Effect of water activity on hydrolytic enzyme production by Fusarium moniliforme and Fusarium proliferatum during colonisation of maize

The effect of different water availabilities (water activity, a(w); 0.98-0.93) and time (up to 15 days) on the production of seven hydrolytic enzymes by strains of F. moniliforme and F. proliferatum during early colonisation of gamma-irradiated living maize grain were examined in this study. Both the total activity (mu mol 4-nitrophenol min(-1) g(-1) maize) and specific activity (nmol 4-nitrophenol min(-1) mu g(-1) protein) were quantified using chromogenic p-nitrophenyl substrates. The dominant three enzymes produced by the fungi on whole colonised maize kernels were alpha-D-galactosidase, beta-D-glucosidase, and N-acetyl-beta-D-glucosaminidase. The other four enzymes were all produced in much lower total amounts and in terms of specific activity (beta-D-fucosidase, alpha-D-mannosidase, beta-D-xylosidase and N-acetyl-alpha-D-glucosaminidase), similar to that in uncolonised control maize grain. There were significant increases in the total production of the three predominant enzymes between 3-15 days colonisation, and between 3-6 days in terms of specific activity when compared to untreated controls. The total and specific activity of the oc-D-galactosidase, beta-D-glucosidase and N-acetyl-beta-D-glucosaminidase, were maximum at 0.98 a(w) with significantly less being produced at 0.95 and 0.93 a(w), with the exception of the total activity of alpha-D-galactosidase which was similar at both 0.95 and 0.93 a(w). Single factors (time, a(w), and inoculation treatment), two- and three- way interactions were all statistically significant for the three dominant enzymes produced except for specific activity of beta-D-glucosidase (two and three-way interactions) and for total activity of alpha-D-galactosidase in the time x a(w), treatment. This study suggests that these hydrolytic enzymes may play an important role in enabling these important fumonisin-producing Fusarium spp. to rapidly infect living maize grain over a wide a(w) range. (C) 1998 Elsevier Science BN.