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, aw; 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 (μmol 4-nitrophenol min−1 g−1 maize) and specific activity (nmol 4-nitrophenol min−1 μg−1 protein) were quantified using chromogenic p-nitrophenyl substrates. The dominant three enzymes produced by the fungi on whole colonised maize kernels were α-d-galactosidase, β-d-glucosidase, and N-acetyl-β-d-glucosaminidase. The other four enzymes were all produced in much lower total amounts and in terms of specific activity (β-d-fucosidase, α-d-mannosidase, β-d-xylosidase and N-acetyl-α-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 α-d-galactosidase, β-d-glucosidase and N-acetyl-β-d-glucosaminidase, were maximum at 0.98 aw with significantly less being produced at 0.95 and 0.93 aw, with the exception of the total activity of α-d-galactosidase which was similar at both 0.95 and 0.93 aw. Single factors (time, aw, and inoculation treatment), two- and three- way interactions were all statistically significant for the three dominant enzymes produced except for specific activity of β-d-glucosidase (two and three-way interactions) and for total activity of α-d-galactosidase in the time ×aw 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 aw range.