Plant Cell Morphogenesis and Cell Wall Mechanics

Increase in size or change of geometry in plant cells requires the controlled yielding of the cell wall to the internal hydrostatic pressure. We use the model system pollen tube to study the role of the mechanical relationship between turgor and cell wall mechanics during plant cell growth.

Mechanics of tip growth

Pollen tubes are tip growing cells. All growth activity - delivery of new cell wall material and cell wall deformation - occurs at the very tip of the cell (Geitmann 1998, Geitmann and Steer 2006, Geitmann and Dumais 2009). The deformation is driven by the turgor pressure. (Geitmann 2009)

Hydrostatic pressure is a non-vectorial force. Therefore, a mechanical prerequisite for the unidirectional and spatially confined growth of pollen tubes must be a softer cell wall at the tip of the cell. This mechanical gradient is generated by the absence or scarcity of callose and cellulose at the tip (Aouar et al. 2009) as well as by the relatively high degree of esterification of the pectin polymers in this region.

Using micro-indentation, we were able to show that this biochemical gradient results in a gradient in cellular stiffness (Geitmann and Parre 2004, Bolduc et al. 2006, Zerzour et al. 2009) that results mainly from the difference in the configuration of the pectin polymers (Parre and Geitmann 2005a). To visualize the different cell wall components we have optimized microwave-enhanced fixation and labeling protocols for single plant cells (Chebli et al. 2008)

Callose, an amorphous cell wall component plays a role in mechanically stabilizing the distal, cylindrical region of the pollen tube against the tensile stress generated in the cell wall by the turgor pressure (Parre and Geitmann 2005b)

Delivery of cell wall material

STICS (spatio-temporal image correlation spectroscopy) showed that transport of the vesicles towards the tip occcurs in the periphery of the cytoplasm. FRAP (fluorescence recovery after photobleaching) revealed that vesicles streaming into the tip hover in an annulus shaped region that is likely to correspond to the principal region of exocytosis (Bove et al. 2008). This pattern fits the theoretically calculated surface strain patterns in tip growing cells (Geitmann and Dumais 2009). We generated a theoretical model for the vesicular transport that predicts the motion patterns of secretory vesicles in growing pollen tubes (Kroeger et al. 2009)

Growth dynamics

Pollen tube growth is rarely steady but displays oscillatory changes in the growth rate (Geitmann 1998, Chebli and Geitmann 2008) that cause the cell wall to display ring-shaped patterns (Geitmann et al. 1995). We investigate the temporal relationship between cell wall mechanics and dynamics of the growth rate (Zerzour et al. 2009). Interfering with the supply of secretory vesicles abolishes these oscillations (Geitmann et al. 1996). We developed mathematical equations based on cellular parameters to propose a theoretical model for the changes in growth behavior (Kroeger et al. 2008)

Mechanical modeling of growth

To better understand the mechanical process of pollen tube elongation under different experimental conditions, we model cellular growth using Finite Element Modeling. This technique is commonly used in engineering and has enormous potential for applications in biology (Bolduc et al. 2006, Fayant et al. 2010, Geitmann 2010).

References

Geitmann A. 2010. Mechanical modeling and structural analysis of the primary plant cell wall. Current Opinion in Plant Biology 13: 693–699

Fayant P, Girlanda O, Aubin C-E, Villemure I, Geitmann A. 2010. Finite element model of polar growth in pollen tubes. The Plant Cell 22: 2579–2593

Kroeger JH, Bou Daher F, Grant M, Geitmann A. 2009. Microfilament orientation contrains vesicle flow and spatial distribution in growing pollen tubes. Biophysical Journal 97: 1822-1831

Zerzour R, Kroeger JH, Geitmann A. 2009. Polar growth in pollen tubes is associated with spatially confined dynamic changes in cell mechanical properties. Developmental Biology 334: 437-446

Geitmann A. 2009. How to shape a cylinder - Pollen tube as a model system for the generation of complex cellular geometry. Sexual Plant Reproduction 23: 63–71

Aouar L, Chebli Y, Geitmann A. 2009. Morphogenesis of complex plant cell shapes - the mechanical role of crystalline cellulose in growing pollen tubes. Sexual Plant Reproduction 23: 15–27

Geitmann A, Dumais J. 2009. Not-so-tip-growth. Plant Signaling & Behavior 4, in press

Chebli Y, Bou Daher F, Sanyal M, Aouar L, Geitmann A. 2008. Microwave assisted processing of plant cells for optical and electron microscopy. Bulletin of the Microscopical Society of Canada 36(3): 15-19

Bove J, Vaillancourt B, Kroeger J, Hepler P, Wiseman PW, Geitmann A. 2008. Magnitude and direction of vesicle dynamics in growing pollen tubes using spatio-temporal image correlation spectroscopy (STICS). Plant Physiology 147: 1646-1658

Kroeger JH, Geitmann A, Grant M. 2008. Model for calcium dependent oscillatory growth in pollen tubes. Journal of Theoretical Biology 253: 363-374

Chebli Y, Geitmann A. 2007. Mechanical principles governing pollen tube growth. Functional Plant Science and Biotechnology 1: 232-245

Bolduc JF, Lewis L, Aubin CE, Geitmann A. 2006. Finite-element analysis of geometrical factors in micro-indentation of pollen tubes. Biomechanics and Modeling in Mechanobiology 5: 227-236

Geitmann A, Steer M. 2006. The architecture and properties of the pollen tube cell wall. In: Malhó R (ed) The pollen tube. Springer Verlag. Plant Cell Monographs 3: 177-200

Parre E,Geitmann A. 2005a. Pectin and the role of the physical properties of the cell wall in pollen tube growth of Solanum chacoense. Planta 220: 582-592

Parre E, Geitmann A. 2005b. More than a leak sealant - the physical properties of callose in growing plant cells. Plant Physiology 137: 274-286

Geitmann A, Parre E. 2004. The local cytomechanical properties of growing pollen tubes correspond to the axial distribution of structural cellular elements. Sexual Plant Reproduction 17: 9-16

Geitmann A. 1998. The rheological properties of the pollen tube cell wall. in: “Sexual Plant Reproduction and Biotechnological Applications”, eds. Cresti M, Cai G, Moscatelli A. Springer Verlag. pp. 283-302

Geitmann A, Li YQ, Cresti M. 1996. The role of the cytoskeleton and dictyosome activity in the pulsatory growth of Nicotiana tabacum and Petunia hybrida pollen tubes. Botanica Acta, 109: 102-109

Geitmann A, Li YQ, Cresti M. 1995. Ultrastructural immunolocalization of periodic pectin depositions in the cell wall of Nicotiana tabacum pollen tubes. Protoplasma 187: 168-171

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