Pectate distribution and esterification in Dubautia leaves and soybean nodules, studied with a fluorescent hybridization probe.

Carbohydrate-hybridization probes (Vreeland and Laetsch, 1989, Planta (177, 423-434) were used to localize the homogalacturonan (pectate) component of pectins in the cell walls of leaves and soybean root nodules. Leaves of two species of the dicotyledon Dubautia were compared; these species contain much pectin but differ in their tissue water relations with respect to their cell-wall properties. Maturation of the primary cell walls in nodules was studied in the Bradyrhizobium japonicum-Glycine max symbiosis. Probe labelling was based on the divalent-cation-mediated association between pectate in tissue sections and fluorescein-conjugated pectate fragments. Pectate was also labelled by mixed-dimer formation with fluorescent polyguluronate derived from alginate. The specificity of the probe for unesterified polygalacturonate was indicated by increased cell-wall labelling after chemical or enzymatic deesterification of tissue sections, in contrast to elimination of labelling by chemical esterification. Postfixation of tissue sections improved retention of soluble pectate. Pectate differences were found in the leaves among cell types, in degree of esterification, and between plant species. The cell walls of soybean nodules were strongly labelled by the pectate probe in nodules one week and three weeks after infection. Pectate was more highly esterified in the central infected zone than in the surrouding cortex. Within the infected zone, walls of uninfected cells and infected cells were similarly labelled by the pectate probe. The results indicate that the pectate molecular probe provides detailed information on pectate distribution at the cellular level for investigations of cell-wall structure, development and physiology.