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Intercellular adhesion in some parenchyma becomes strengthened in response to stress. The present study provides an approach to investigate this phenomenon (usually attributed to pectin methyl esterase and binding of Ca(2+) and/or rhamnogalacturonan-II-borate) through reliable stress simulation by probing organic acid molecules in potato tuber parenchyma. Short-chain monocarboxylic acids induce consistent intercellular adhesion strengthening (3.8-5.3 newton) at pH >or= 3 < pK(a), where pectin methyl esterase activity and Ca(2+) or borate binding are limited, and vice versa at pH > pK(a) with a strength of 1.4-2.0 newton as compared to 0.3-0.4 newton for the nonincubated control. Strengthening of intercellular adhesion is characterized by prominent staining of pectin and protein and immunogold labeling of pectin in the cell wall and the middle lamellar complex, particularly after boiling. Pectin confers strengthening to the primary cell wall, as reflected by: (i) prominent immunogold labeling following boiling; and (ii) puncturing macerated cells by starch gelatinization pressure after enzymatic pectin removal.

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