaliana), the model for plant molecular biology studies (Arabidopsis Genome Initiative, 2000), there is a striking lack of quantitative information concerning the composition of the pollen tube cell wall as well as the spatial distribution of its components.…”