Poster: Cell Walls

Abs # 1250: Acid-induced creep and cell wall ultrastructure

Presenter:	Marga, Francoise S, margaf@missouri.edu
Authors	Marga, Francoise S (A)   Grandbois, Michel  (A)   Cosgrove, Dan J (B)   Baskin, Tobias I (A)
Affiliations:	(A): University of Missouri - Columbia (B): Pennsylvania State University

The primary cell wall can extend considerably when stressed, even in dead cells. This extension involves expansin, requires acidic pH and correlates with the growth behavior of living cells; however, little is known about the spatial dynamics of polymer movements within cell wall during expansion. To find out, we have taken an ultrastructural approach. Cucumber hypocotyl segments were frozen, thawed, bisected and either allowed to extend up to 30% of their length (pH 4.5) or not (pH 6.8). Subsequently, the innermost layer of the cell wall was imaged by either field emission scanning electron microscopy (FESEM) or atomic force microscopy (AFM). Samples for AFM remained hydrated. Cell walls in FESEM appear more organized than in AFM, which suggests that processing (fixation, dehydration, critical-point drying) partially rearranges the polymers. However, in both FESEM and AFM, samples from extended and not-extended cell walls appeared indistinguishable by eye. To quantify the order within the cell wall, we analyzed the fast Fourier transform (FFT) of the image. The shape of the FFT was characterized as a function of spatial frequency by thresholding the FFT at successive gray levels and fitting an ellipse to each thresholded FFT. For spatial frequencies larger than 5 nm, the ellipse became more eccentric for non-extended samples when imaged in FESEM, indicating that the extension degraded the parallel order among cell wall components. However at all spatial frequencies, the transforms of the AFM images of non-extended cell walls were nearly the same as those of the extended samples, suggesting that the difference seen in FESEM was induced during sample preparation. We conclude the overall structure of the cell wall remains surprisingly constant during extension in vitro.