Growing straight through walls (original) (raw)

The architecture and properties of the pollen tube cell wall

Pollen Tube: Cellular and Molecular Perspective, 2006

The pollen tube wall differs in both structure and function from walls of vegetative plant cells. Cellulose represents only a small portion of the cell wall polymers, so an organized microfibrillar system has not been identified yet. The initial wall, formed by secretion at the growing tip, is mostly composed of methyl esterified pectins. During cell wall maturation, concomitant with its translocation from apex to shank, these are demethylated by pectin methylesterase to yield carboxyl groups which have the potential to bind calcium ions, adding mechanical strength to the gel. Callose synthase activity is established close to the growing tip, and builds a callose layer beneath the fibrous pectic layer. The mature wall also contains proteins, arabinogalactan proteins and pollen extensin-like proteins. The mature wall is a cylinder that resists turgor expansion, but is stronger at the base than the tip due to the presence of the callose layer and the gelation of pectin polymers in the shank. Permeability of the wall is essential, to allow passage of both ions and sporophytic proteins that determine compatibility in many species. Influx of calcium ions affects the tip cytoplasm, especially the cytoskeleton, and oscillatory changes in these fluxes are involved in the "pulsatile" mode of growth. This process deposits extra wall material during the "slow" growth phase, which generates rings of increased density in the walls that can be readily seen with appropriate antibodies.

Directional Guidance of Nicotiana alata Pollen Tubes in Vitro and on the Stigma

PLANT PHYSIOLOGY, 1998

Pollen tubes navigate the route from stigma to ovule with great accuracy, but the cues that guide them along this route are not known. We reproduced the environment on the stigma of Nicotiana alata by immersing pollen in stigma exudate or oil close to an interface with an aqueous medium. The growth of pollen in this culture system mimicked growth on stigmas: pollen grains hydrated and germinated, and pollen tubes grew toward the aqueous medium. The rate-limiting step in pollen germination was the movement of water through the surrounding exudate or oil. By elimination of other potential guidance cues, we conclude that the directional supply of water probably determined the axis of polarity of pollen tubes and resulted in growth toward the interface. We propose that a gradient of water in exudate is a guidance cue for pollen tubes on the stigma and that the composition of the exudate must be such that it is permeable enough for pollen hydration to occur but not so permeable that the supply of water becomes nondirectional. Pollen tube penetration of the stigma may be the most frequently occurring hydrotropic response of higher plants.

The pollen tube: a soft shell with a hard core

The Plant Journal, 2013

Plant cell expansion is controlled by a fine-tuned balance between intracellular turgor pressure, cell wall loosening and cell wall biosynthesis. To understand these processes, it is important to gain in-depth knowledge of cell wall mechanics. Pollen tubes are tip-growing cells that provide an ideal system to study mechanical properties at the single cell level. With the available approaches it was not easy to measure important mechanical parameters of pollen tubes, such as the elasticity of the cell wall. We used a cellular force microscope (CFM) to measure the apparent stiffness of lily pollen tubes. In combination with a mechanical model based on the finite element method (FEM), this allowed us to calculate turgor pressure and cell wall elasticity, which we found to be around 0.3 MPa and 20-90 MPa, respectively. Furthermore, and in contrast to previous reports, we showed that the difference in stiffness between the pollen tube tip and the shank can be explained solely by the geometry of the pollen tube. CFM, in combination with an FEM-based model, provides a powerful method to evaluate important mechanical parameters of single, growing cells. Our findings indicate that the cell wall of growing pollen tubes has mechanical properties similar to rubber. This suggests that a fully turgid pollen tube is a relatively stiff, yet flexible cell that can react very quickly to obstacles or attractants by adjusting the direction of growth on its way through the female transmitting tissue.

Cytoskeletal organization and pollen tube growth

Trends in Plant Science, 1997

The growth of pollen tubes is characterized by intense secretory activity in the tip region. This process of vesicle-mediated secretion and tip growth is strongly influenced by calcium gradients. The cytoskeletal apparatus is also critically involved, as it is required for the translocation of organelles along the tube (a prerequisite for tube extension) and for the transport of the generative/sperm cells. The microtubules and actin filaments probably have distinct functions that relate to different, but related, cytological events within the pollen tubes. Both systems, as well as cytoskeleton-based motor proteins, are necessary for the proper development and growth of the pollen tubes. Different approaches have allowed the roles of several cytoskeletal components to be deciphered, and it is now possible to speculate how they might interact.