| Various biochemical factors were studied in the interaction between soybean and P. sojae. These are: (1) the toxicity of various flavonoids and isoflavonoids against P. sojae, (2) the metabolism of these compounds by P. sojae, (3) the role of an enzyme, catalase, in P. sojae pathogenicity or virulence and (4) the role of various pectolytic enzymes produced by P. sojae in soybean tissue maceration.;Fungal growth measurements and microscopic observations were used to determine the effects of twelve different flavonoids and isoflavonoids against three P. sojae isolates. This study showed that flavonoids and isoflavonoids have various effects on P. sojae, ranging from direct toxicity to growth inhibition to more subtle effects on hyphal morphology and development of sporangia and oogonium. A major finding was that the soybean metabolite, genistein, is highly toxic to P. sojae and may play a role in antibiotic containment of P. sojae. High performance liquid chromatography (HPLC) profiles showed that many of these compounds were partially or completely metabolized by P. sojae races. P. sojae rapidly hydrolized all flavonoid glycosides and various non-glycosylated flavonoids to non-aromatic compounds. Isoflavonoid glycosides were partially metabolized to their aglycones but very little metabolism occurred after that. Aglycones of those highly inhibitory compounds were not degraded. The metabolism of the various compounds occurred at the early stages of growth and appear to be associated with the hyphal tips.;Another biochemical factor considered was the production of catalase, a peroxide consuming enzyme, by different P. sojae isolates. We found that low levels of catalase were constitutively present in all P. sojae isolates examined. Low levels of activity were also detected on induction experiments. These observations make us believe that catalase does not play a role in P. sojae pathogenicity or virulence under the conditions of the experiments.;The role of four pectolytic enzymes (pectate lyase (PL), pectin lyase (PNL), polygalacturonase (PG) and pectin methyl esterase (PME)) in soybean tissue maceration was examined. Of all the enzymes tested, PNL seemed to play a major role in pectic fraction degradation. It is expressed in a non race specific manner, suggesting a role in pathogenicity but not in virulence. PL, PME and PG did not show significant activity under any of the conditions studied. |
