Pectin methylesterase selectively softens the onion epidermal wall yet reduces acid-induced creep

Abstract

De-esterification of homogalacturonan (HG) is thought to stiffen pectin gels and primary cell walls by increasing calcium cross-linking between HG chains. Contrary to this idea, recent studies found that HG de-esterification correlated with reduced stiffness of living tissues, measured by surface indentation. The physical basis of such apparent wall softening is unclear, but possibly involves complex biological responses to HG modification. To assess the direct physical consequences of HG de-esterification on wall mechanics without such complications, we treated isolated onion (Allium cepa) epidermal walls with pectin methylesterase (PME) and assessed wall biomechanics with indentation and tensile tests. In nanoindentation assays, PME action softened the wall (reduced the indentation modulus). In tensile force/extension assays, PME increased plasticity, but not elasticity. These softening effects are attributed, at least in part, to increased electrostatic repulsion and swelling of the wall after PME treatment. Despite softening and swelling upon HG de-esterification, PME treatment alone failed to induce cell wall creep. Instead, acid-induced creep, mediated by endogenous α-expansin, was reduced. We conclude that HG de-esterification physically softens the onion wall, yet reduces expansin-mediated wall extensibility.

Preferred crystallographic orientation of cellulose in plant primary cell walls

“…Thus, at X-ray energies of 10 and 12 keV, critical angles are 0.148°and 0.116°, respectively. Despite the roughness of cell walls, e.g., approximately 10 nm root mean square from AFM images of onion cell wall previously reported 37 , GIWAXS intensities peak at these incidence angles ( Supplementary Fig. 2).…”

Mechanical feedback-loop regulation of morphogenesis in plants

“…Based on previous X-ray diffraction studies performed in vitro, the study proposes that HGs exists as quaternary structures (Walkinshaw and Arnott, 1981). The study also proposes that de-methyl-esterification of HGs causes a change in conformation of its quaternary structure, resulting in expansion of the filamentous structure, as opposed to the electrostatic repulsion-based swelling shown previously (Wang et al, 2020). As pectin is found in close proximity to cellulose, the authors speculate that pectin and cellulose could co-exist in such higher-order structures, which in turn has important implications in regulating the mechanics of the cell wall.…”

“…isolated cell wall fractions has indeed shown that de-methylesterification results in cell wall softening by increasing plastic compliance without influencing the elastic compliance (Boxes 1 and 2) (Wang et al, 2020). As cell wall polymers are heterogeneous polyelectrolytes, this softening effect comes about by the swelling of the wall due to enhanced electrostatic repulsion between pectin carboxylates.…”

Expansin-controlled cell wall stiffness regulates root growth inArabidopsis

“…CWs may become mechanically softer (meaning more easily deformed by mechanical force), but they do not necessarily result in an increase in wall relaxation and growth, e,g, lytic enzymes may soften CW but do not stimulate cell growth. On the other hand, -expansins cause stress relaxation and prolonged enlargement of CWs, but they lack wall lytic activity and they do not soften the wall, as measured by tensile tests (Cosgrove 2018b, Wang andCosgrove, 2020). An example of such observations was made by Wang and Cosgrove (2020) with pectin methylesterase (PME) that selectively softened the onion epidermal wall yet reduced expansinmediated creep.…”

“…On the other hand, -expansins cause stress relaxation and prolonged enlargement of CWs, but they lack wall lytic activity and they do not soften the wall, as measured by tensile tests (Cosgrove 2018b, Wang andCosgrove, 2020). An example of such observations was made by Wang and Cosgrove (2020) with pectin methylesterase (PME) that selectively softened the onion epidermal wall yet reduced expansinmediated creep. After enzymatic de-esterification (without added calcium), the onion epidermal wall swelled and became softer, as assessed by nanoindentation (AFM) and tensile plasticity tests, yet exhibited reduced acid-induced creep.…”