1,3-BETA-GLUCAN SYNTHASE ON THE PERIBACTEROID MEMBRANE (SYMBIOSOME MEMBRANE) FROM SOYBEAN ROOT-NODULES

1,3-β-glucan synthase activity, previously recognized as a plasma membrane marker, was observed on the peribacteroid membrane from root nodules of soybean (Glycine max cv. Maple Arrow) infected by Bradyrhizobium japonicum. Synthesis of the 1,3-β-glucan callose was monitored using intact symbiosomes and peribacteroid membrane suspensions isolated via sucrose or Percoll gradient systems. The specific activity achieved by the two different gradient types is 83·1 mU mg protein-1 and 58·6 mU mg protein-1 respectively, with an optimum at pH 7·3. It is markedly stimulated by Ca2+. The apparent Km for the incorporation of UDP-glucose into the polyglucan is 0·21 mm. Deposition of callose by isolated symbiosomes was detected by its specific fluorescence with sirofluor. The reaction product was determined to be a 1,3-β-glucan by methylation analysis and by glucanase digestion. Thin sections of soybean root nodules revealed that callose deposits only occur in some incompatible interactions between host and microsymbiont suggesting that 1,3-β-glucan synthases are activated when the normal balance of the symbiosis is disturbed. © 1992.