Immunocytochemistry of pectins in shoot apical meristems: consequences for intercellular adhesion

The nature of pectins ( acidic, methyl-, or acetyl-esterified) in the shoot meristem of Sinapis alba was assessed by immunocytochemistry with the 2F4 monoclonal antibody in light and electron microscopy. This antibody is specific for egg-boxes'' - the polygalacturonic acid conformation induced by calcium as described in Liners et al. ( Plant Physiol. 99: 1099 - 1104, 1992). Hardly any acidic pectin was detected in meristem walls; the pectins were largely methyl- esterified and esterified by acetyl groups and/or other esters. After in situ chemical or enzymatic de-esterification, labeling was distributed over the primary wall and the middle lamella of meristematic cells. Acidic pectin and Ca(2+)-cross-linked homo-galacturonans were absent from the pit fields, where plasmodesmata traverse the middle lamella. The type and distribution of pectins are discussed in relation to cellular adhesion between active meristem cells.