Cell‐cell adhesion in fresh sugar‐beet root parenchyma requires both pectin esters and calcium cross‐links

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Abstract
Multicellular plants depend for their integrity on effective adhesion between their component cells. This adhesion depends upon various cross‐links; ionic, covalent or weak interactions between the macromolecules of the adjacent cell walls. In sugar‐beet (Beta vulgarisL. Aztec) root parenchyma, cell‐cell adhesion is disrupted by successive extractions with a calcium‐chelating agent (imidazole) and a de‐esterifying agent (sodium carbonate) but not by the calcium‐chelating agent or the de‐esterifying agent alone. Cell‐cell adhesion in sugar‐beet parenchyma thus depends upon both ester and Ca2+cross‐linked polymers. Pectic polysaccharides are removed by these treatments. Both parallel‐electron energy‐loss spectroscopy (PEELS) and Image‐EELS show that calcium‐binding sites are removed from the wall by imidazole. Using a monoclonal antibody that recognizes a relatively unesterified epitope of homogalacturonan, JIM 5, we show that a subset of JIM 5‐reactive antigens remain in the middle lamella after Ca2+chelation and that this subset is removed by cold (4° C) Na2CO3‐induced breakage of ester bonds. Fourier transform infrared, nuclear magnetic resonance, and spectrophotometric assays show that methyl and feruloyl esters are removed from the wall by Na2CO3but acetyl esters remain. Sodium carbonate extraction at 20° C removes cell wall autofluorescence and most of the feruloylated moieties from the wall. We propose that the chelator‐resistant subset of ester‐linked JIM 5‐reactive pectins are important for cell‐cell adhesion.