The location of sucrose synthase in root nodules of white clover



Immunogold labelling was used to identify the location of the sucrose hydrolytic enzyme sucrose synthase in the N2-fixing root nodules of white clover. This is the major enzyme of sucrose cleavage in clover nodules and might be involved in the modulation of N2 fixation by controlling the rate of utilization of photosynthetic products. Knowledge of the precise cellular location of this enzyme in relation to the point of delivery (the vascular bundles) and the site of utilization of the catabolic product (malate in the bacteroids) might aid our understanding of the metabolic communication between different cell types of the nodule. Gold particle density was greatest in the cytosol of uninfected cells of the central region and in the layer of cortical cells in direct contact with infected cells. By contrast, relative gold particle density was 2–3 fold lower in the cytosol of infected cells. The distribution of sucrose synthase is discussed in relation to sucrose transport, starch metabolism and the provision of carbon and energy for N2 fixation.