Shomer, I., Agricultural Research Organization, The Volcani Center, P.O. Box 6, Bet Dagan 50250, Israel
Kaaber, L., Norwegian Food Research Institute, MATFOSK, Osloveien 1, N-1430 Ås, Norway

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 Ca2+ 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 ≥ 3 < pKa, where pectin methyl esterase activity and Ca2+ or borate binding are limited, and vice versa at pH > pKa 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. © 2006 American Chemical Society.