title: Heterogeneous distribution of pectin epitopes and calcium in different pit types of four angiosperm species.
abstract: Intervessel pits act as safety valves that prevent the spread of xylem embolism. Pectin-calcium crosslinks within the pit membrane have been proposed to affect xylem vulnerability to cavitation. However, as the chemical composition of pit membranes is poorly understood, this hypothesis has not been verified. Using electron microscopy, immunolabeling, an antimonate precipitation technique, and ruthenium red staining, we studied the distribution of selected polysaccharides and calcium in the pit membranes of four angiosperm tree species. We tested whether shifts in xylem vulnerability resulting from perfusion of stems with a calcium chelating agent corresponded with the distribution of pectic homogalacturonans (HG) and/or calcium within interconduit pit membranes. No HG were detected in the main part of intervessel pit membranes, but were consistently found in the marginal membrane region known as the annulus. Calcium colocalized with HG in the annulus. In contrast to intervessel pits, the membrane of vessel-ray pits showed a high pectin content. The presence of two distinct chemical domains, the annulus and the actual pit membrane, can have substantial implications for pit membrane functioning. We propose that the annulus could affect the observed shift in xylem vulnerability after calcium removal by allowing increased pit membrane deflection.
authors.affiliation: Department of Renewable Resources, University of Alberta, 442 Earth Sciences Building, Edmonton, AB, Canada, T6G 2E3.
authors.forename: Lenka
authors.initials: L
authors.lastname: Plavcová
authors.affiliation: Department of Renewable Resources, University of Alberta, 442 Earth Sciences Building, Edmonton, AB, Canada, T6G 2E3.
authors.forename: Uwe G
authors.initials: UG
authors.lastname: Hacke
mesh_terms: D000918:Antibody Specificity / Q000276:immunology; D002118:Calcium / Q000378:metabolism*; D000939:Epitopes / Q000276:immunology*; D004951:Esterification; D005936:Glucans / Q000276:immunology; D019684:Magnoliopsida / Q000276:immunology* / Q000378:metabolism / Q000648:ultrastructure; D008745:Methylation; D010368:Pectins / Q000276:immunology*; D012430:Ruthenium Red / Q000378:metabolism; D013045:Species Specificity; D013194:Staining and Labeling; D014990:Xylans / Q000276:immunology; D052584:Xylem / Q000276:immunology* / Q000378:metabolism / Q000648:ultrastructure
chemical_list: D000939:Epitopes; D005936:Glucans; D014990:Xylans; C042491:rhamnogalacturonan I; D012430:Ruthenium Red; C029353:xyloglucan; D010368:Pectins; D002118:Calcium; C003181:polygalacturonic acid