Inhibitory effects of South African plants against mycotoxigenic phytopathogens

Abstract

Mycotoxigenic fungal strains pose a significant threat to food crops consumed by humans and animals, leading to reduced yields and compromised food quality as reported in almost 25% of maize products worldwide. The harmful effects following the ingestion of mycotoxin-contaminated food include inducing cancers, mutagenicity, immune suppression, and toxicities to target organs of the digestive, cardiovascular, and central nervous systems. Synthetic fungicides are associated with a high potential for toxic residues in food and the development of fungal resistance. Genetically modified crops can also reduce crop contamination. However, these strategies are inaccessible and expensive resources for most subsistence farmers. To address these issues, the use of plants as potential biopesticides to counter fungal infections on crops emerges as a possible solution to enhance food safety and security. This study aimed to determine the antifungal activities of Bauhinia galpinii N. E. Br., Combretum caffrum (Eckl. & Zeyh.) Kuntze, Markhamia obtusifolia (Baker) Sprague, and Maytenus undata (Thumb.) Blakelock, against mycotoxigenic fungi and potentially develop plant-based bio-fungicides. The leaves of the ethnobotanically selected plants were extracted using methanol and tested for antifungal activity against phytopathogenic strains of Aspergillus parasiticus, Aspergillus nomius, Fusarium verticilloides, Fusarium graminearum, Fusarium oxysporum, Penicillium haloterans and Cladosporium cladospoides. The antioxidant activities of the extracts were tested using the 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azinobis (3-ethylbenzthiazoline-6-suphonic acid) (ABTS) assays. The MTT (3-(4,5-dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide) cytotoxicity assay was used to investigate the effects of the extracts on the viability of the African green monkey kidney (Vero) cells and the human colorectal adenocarcinoma (Caco-2) cells. Additionally, the micronucleus test was used to examine the potential of these plants to induce genotoxicity in Vero cells. The identity and quantity of phytocompounds in the plant extracts were analysed using gas chromatography (GC-ToF-MS) mass spectrometry and liquid chromatography time-of-flight mass spectrometry (LC-TOF-MS) analysis. The highest percentage extract yield was recorded from Combretum caffrum (23%), while Markhamia obtusifolia yielded 10.7%. Bauhinia galpinii methanol extracts had the lowest MIC value of 0.16 mg/ml against Furasium graminearum, Furasium oxysporum, Penicillium haloterans, and Cladosporum cladospoides at 24-hour incubation period. C. caffrum had a MIC value of 0.31 mg/ml against F. vercitilloides and F. graminearum. C.caffrum had the highest total activity (TA) of 1437.5 ml/g against the fungal pathogen, Aspergillus nomius. In the antioxidant studies, C. caffrum had good antioxidant activity against DPPH with 50% inhibitory concentration (IC50) value of 10 μg/ml, while B. galpinii had IC50 value of 50 μg/ml against free radicals of ABTS. Based on the ABTS/DPPH correlations (ADC), M. undata exhibited a notable ADC correlation of 2.33, which is over three-fold that of the ADC of ascorbic acid (control drug). Furthermore, all extracts were assayed for cytotoxicity and genotoxicity using the MTT and micronucleus assays, respectively. In the cytotoxicity studies, B. galpinii, C. caffrum, and M. undata extracts were cytotoxic to Vero cells at 500 μg/ml. C. caffrum and M. undata extracts demonstrated cytotoxic effects on human colorectal adenocarcinoma cells in a dose-dependent manner. C. caffrum extract significantly reduced cell viability by up to 89.61% at the highest concentration of 500 μg/ml. M. undata extract was also cytotoxic at the highest tested concentration of 500 μg/ml. B. galpinii extracts increased the viability of the cancer cells, indicating a potential stimulation of cell proliferation. The extract from M. obtusifolia had no effect on the cells at the tested concentrations of 250 and 500 μg/ml. These findings suggest that C. caffrum and M. undata extracts have potential as cytotoxic agents against colorectal adenocarcinoma cells, while B. galpinii extracts may not be suitable for cancer treatment. All tested plant extracts were genotoxic at the highest tested concentration of 500 μg/ml. C. caffrum showed genotoxicity at 250 μg/ml and reduced cell numbers to less than 50% of the control at 500 μg/ml, indicating its significant cytotoxic potential. Three of the four medicinal plants reported a higher phenolic content (TPC) compared to their total flavonoid compounds. These higher TPC contents may well explain the antimycotoxigenic and antioxidative ability of the medicinal plants selected for the current study. The GC-ToFMS analysis of the selected medicinal plants revealed that all the extracts contained Hexadecanoic acid (HA) and methyl ester. B. galpinii and C. caffrum exhibited the presence of lupeol, while M. obtusifolia and M. undata contained phytol. Kaempferitrin was identified following the LC-TOF-MS analysis of both B. galpinii and M. obtusifolia. Quercitrin was identified from C. caffrum, while rutin and pisumionoside were identified from M. undata. Overall, the study concluded that the strong inhibitory effects against various fungal pathogens shown by some of the tested plants indicate that investigating plants as potential solutions for fungal infections in crops offers a promising avenue to enhance food safety and security in developing and underdeveloped countries. Therefore, it is important to intensify efforts to explore the use of plants as potential biopesticides. To further advance the findings of the current study, continued research and safety assessments of the tested South African plant extracts are essential to fully exploit their potential benefits in agriculture and healthcare while mitigating any risks to human health and the environment.