2 Oct 2010

Modeling Polar Growth of Plant Cell Walls

Nancy A. Eckardt

Senior Features Editor

This e-mail address is being protected from spambots. You need JavaScript enabled to view it

Finite element analysis is a numerical technique for modeling the behavior and mechanics of physical systems, especially complicated systems that change over time, such as load-bearing structures (it is used extensively in civil engineering and aeronautics), but also fluid flow, weather patterns, and a variety of other phenomena. The method involves finding approximate solutions to complex problems, which allows for modeling the behavior of structures with complicated geometry and material properties. Given this description, it is no surprise that finite element analysis might be a good approach for modeling the behavior of plant cell growth. Fayant et al. (pages2579–2593) explore the use of finite element analysis to model the polar growth of pollen tubes. The authors argue that understanding the biomechanical underpinnings of cellular growth will help to focus attention on key biochemical and molecular pathwaysthat govern this process. The resulting model shows how the distribution of mechanical properties at the pollen tube apex controls its shape and growth and further predicts that the biochemical properties of pectin play a key role in determining cell shape.

The authors constructed the model by subdividing the pollen tube tip region into a number of ring-shaped domains, such thatdifferent mechanical properties could be assigned to the different domains (see figure). Mechanical properties known to be important in the growing tip region include pectin distribution and orientation of cellulose microfibrils. Consideration of these properties was used to define Young's modulus, which is a measure of the stiffness of an elastic material. The model also took into account turgor pressure, geometry, and defined boundary conditions. A key objective was to build a model capable of representing perfectly self-similar growth because tip-growing cells have been shown to grow in a self-similar manner when undisturbed (meaning that the shape of the apical region of the cell stays largely constant over time). To assess the quality of simulations, the authors developed two quantitative validation methods to determine how closely the model approximated the behavior of real cells. In addition, model predictions were compared with experimental data on the spatial distribution of major cell wall components, obtained by labeling lily pollen tubes with fluorescent markers for cellulose, callose, and pectins in an in vitro system.

 

View larger version(30K): [in this window] [in a new window] [as a PowerPoint slide] Finite element structure of a tip-growing cell. Initial structure (A) and structure after 50 load cycles (B) with self-similar elongation. The apical region of the cell wall was divided into six ring-shaped surface domains defined by the angles 1 through 6 originating from the center of the prolate spheroid (C). [Adapted from Figure 2 of Fayant et al. [2010].)

A strong correlation was found between model predictions and the spatial distribution of the pectin label, and this was furthertested experimentally by treating lily pollen tubes with pectinase to remove pectin. The resulting swellings observed in the apical region of such pollen tubes were in close agreement with model simulations in which mechanical properties described by Young's modulus were similarly altered. The model further suggested that a dramatic increase in cell wall stiffness at the base of the apical dome is essential for tip growth, which was consistent with experimental data showing enhanced pectin deesterification at this precise location. Thus, the model predicted the importance of pectin and its biochemical conformation for determining the shape of the elongating pollen tube, which was borne out by experimental observation.

The work of Fayant et al. provides an excellent example of the utility of finite element modeling and its application to understanding complex cellular morphogenesis in walled cells. Although the model was validated only for pollen tubes in this work, it could be easily adapted for other types of tip-growing walled cells, such as root hairs and fungal hyphae.

Footnotes

www.plantcell.org/cgi/doi/10.1105/tpc.110.220811

REFERENCES

Fayant, P., Girlanda, O., Chebli, Y., Aubin, C.-E., Villemure, I., and Geitmann, A. (2010). Finite element model of polar growth in pollen tubes. Plant Cell 22: 2579–2593.[Abstract/Free Full Text]