On Pectin Methyl-esterification: Implications for In vitro and In vivo Viscoelasticity

Abstract

Pectin is a major component of the primary plant cell wall and is important for cell expansion. However, the relationship between its chemistry and mechanical properties is not fully understood, especially in vivo. In this study, a protocol for viscoelastic micro-indentation using atomic force microscopy (AFM) was developed and applied to pectin in vitro and in vivo. After determining that linear viscoelasticity was a suitable theoretical framework for in vitro pectin analyses were conducted with both a standard linear solid and fractional Zener model. These indicated a strong coupling between elastic and viscous properties over a range of degrees of methyl-esterification (DM). Both elasticity and viscosity were found to vary non-linearly with DM which had interesting consequences for pectin gels of mixed DM. In Arabidopsis cell walls, the standard linear solid model was found to be appropriate. In this in vivo composite material a weaker elastic-viscous coupling was exhibited, correlated with DM. The viscoelastic testing in vivo of rapidly elongating cell walls, rich in high DM pectin, displayed a longer viscous time-scale. The implications of the testing method and results are discussed in the context of mechanobiology, mechano-chemistry, and cell growth.