Biophysical consequences of remodeling the neutral side chains of rhamnogalacturonan�I in tubers of transgenic potatoes

Abstract

Two lines of transgenic potato (Solanum tuberosum L.) plants modified in their cell wall structure were characterized and compared to wild type with regard to biomechanical properties in order to assign functional roles to the particular cell wall polysaccharides that were targeted by the genetic changes. The targeted polymer was rhamnogalacturonan I (RG-I), a complex pectic polysaccharide comprised of mainly neutral oligosaccharide side chains attached to a backbone of alternating rhamnosyl and galacturonosyl units. Tuber rhamnogalacturonan I molecules from the two transformed lines are reduced in linear galactans and branched arabinans, respectively. The transformed tuber tissues were found to be more brittle when subjected to uniaxial compression and the side-chain truncation was found to be correlated with the physical properties of the tissue. Interpretation of the force-deflection curves was aided by a mathematical model that describes the contribution of the cellulose microfibrils, and the results lead to the proposition that the pectic matrix plays a role in transmitting stresses to the load-bearing cellulose microfibrils and that even small changes to the rheological properties of the matrix have consequences for the biophysical properties of the wall.

Mechanical Properties of Plant Cell Walls Probed by Relaxation Spectra

“…We previously inferred that in T13.1, a slowly relaxing component had been lost (Ulvskov et al, 2005). Our relaxation spectra here confirm this, indicating that in T13.1, a component with a t value of around 1,000 s is greatly reduced.…”

“…Eliminating side chains would then increase these associations and stiffen the wall, as indicated for arabinan side chains by Jones et al (2003) for stomatal guard cell walls, and as our previous experiments suggested (Ulvskov et al, 2005). Wild-type component(s) with t values approximately 1,000 s would, in T13.1, have become immobile enough that their relaxation lies beyond the upper t limit of our spectrum's reliable range.…”

“…We suggested earlier (Ulvskov et al, 2005) that a difference in wall water status between the wild type and the transformant might contribute to the observed wall mechanical effects. Evered et al (2007) demonstrated that hydration state influences primary wall mechanical properties.…”

“…Compared with the wild type, the transformants are reduced either in RG-I b-1,4-galactans (line T13.1; Sørensen et al, 2000) or in RG-I a-1,5-arabinans (line T7.2; Skjøt et al, 2002). Previous investigation (Ulvskov et al, 2005) by more conventional methods suggested that T13.1 cell walls differ mechanically from those of the wild type. Potato tubers afford ample amounts of relatively homogenous tissue suitable for frequency sweep measurements in standard rheometers used in the food industry.…”

Arabidopsis thaliana RGXT1 and RGXT2 Encode Golgi-Localized (1,3)-α-d-Xylosyltransferases Involved in the Synthesis of Pectic Rhamnogalacturonan-II

“…However, the recent finding by Zykwinska et al (2005) that both galactan and to a lesser extent arabinan side chains of RG-I bind specifically to cellulose lead the authors to propose a CW model in which the cellulose microfibrils are tethered by parallel networks of hemicellulose and pectin. The significance of RG-I side chains to physical wall properties have been shown by Jones et al (2003) and Ulvskov et al (2005). The structural significance of RG-II to CW integrity and cell-cell adhesion is well established, and the measurements of tensile strength of wild-type and mutant CWs by Ryden et al (2003) showed that both xyloglucan cross-linking and RG-II borate complex formation are essential for wall strength.…”