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.
Highlights
This study attempts to resolve some perplexing and apparently contradictory results concerning the influence of pectin de-esterification on the mechanics and extensibility of growing cell walls.Pectins are acidic polysaccharides that constitute a large proportion of the primary cell wall of many plant species and are dominant in cell walls of some Charophycean algae (Domozych et al, 2014) and the tips of pollen tubes (Chebli et al, 2012)
Wall strips were incubated for 2 h in 20 mM HEPES buffer, pH 7.5, ± 50 μg/mL pectin methylesterase (PME), which hydrolyzes methyl esters of HG
We conclude that PME may selectively soften the epidermal wall under calcium-limited conditions, but we found no evidence for wall loosening by PME
Summary
This study attempts to resolve some perplexing and apparently contradictory results concerning the influence of pectin de-esterification on the mechanics and extensibility of growing cell walls. Pectins are acidic polysaccharides that constitute a large proportion of the primary cell wall of many plant species and are dominant in cell walls of some Charophycean algae (Domozych et al, 2014) and the tips of pollen tubes (Chebli et al, 2012). Homogalacturonan (HG) constitutes the most abundant pectic component of the primary wall (Atmodjo et al, 2013). It is synthesized in methyl esterified form in the Golgi system, deposited to the cell wall, and subsequently de-esterified in muro by pectin methylesterases (PMEs) (Wolf et al, 2009). In the special case of pollen tubes, regions of low HG esterification on the flanks of the tip coincide with cell wall regions of high stiffness and reduced surface enlargement, potentially connected with calcium crosslinking of HG (Chebli et al, 2012; Zerzour et al, 2009)
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