Inhibition of Rhizopus stolonifer Growth and Pectin Degrading Enzymes by Tannic Acid with an in vitro, in vivo, and in silico Assessment

Abstract

Tannic acid has attracted considerable attention as a natural inhibitor of fungi. The antifungal activity of tannic acid was evaluated by assessing fungal growth and the activities of polygalacturonase and pectin methylesterase. Fungal growth was stimulated at 200 μg mL⁻¹ (112.6%), but significantly reduced at higher concentrations, reaching 12.4% at 1000 μg mL⁻¹ (87.6% inhibition) of tannic acid. In vivo, tannic acid effectively controlled postharvest soft rot in muskmelon, achieving up to 81.4% disease control. Enzyme activities in the culture medium declined markedly, reduced to 15.3% for polygalacturonase and 20.1% for pectin methylesterase. However, it should be noted that this reduction may result from either direct inhibition of enzyme catalytic activity or reduced enzyme secretion due to suppressed fungal growth, or a combination of both. Molecular docking was performed using heterologous protein structures (endopolygalacturonase from Fusarium verticillioides, PDB: 1HG8; pectin methylesterase from Daucus carota, PDB: 1GQ8) as representative models. Because these proteins are not from R. stolonifer, the docking results have no direct biological relevance to the target pathogen and cannot be used to infer binding to R. stolonifer enzymes. Thus, the experimental data demonstrate that tannic acid reduces extracellular pectin-degrading enzyme activity in fungal cultures, but the mechanism remains unresolved.