Generation of β-glucan elicitors by plant enzymes and inhibition of the enzymes by a fungal protein

Abstract

The biosynthesis and accumulation of phytoalexins is a well-studied plant defense response. Plants synthesize and accumulate phytoalexins in response to microbial infection or elicitor treatment. The β-glucan heptaglucoside is a well-studied phytoalexin elicitor isolated from partial acid hydrolysates of Phytophthora sojae f.sp. glycines (Psg) mycelial walls. Using the soybean – Psg system, we have demonstrated that endo-1,3-β-glucanases (EC 3.2.1.39) are the principal soybean enzymes involved in generating phytoalexin oligoglucoside elicitors from mycelial walls. We have also recently observed that Psg secretes a protein that inhibits the soybean endo-1,3-β-glucanase activity that could release elicitors from fungal mycelial walls. This inhibitor protein, which has been purified to homogeneity, does not inhibit endo-1,3-β-glucanases of the fungus or a tobacco pathogenesis-related endo-1,3-β-glucanase. The existence of the inhibitor protein in Psg suggests that pathogens have evolved specific proteins to inhibit the fungal wall-degrading enzymes of their host plants, just as plants have evolved proteins (e.g., pectic enzyme inhibitors) to inhibit plant cell wall degrading enzymes secreted by their pathogens. It seems possible that pathogens secrete inhibitors of other pathogenesis-related proteins (e.g., chitinases) and that the interplay of hydrolases and their inhibitors could determine the outcome of plant – pathogen interactions. Key words: oligosaccharin, phytoalexin, endo-1,3-β-glucanase, glucanase inhibitor protein, elicitor.