Stress metabolite accumulation, bacterial growth and bacterial immobilization during host and nonhost responses of soybean to bacteria

Hypersensitive response inducing (HR-inducing) strains of the heterologous (non-soybean) pathogens Pseudomonas syringae pv. phaseolicola and P. syringae pv. syringae attained populations in soybean leaves only three-fold to five-fold less than a compatible race of the soybean pathogen P. syringae pv. glycinea. Incompatible (HR-inducing) races of P. syringae pv. glycinea and a non-HR-inducing strain of the heterologous pathogen Corynebacterium flaccumfaciens pv. flaccumfaciens showed more restricted growth. A non-HR-inducing strain of the heterologous pathogen Erwinia carotovora subsp. atroseptica and a strain of the saprophyte Bacillus cereus did not grow in soybean leaves. Incompatible races of P. syringae pv. glycinea and the heterologous pseudomonads induced accumulation of isoflavonoids (daicizein, formononetin, genistein, glyceollin) and isoflavone glucosides (daidzin, genistin, ononin). Bacillus cereus induced accumulation of isoflavone glucosides alone, and C. flaccumfaciens pv. flaccumfaciens and E. carotovora subsp. astroseplica did not induce accumulation of either type of stress metabolite. Of the purified compounds tested, glyceollin alone had significant antibacterial activity, but only against Cram-positive bacteria. By use of in vitro bioassays, no evidence was obtained to indicate that induction of additional inhibitory compounds or inhibitory activity due to total isoflavonoid content of inoculated leaf tissue was responsible for resistance.

Immobilization of bacteria by highly electron-dense material in intercellular spaces of soybean leaves occurred to the greatest extent with E. carotovora subsp. atroseptica and B. cereus, and to a lesser extent with C. flaccumfaciens pv. flaccumfaciens. No bacterial immobilization was evident at 48 h after inoculation with incompatible races of P. syringae pv. glycinea or heterologous pseudomonads.