Pectin immunolocalization and calcium visualization in differentiating derivatives from poplar cambium

Calcium distribution and pectin esterification patterns in the cambial zone of poplar branches were studied with ionic microscopy and immunological tools respectively. Dynamic changes cor relating with cell growth and cell differentiation were observed both on the xylem and on the phloem sides. In expanding cell walls of xylem derivatives, unesterified pectins were restricted to cell junctions and middle lamellae, occasionally accompanied by calcium ions. In contrast, in differentiating and mature phloem cells, acidic pectins and Ca2+ were present all over the walls leading to early stiffening of the polysaccharide network. Significant labelling was detected with JIM5 antibodies in some dictyosomes suggesting exocytosis of low methylated polymers towards the cell walls. At cell junctions, unesterified pectins might originate from the activity of pectinmethylesterases localized in these areas. Thus un-and deesterified pectins might be located in different cell wall domains whose distribution, varying with cell type, will confer specific extensibility to the wall matrix.