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Abstract:
The best characterized elicitors of the oomycete Phytophthora sojae for activating a multicomponent defense response, including the production of phytoalexins, in soybean (Glycine max L.) are the branched 1,3- and 1,6-linked beta-glucans that are structural polysaccharides of the hyphal walls of the pathogen. The soybean microsymbiont Bradyrhizobium japonicum synthesizes cyclic 1,3-1,6-beta-glucans that suppressed the fungal beta-glucan-induced phytoalexin response in soybean, indicating a novel mechanism by which the symbiotic bacteria might avoid a defense response that is normally activated in pathogenic interactions. A putative receptor (M-r = 75 kDa) for the P. sojae beta-glucans was isolated from soybean membranes and partially characterized. Partial amino acid sequences were used to raise an anti-peptide antiserum and to generate two oligonucleotides which served as primers for PCR using soybean cDNA as template. Northern blot experiments with a PCR product indicated that the glucan-binding protein mRNA has a size of about 2.4 kb. Since several species of the plant family Fabaceae were shown to possess related high-affinity beta-glucan-binding sites, glucan-based perception mechanisms may be a more common feature of this plant family. Following plasma membrane binding of the beta-glucan elicitor, the subsequent signal transduction might involve a rapid, transient increase in the cytosolic Ca2+ level and in permeability changes of the plasma membrane to Ca2+, H+, and Cl-. Production of the isoflavonoid phytoalexins, glyceollins, in soybean involves a multi-step biosynthetic pathway. Among the pathway enzymes are several cytochrome P450s. A gene family-specific differential display approach was used in the identification of several elicitor-inducible P450 cDNAs.