CYCLIC BETA-1,6-1,3-GLUCANS OF BRADYRHIZOBIUM-JAPONICUM USDA-110 ELICIT ISOFLAVONOID PRODUCTION IN THE SOYBEAN (GLYCINE-MAX) HOST

High levels of cyclic beta-1,6-1,3-glucans (e.g. 0.1 mg mg(-1) of total protein) are synthesized by free-living cells as well as by bacteroids of Bradyrhizobium japonicum USDA 110 (K.J. Miller, R.S. Gore, R. Johnson, AJ. Benesi, V.N. Reinhold [1990] J Bacteriol 172: 136-142; R.S. Core and KJ. Miller [1993] Plant Physiol 102: 191-194). These molecules share structural features with glucan fragments isolated from the mycelial cell wall of the soybean (Glycine max) pathogen Phytophthora megasperma. These latter glucans have been shown to be potent elicitors (at nanogram levels) of the phytoalexin glyceollin in G. max. Using the well-characterized soybean cotyledon bioassay, we now show that the cyclic beta-1,6-1,3-glucans of B. japonicum USDA 110 are also biologically active elicitors of glyceollin production (but at microgram levels). We further show that both classes of beta-glucans elicit the production of the isoflavone daidzein within soybean cotyledon wound droplets.